With Pala Presents, we offer selections from the library of Pala International’s Bill Larson, who shares with us some of the wealth of information in the realm of minerals and mineralogy.
Gem- and Lithium-Bearing Pegmatites of the Pala District, Part 2
Richard H. Jahns
Lauren A. Wright
Prepared in Cooperation With the
United States Geological Survey, June 1951
The pegmatites in the Pala district occur in a curving belt that occupies an area of about 13 square miles. The belt extends from Pala Canyon south-southeastward to the southern base of Pala Mountain, and within it are exposed at least 400 pegmatite dikes. Other pegmatites are scattered through the surrounding area. Within the district, most of the deposits of known commercial interest lie north and northeast of Pala, on the slopes and crests of Queen Mountain, Chief Mountain, Little Chief Mountain, and Hiriart Mountain. These mountains and most of the pegmatites exposed thereon are shown on plate 2.
|Principle pegmatite dikes, mines, and prospects in main part of Pala district|
|Mountaina||Pegmatite dike or dike groupa||Mines and/or prospectsb||No. on plate 2||Principal outputc|
|Queen Mountain||West Canyon||West Canyon (Freak) prospect||1||--------|
|Happy Hooligan||Happy Hooligan prospect||3||--------|
|Tourmaline King||Tourmaline King (Wilke, Schuyler) mine||4||Tourmaline, quartz|
|Ed Fletcher mine||5||Tourmaline, quartz|
|White Cloud||White Cloud (Buster Brown) mine||6||Tourmaline, quartz|
|Emerald||Enerald (Upper Queen) prospects||7||--------|
|Tourmaline Queen||Tourmaline Queen mine||8||Tourmaline, quartz|
|Queen Extension prospects||9||--------|
|Pala View (Sholder-Trotter) mine||10||Tourmaline, quartz|
|Mission||Mission mine||11||Tourmaline, quartz|
|Pala King||Pala King (Spring Bank, Wedge) prospect||12||--------|
|Stewart||North Star mine||13||Tourmaline, quartz|
|Gem Star (Loughbaugh) mine||14||Tourmaline, quartz|
|Stewart mine||15||Lepidolite, tourmaline|
|Alvarado mine||16||Lepidolite, tourmaline|
|Stewart Extension||Stewart Extension prospect||17||--------|
|Douglass||North Douglass prospect||19||--------|
|Douglass Extension prospect||20||--------|
|Douglass mine||21||Tourmaline, quartz|
|Pala Douglass mine||23||Tourmaline, quartz|
|Mountain||Pegmatite dike or dike group||Mines and/or prospects||No. on plate 2||Principal output|
|Chief Mountain||Salmons View||Upper Salmons View prospect||24||--------|
|Lower Salmons View prospect||25||--------|
|Redlands King prospect||26||--------|
|Blanket||Lower Blanket prospects||27||--------|
|Upper Blanket prospects||28||--------|
|Hazel W.||Hazel W. prospect||29||--------|
|West Chief||Canyon King prospect||30||--------|
|West Knickerbocker prospect||31||--------|
|Margarita mine||32||Beryl, tourmaline, quartz|
|Crystal King prospects||33||--------|
|Chief Extension prospects||36||--------|
|Pala Chief||Pala Chief mine||37||Spodumene, tourmaline, beryl, quartz|
|Verdant View (Anita) prospect||38||--------|
|Chief Ridge||Chief Ridge prospects||39||--------|
|East Knickerbocker prospect||40||--------|
|Ocean View||Meadow prospects||41||--------|
|Poison Oak prospects||42||--------|
|Ocean View mine||43||Beryl, quartz|
|Redwing (Redlands) prospects||44||--------|
|Jackpot Tunnel (Butterfly) prospect||45||--------|
|North End||North End prospects||46||--------|
|Mountain||Pegmatite dike or dike group||Mines and/or prospects||No. on plate 2||Principal output|
|Little Chief Mountain||Big Slope||Big Slope prospect||49||--------|
|Little Chief||Little Chief prospects||50||--------|
|Hiriart Mountain||Anita||Chaparral prospects||52||--------|
|Anita mine||53||Beryl, spodumene, quartz|
|Spar Cut prospect||54||--------|
|Snake Den||Snake Den prospects||55||--------|
|Center Drive||Center Drive prospect||56||--------|
|Upper Katerina prospect||57||--------|
|Senpe||Senpe (Sempa) mine||58||Beryl, lepidolite, quartz, tourmaline|
|El Lobo prospects||59||--------|
|White King||Pluto prospect||60||--------|
|White King prospect||61||--------|
|White Queen||White Queen mine||62||Spodumene, beryl, quartz|
|Spar Pocket||Spar Pocket mine||63||Beryl, quartz|
|Vanderburg-Katerina||San Pedro mine||64||Beryl, spodumene, quartz, tourmaline|
|Vanderburg (Naylor-Venderburg, Sickler) mine||66||Spodumene, beryl, quartz, tourmaline|
|Katerina (Ashley, Catherina, Katrina) mine||67||Spodumene, lepidolite, beryl, quartz, tourmaline|
|Landslides from Vanderburg-Katerina dike group||Hiriart prospects||68||--------|
|Hiriart mine||69||Spodumene, quartz|
|Fargo||Fargo mine||70||Tourmaline, quartz|
|El Molino||Canyon prospect||71||--------|
|Naylor mine||72||Tourmaline, quartz|
|El Molino mine||74||Beryl, quartz|
|a Listed from west to east.
b Listed from north to south, within each dike or dike group.
c Does not include material obtained by mineral collectors. Principal output of minerals other than lepidolite is gem and specimen material.
The pegmatites on Queen Mountain, westernmost and largest of the four mountains, are best known as commercial sources of lithium minerals, gem tourmaline, and quartz crystals. One deposit, the Stewart, has yielded more than 20,000 tons of lepidolite, some amblygonite and other lithium minerals, bismuth minerals, and a remarkable suite of phosphate minerals. Another, the Tourmaline Queen, has been the leading source of pink, green, and blue tourmaline in the district. Three pegmatite dikes, the Tourmaline King, Tourmaline Queen, and Stewart, have been extensively mined, and many others have been prospected or mined on a small scale. Most of these dikes are listed in table 2.
The Pala Chief mine, on Chief Mountain, has been the world’s greatest source of gem spodumene. In addition it has yielded gem beryl and some gem tourmaline. These minerals have been recovered also in several prospects and small mines elsewhere on this hill, where the number of pegmatite dikes is considerably greater than on Queen Mountain to the west. Many pegmatites are exposed also on Little Chief Mountain to the south, but no mining and very little prospecting have been done there.
Pegmatite dikes are also closely spaced on Hiriart Mountain, on whose slopes they appear as well-defined riblike projections. Gem spodumene and beryl, quartz crystals, lepidolite, and some green tourmaline of gem quality have constituted the commercial output from the pegmatites on this mountain. Other lithium minerals, and several species of columbium-tantalum, bismuth, and phosphate minerals occur in the Katerina-Vanderburg dike group. The pegmatites most extensivelv worked include the Senpe, San Pedro, Katerina-Vanderburg, and El Molino (pl. 2), and there are at least 70 prospect openings in other dikes on the mountain.
A few outlying pegmatites are on the rugged hills immediately north of those described above. Most of them are small, and few are traceable for strike distances of more than 200 feet. The only dikes with marked continuity are exposed on the low hills north of Hiriart Mountain and immediately east of Chief Mountain. Even these dikes, however, do not appear to contain more than traces of lithium or gem minerals.
Many pegmatites occur on Pala Mountain, especially on its southwestern and eastern slopes. Some of them are shown in plates 1 and 2. Several contain quartz crystals and massive rose-colored quartz, and both tourmaline and beryl have been reported. Very little commercial production has been obtained from any of these dikes.
With only a few exceptions, the pegmatites are restricted to areas underlain by gabbroic rocks. This generalization applies particularly to pegmatites that contain deposits of commercial interest. The pegmatite areas coincide with the outcrop belt of one, or possibly two large plutons of gabbroic rocks, which in general are bounded on the north and east by granodiorite and metasedimentary rocks, and on the south and west by tonalite. Other, nearby intrusive masses of gabbro, though apparently similar in petrology and structure, contain very few pegmatites.
The intimate relation between pegmatite and gabbroic rocks is not ascribable to the relative age of these gabbroic rocks, but appears instead to be a reflection of some preferred structural feature in the gabbros and norites. The metasedimentary rocks of the pre-batholithic series, for example, antedate even the gabbro, and yet they are hosts to very few pegmatites. Moreover, the pegmatities are younger than the Bonsall tonalite, as they cut that rock on Little Chief Mountain and transect tonalite dikes in many other places (pl. 1). That they are also younger than the Woodson Mountain granodiorite is shown by their occurrence in a large mass of that rock on the southeastern part of Hiriart Mountain, the south edge of Carver Mountain, and at several other localities. They even cut across dikes of fine-grained granodiorite that are younger than the typical Woodson Mountain granodiorite.
Most of the pegmatites are well exposed, despite the thick cover of vegetation in much of the area. They are more resistant to erosion than most of the country rock, and thus typically form low knobs and rib-like protuberances on the hillsides. Such series of projecting outcrops are especially prominent on the east slopes of Little Chief and Hiriart Mountains. The pegmatite minerals are little altered by weathering, although much near-surface pegmatite is severely broken and otherwise mechanically disturbed. This disturbance is particularly common in exposures on and immediately below old erosion surfaces along the southwestern side of Queen Mountain and the west side of Pala Mountain. The adjacent gabbro is partly decomposed to depths of at least 30 feet in many places.
Excellent sections of most of the pegmatites may be seen where the dikes crop out on northeast, east, and southeast slopes, and in such places their structure and mineralogy can be observed. Studies in three dimensions are possible where the slopes are cut by narrow, steepsided ravines, or where there are accessible mine workings.
The pegmatites are strikingly uniform in shape. Nearly all are markedly tabular masses, and have gentle to moderate westerly dips. They range from stringers less than an inch thick to large dikes with bulges nearly 100 feet in maximum thickness. Most dikes of commercial interest are 5 to 25 feet thick, with an average thickness of slightly less than 10 feet. Most of them also are continuous, and can be traced along their strike for distances of half a mile or more. Most dikes that are thicker than 3 feet appear to be more than 400 feet long, and even the thinnest stringers are remarkably persistent.
A general uniformity in attitude is characteristic. The dikes range in strike from north-northwest to north-northeast, but most of them trend within a few degrees of due north. They dip westward at angles ranging from 5 to 60 degrees, but the dips of nearly all lie within the range of 10 to 35 degrees. The average value is about 20 degrees.
Two general types of irregularities in attitude, distinguished by a considerable difference in scale, are widespread in the area. The walls of some dikes are marked by numerous septa and tabular inclusions of country rock, most of which are less than 6 feet in maximum dimension. Other wallrock contacts are serrate on a small scale, with the amplitude of the irregularities rarely greater than half an inch. Most of the contacts, however, are remarkably regular in detail.
In contrast to their detailed regularity, nearly all the pegmatite dikes bend broadly in both strike and dip. Such variations are confined to a small range of attitudes, and generally form gentle rolls and terracelike features. These features are 20 feet to 600 feet or more wide, and 40 feet to more than 1,000 feet long. The axes of some are parallel to the strike of the pegmatite, and hence they appear as broad, gentle steps on the dike. Such essentially horizontal benches are most common on the west knob of Hiriart Mountain, where several pegmatites flatten in dip by as much as 40 degrees. Some of these structural terraces are 500 feet to 800 feet wide, and extend for distances of more than 1,500 feet along the strike of the pegmatite dikes. Similar features occur elsewhere in the district, but in general are much smaller.
The axes of other rolls are inclined, so that the simple form of the west-sloping dikes is complicated by broad corrugations. These axes plunge directly down the dip, or very nearly so. Such rolls are best exposed on the west slopes of Chief and Little Chief Mountains, where there are several broad dip slopes of pegmatite. The gabbroic wallrock is exposed as elongate but irregular patches where the crests of the rolls have been breached by erosion, or where the rock is preserved as hanging-wall remnants along the troughs of adjacent rolls. Both the plunging rolls and the terracelike features may have been partly responsible for the localization of commercially desirable minerals in certain parts of the pegmatite dikes.
Some of the major pegmatites, like the Tourmaline Queen, Stewart, and Douglass on Queen Mountain, are essentially isolated. Little or no pegmatite occurs in the nearby country rock, even as minor lenses or stringers. Other pegmatites form swarms of subparallel dikes that are separated by only a few feet or a few tens of feet of country rock. Such dike groups are widespread throughout the north-central and northeastern parts of the district.
An exceptional exposure of parallel dikes is on the southeastern face of Little Chief Mountain. Sub-parallel dikes are also exposed on Hiriart Mountain, but these dikes commonly branch out and converge, as traced along their strike. Markedly anastomosing patterns appear locally, but a general parallelism remains the most consistent feature. Three-dimensional exposures provided by canyons and by mine workings indicate that many of the pegmatites branch out or join one another in a down-dip direction. Many of those that are very close together converge to form thick composite dikes. Although such juxtaposed dikes crop out as single tabular masses of pegmatite, they commonly retain their individual identities, as shown by their internal structure and by the slivers and thin plates of country rock between some of them.
The dikes characteristically taper at their ends, forming thick stubs or thin, elongate prongs in the country rock. Some terminations are less simple, ranging from forked tongues to complex groups of pegmatite stringers. Locally these stringers form stockworks, especially where the country rock is quartzite or schist. In general the ends of the thickest dikes are most complex. They characteristically form series of subparallel septa and lenses along well-defined horizons in the country rock. This feature is shown in several parts of the underground workings at the Tourmaline King mine.
The pegmatites are not systematically related to the truly primary structural features in the enclosing rocks. Their general attitude, for example, is wholly independent of foliation, schistosity, and lineation in the plutonic and pre-batholithic rocks, and neither their form nor their size appears to be a function of such features in the igneous country rocks. The few pegmatites in the least quartzitic parts of the metamorphic series do reflect changes in structure and lithology of the wall rocks. They pinch and swell more than the other dikes, and commonly are distinguished by bulges and irregular protuberances where they cross the less competent layers of the wall rock. In general, however, the pegmatites in the quartzitic, or dominant part of the Julian sequence exposed in the Pala district, are little different in form and structure from those in the nearby batholithic rocks.
The striking uniformity in shape and attitude of the pegmatite dikes seems best attributable to their emplacement along a well-developed set of fractures. These fractures are present throughout the Pala district, and occur also in adjacent areas and in adjacent pegmatite districts. In most places they do not contain pegmatite dikes, although of course they are most conspicuous where pegmatites have been injected along them. They are not related to individual plutons or other masses of country rock, but instead appear to have been superimposed uniformly upon groups of these masses and upon all earlier structural features within the masses. The fractures transect contacts between major rock units, and vary little in general attitude over areas of several square miles. They appear to be most abundant and closely spaced in the gabbroic rocks.
|click to enlarge) of northern part of Pala district, showing contrast between the smooth slopes of Queen Mountain, Hiriart Mountain, and other hills underlain by gabbroic rocks and the adjacent more bouldery slopes underlain by granodiorite. View is toward the west-northwest. QM, Queen Mountain; CM, Chief Mountain; LCM, Little Chief Mountain; HM, Hiriart Mountain; SM, Slice Mountain; MF, McGee Flats; WC, White Cloud mine; TQ, Tourmaline Queen mine; S, Stewart mine; GS, Gem Star mine; K, Katerina mine; V, Vandenburg mine; EM, El Molino mine; G, Gabbro quarry. Pacific Air Industries photoAerial view (|
The origin of the pegmatite-controlling fractures is not clear. Attempts have been made to correlate them with the Elsinore fault and other major structural breaks in the region, but without success. They cannot be satisfactorily related to such faults in terms of abundance or trends in attitude, nor can they be well explained as fillings of gash or shear fractures in terms of known movements along a given fault. Finally, the pegmatites themselves, which fill some of the fractures, antedate much if not all of the fault displacement.
The fractures and pegmatites are almost certainly pre-Tertiary in age, but are distinctly younger than the individual units of batholithic rocks in which they occur. Perhaps the fractures are purely tensional features, developed by an almost regional subsidence during the end stages of cooling in the southern California batholith. They may well have been tilted westward since their formation, as the entire Peninsular Range province is thought to have been tilted in this direction to various degrees during middle and late Tertiary time.
Most irregularities in the pegmatite dikes are attributable to irregularities in the pattern of host fractures. In general, the branching pegmatites appear to have been emplaced along branching, subparallel fractures, even though some pegmatites do not follow these fractures in detail. Some bulges in the dikes probably have a similar origin, but others evidently were developed along intersections with different sets of fractures. The origin of still others is not clear. The structural terraces and rolls previously described appear to be direct reflections of variations in attitude of the fractures.
Graphic granite forms the most widespread rock type in the Pala pegmatites. Typically it consists of large crystals of perthitic microcline, in which many spindlelike and rodlike anhedral grains of quartz lie essentially parallel to one another. Most of this rock is easily recognized in the field because of the quartz rods, although some varieties are so fine grained and dense appearing that at first they may be mistaken for other types of fine-grained pegmatite.
The graphic granite is ordinarily resistant to erosion, and forms bold, somewhat rounded outcrops. Together with massive quartz, it forms the best-exposed parts of the pegmatite dikes. In general it occurs along and near the hanging-wall parts of these dikes, but is by no means restricted to them.
The host crystals of perthite are white, gray, and tan, and the deep-flesh and reddish colors common in other pegmatite districts are rare here. The crystals are anhedral to euhedral; most are subhedral. They yield broad, fairly straight cleavage surfaces 2 inches to more than 8 feet in diameter, with an average diameter of less than a foot. The microcline encloses typical blebs and tablets of albite, which in general are less than 1 millimeter thick. They range considerably in size, however, and the largest ordinarily are in the coarsest crystals of potash feldspar.
The quartz rods in the graphic granite that has the most regular structure are characteristically many times as long as they are thick. They range from about half an inch to 14 inches in maximum dimension, and have cross sections ranging from slightly less than 1⁄16 inch to nearly an inch in diameter. Some of them are truly rodlike, with triangular to subrounded sections. Others are markedly platy, and still others have V- or L-shaped sections. Many are uniform in the direction of their elongation, but others bulge in the middle and taper near their ends. Some are closely spaced, and even touch one another, but most of them are at least as far apart as their own thicknesses. They are therefore separate masses, rather than elements of a single skeletal crystal, although their orientation is remarkably consistent.
In general the c-axes of the rods are nearly parallel to the a-axis of a given host perthite crystal, and slope gently toward the observer as the crystal is viewed from the front along its a-axis. In some perthite crystals, however, the rods are much less regularly disposed. Many are plate-like masses, and are grouped in a rudely radial pattern with respect to the a-axes of some host crystals, and the c-axes of others. Many rods appear to be perpendicular or nearly perpendicular to the walls of the enclosing pegmatite dikes, although exceptions are not uncommon. This orientation seems to be in part a reflection of preferred growth of the host perthite crystals along their a-axes.
The coarsest varieties of graphic granite appear to be the least regular in structure. They ordinarily contain rods of bulbous shape, which are distributed in an almost random position in many crystals. In other crystals the rods have a crude radial pattern. Some of the rods are not simple, but instead appear to be skeletal crystals of quartz, with abundant interstitial feldspar. Most are not so elongate as those in the more uniform graphic granite.
Ordinarily the graphic granite contains 15 to 25 percent quartz, although some of it is richer, and consists of unusually large, tapering quartz rods packed very close together. In contrast, much of the coarse perthite in the interior parts of the dikes contains only a few scattered rods. Commonly the rods in such feldspar die out abruptly, either along crystal boundaries or along planes parallel to crystal boundaries but within individual crystals. Elsewhere they decrease in number and die out gradually, so that the graphic granite merges into coarse, blocky perthite without quartz rods.
Although much of the graphic granite is remarkably free from other minerals, particularly in the upper parts of many pegmatites, it is commonly associated with finer-grained aggregates of quartz, muscovite, and albite. These are interstitial to the crystals  or crystal groups of graphic granite. Many of these aggregates also contain abundant prisms of schorl, and others are rich in garnet. Muscovite, in foils and plates as much as 5 inches in maximum dimension, commonly forms radiating sprays within graphic granite, where it is characteristically associated with fine-grained, sugary albite. In this form it generally has been termed “plumose muscovite.” Such fine- to medium-grained mineral aggregates as these ordinarily constitute 5 percent or less of the containing graphic-granite pegmatite, although in some places, particularly in and near those parts of pegmatite dikes of greatest commercial interest, they are more abundant.
The graphic granite of many pegmatites is transected by individual fractures and groups of parallel fractures that in general are conformable with the pegmatite walls. Along many of these fractures are aggregates of quartz, muscovite, albite, and other minerals, chiefly tourmaline and garnet. Some of the fractures contain aggregates of sugary albite, and little else. Fine-grained albite also is widespread along shorter, more irregular fractures, and also occurs as impregnations of irregular shape. Many of these impregnations are extensive, appearing as blanketlike masses as much as 8 feet in diameter.
Extremely coarse-grained varieties of pegmatite that contain little or no graphic granite occur in many of the dikes, including most of those that have yielded commercial minerals. These varieties are known throughout the district as “giant pegmatite.” They consist mainly of quartz or perthite, or both, and in a few pegmatites also contain spodumene. They characteristically form very conspicuous units in the inner parts of the pegmatite dikes. Such units are largest in the thickest dikes, or in bulges in thinner dikes. Where they are chiefly massive quartz, these units form conspicuous white patches on the hillsides, and in many places form small ridges and knobs. Units that are rich in perthite form more subdued, irregular slopes with abundant feldspathic rubble. Many of the exposures are similar in general appearance to exposures of graphic granite. The spodumene-bearing units, in contrast, rarely crop out, even on the steepest slopes.
Most abundant in the thickest dikes are aggregates of very coarse, blocky perthite, occurring as crystals 2 or 3 inches to as much as 8 feet in diameter. The average diameter is about 2 feet. Most of these crystals are equant, but some are elongate in directions parallel to their a-axes. Most of them are subhedral, and are best recognized in outcrops and mine workings by their straight and uniform cleavage faces. The most characteristic colors are white, flesh, and an unusually dark gray. The gray is characteristic of many of the coarsest masses, particularly in pegmatites with abundant lithium minerals, and is in marked contrast to the lighter colors typical of the graphic granite.
Some of the blocky perthite is fairly pure, occurring as very coarse aggregates of dark-gray color. The included plates of plagioclase stand out prominently in such material. Other groups of perthite crystals are associated with much finer-grained aggregates of quartz, sugary albite, muscovite, and tourmaline. Much of the perthite has been fractured, and subsequently mineralized along these fractures, in the same general manner as the graphic granite described above.
Another common type of very coarse-grained, or giant pegmatite consists of individual perthite crystals, or aggregates of several crystals, scattered through a matrix of anhedral quartz. They are typically euhedral and range in diameter from 1 inch to as much as 10 feet. Most masses are 12 to 18 inches in diameter The color of these crystals ranges from white through flesh and tan to dark gray. With decreasing quartz content, this rock type grades into the coarse, blocky perthite just described. Toward the centers of some pegmatite dikes, the proportion of perthite decreases, and the rock grades into massive quartz.
Some masses of quartz with individual perthite crystals are fairly pure, but most of them contain small grains of other minerals. Some of the small grains are along continuous and regular fractures, and some are near or along contacts between quartz and perthite. A few pegmatites have anhedral to euhedral crystals of beryl in this quartz-perthite unit. The most characteristic habit of the beryl is prismatic, and some crystals are as much as 2 feet long. This beryl is white, light gray, and pale yellowish green to dark bluish green.
Perhaps the most spectacular type of coarse-grained pegmatite is the so-called massive quartz, which appears as homogeneous masses of gray to milky-white color. Actually, these masses are aggregates of' subhedral to anhedral quartz crystals that are 3 inches to 6 feet in diameter. The quartz is characterized by unusually well developed rhombohedral cleavage and very coarse lamellar twinning. In addition it exhibits many growth lines, lines of bubbles and inclusions parallel to crystal boundaries, and layers, 1⁄16 to 3⁄16 inch thick, of alternately milky and clear material. Some of the quartz contains scattered crystals of feldspar, and grades into the units previously described. Other masses grade into quartz-spodumene aggregates, and still others contain scattered crystals of beryl.
A very striking rock type in many but not in the majority of the pegmatites, consists of lathlike crystals of spodumene in a mosaic of anhedral quartz crystals slightly less than a foot in average diameter. The spodumene ranges from equant crystals about half an inch in maximum dimension to blades and laths as much as 7½ feet long and 2 by 14 inches in section. The average dimensions of the laths, however, are about 18 inches and ½ inch by 3 inches, respectively. Some of the spodumene is relatively fresh, although it appears dull and opaque. Very light gray to yellowish gray are the most common colors. Most of the crystals are considerably altered, and occur in the light to dark-gray quartz as soft, chalky stripes and streaks. Few of them appear to be oriented in any systematic way, and the arrangement in most large exposures suggests groups of jackstraws. Typical exposures are present in the Stewart, Pala Chief, and Katerina mines.
The quartz-spodumene pegmatite in some dikes contains coarse, irregular masses of amblygonite. A few individual crystals are as large as 3 feet, and one aggregate of coarse crystals 18 feet in maximum dimension was encountered in the Stewart mine. Crystals and crystal groups of lithiophilite, some as much as 15 inches in diameter, are associated with the quartz-spodumene rock in a few pegmatites, and occur also in the nearby perthite-bearing parts of the dikes. These coarse crystals are especially common in the Stewart and Katerina mines. Beryl, generally in white to very pale pink, equant crystals, is scattered through the quartz-rich parts of some quartz-spodumene units, but it is not at all common.
|click to enlarge) of Little Chief and Chief Mountains, showing westerly dip of the pegmatite dikes. A belt of bouldery granodiorite crosses the area in the middle distance. View is toward the north. SC, Salmons City (abandoned); PC, Pala Chief mine; WC, West Chief pegmatite (broad dip slope); MF, McGee Flats. Pacific Air Industries photoAerial view (|
The lepidolite and gem-bearing part of a given pegmatite dike in the Pala district is referred to locally as the “pocket zone,” “pay streak,” “clay layer,” “gem seam,” or “gem strip.” This type of pegmatite ordinarily occurs in the central parts of the dikes, although in some it is within a foot of the hanging wall. It is characterized by fine-grained to very coarse-grained minerals, many of which contain lithium, beryllium, bismuth, boron, caesium and rubidium, columbium and tantalum, flourine, manganese, phosphorus, or combinations of these elements. The most common minerals are quartz, albite, orthoclase and subordinate microcline, muscovite, lepidolite, and tourmaline. Spodumene forms as much as 25 percent of some gem units, but most of the spodumene-bearing rock contains no gem material.
In the strictest sense, the term “pocket pegmatite” is a misnomer, as actual cavities are not particularly abundant in most of the pegmatites. In many of the so-called pockets, the minerals are markedly euhedral, but most of the minerals are in contact with one another, and where open space does exist, its relative volume is very small. Other pockets are partly or completely filled with so-called pocket clay, which is characteristic of most of the gem-bearing units in the pegmatites.
Well-formed crystals of quartz are abundant in the pocket pegmatite. Most of them are associated with coarse cleavelandite and aggregates of large foils and plates of muscovite. Well-formed crystals of perthitic orthoclase are abundant in some pegmatites. Few of these crystals exceed 8 inches in maximum dimension, and most are less than 5 inches. Some of the quartz crystals, in contrast, are as much as 3 feet long, and several weighing 100 pounds or more have been recovered during mining operations. Sugary albite also is abundant, and is generally associated with irregular small masses of quartz and aggregates of muscovite, schorl, lepidolite, or combinations of these minerals. Schorl, some of it in prisms as much as 4 feet long, is particularly abundant around the margins of units of pocket pegmatite. In contrast, the green, blue, white, and pink varieties of tourmaline form the central parts of such units.
Marked concentrations of coarse muscovite and schorl, in places associated with garnet, are generally most characteristic of the outer parts of gem-bearing masses. These masses generally contain both very coarse-grained and very fine-grained minerals in close association. The fine-grained minerals, chiefly albite, lepidolite, and muscovite, form compact aggregates, to which coarser crystals of other minerals are commonly attached. Elsewhere these aggregates appear to cover earlier formed, coarser crystals.
Many gem crystals of tourmaline, beryl, and spodumene are attached to quartz or to other typical pocket minerals, and such material is ordinarily referred to as “frozen.” The gem material of highest quality, however, is not attached, but is commonly arranged without known systematic distribution within the masses of clay that fill the cavities.
Unlayered and Poorly Layered Varieties. Fine-grained rocks with typical granitoid texture arc well represented in most of the pegmatite dikes, although in general they are not as well exposed as the graphic granite and other coarser varieties of pegmatite. They are most common in the footwall parts of the dikes, and have formed entire dikes in some areas, particularly on Hiriart Mountain and on the north slope of Pala Mountain.
Nearly all these rocks, known in the district as granite, sugar granite, mica aplite, or albitite, are composed mainly of quartz and albite, with or without microcline, muscovite, schorl, garnet, biotite, or other minerals. A sugary texture is characteristic, and the average diameter of individual mineral grains is about 2 millimeters. The garnet crystals ordinarily are smaller, but some of the mica foils and strips are much larger, in places reaching maximum dimensions of half an inch to an inch. Despite these local variations, the rocks are fairly even-grained. Their average color is light gray, and nearly white feldspars contrast sharply with the gray of the quartz.
The uniform, unlayered appearance of these rocks is broken in some places by a very crude but broadly regular planar structure. This structure is commonly formed by ill-defined layers, 1⁄32 inch to several inches thick, that are relatively rich in scattered crystals of garnet. Garnet is by no means restricted to such layers, however, but is disseminated through most of the rock. Some layers are relatively rich in mica, and a few in fine-grained schorl. Other layers are relatively rich in quartz, and the layering in still other types of rock is caused by alternations of coarser-grained and finer-grained material of nearly uniform composition.
Ordinarily the groups of garnet-, mica-, and schorl-rich layers interrupt rather homogeneous, unlayered rock. Individual layers are at least ½ inch apart, and are much thinner than the material that separates them. They are uniformly flat, or nearly so, for great distances, but in some places are characterized by broad rolls. Locally they appear to have been folded on a smaller scale, and in a few places are even plicated. The planar structure in some of these fine-grained rocks is emphasized by stringers of younger pegmatite that may have been emplaced along fractures. The fractures follow closely the layering of the rocks, and it is only on a small scale that the later injected material cuts across this structure.
Where the pegmatite dikes are simple in internal structure, the fine-grained granitoid rocks generally pass abruptly upward or downward into graphic granite. As traced along the strike of the dikes, the contact relations at the ends of the fine-grained units are similarly abrupt in some places, but in others the fine-grained rock fades into graphic granite over distances of 2 feet to as much as 20 feet. Where the fine-grained rock is layered, the layers fade out with diminution of their characteristic mineral constituents, rather than taper out sharply.
Much of the fine-grained, albite-rich rock contains ragged masses of graphic granite ranging in maximum diameter from ¼ inch to nearly 3 feet. The average size of these masses, some of which appear to be remnants of once more extensive material, is slightly less than an inch. Graphic granite occurs also as tabular masses, some of them discordant but most of them parallel to whatever layering is present in the fine-grained host rock. These younger masses range from stringers less than ¼ inch thick to sills and dikes as much as 8½ feet thick. Many of the thicker dikes of graphic granite are pegmatities in their own right, and show internal structure similar to that of dikes enclosed by gabbroic rocks.
In many places the fine-grained rocks are cut by stringers of aplitic material of similar texture and mineralogy. These stringers are somewhat less regular than the graphic-granite units described above, and most of them fade out along their strike and down their dip. They are similar in many respects to the “auto-injection” material so widespread in the earlier gabbroic rocks of the district.
Layered Varieties: Line Rock. One of the most widespread and distinctive lithologic units in the Pala pegmatites is line rock, also well-known as albite aplite, garnet aplite, albitite, zebra rock, stripe rock, garbandite, and bottom rock. Several of these names are derived from its strikingly layered structure, which forms closely spaced, nearly parallel lines on most exposed surfaces.
The line rock is not so resistant to erosion as the graphic granite, but nevertheless in some dikes forms many outcrops. The best exposures of the more resistant, garnet-rich types of this rock are the surfaces of the very large boulders that veneer several landslide masses on the slopes of Hiriart Mountain, and the boulders immediately northeast of Ashley Ranch (pl. 1). This garnet-rich type of line rock seems to be in a greater proportion of the pegmatites on Hiriart Mountain than elsewhere in the district.
The line rock is much like the other varieties of fine-grained granitoid rocks noted above, and indeed grades into them in many places. In general, however, it is finer grained and more distinctly layered. The planar structure is formed by alternations of garnet-rich and garnet-poor layers that generally are 0.5 millimeter to 1 centimeter thick. The average thickness is between 2 and 3 millimeters. These layers occur in a uniformly finegrained mosaic of quartz, albite, and some muscovite and microcline. A few varieties of line rock are characterized by schorl-rich layers. Beryl, apatite, and monazite are rare accessory constituents of such rock. The garnet-rich and schorl-rich layers are ordinarily separated by several times their thicknesses of quartz-albite rock.
The layers are very uniform in width and spacing for considerable distances along their strike, and in general they are parallel to the walls of the enclosing pegmatite dikes. They are strikingly similar in all respects to the banded garnet phase of some pegmatites in the Owl Creek Mountains of Wyoming, as described by McLaughlin.  Some of the layers die out abruptly as traced along their strike, but others fade gradually into a more homogeneous aplitic rock, or into graphic granite. In general the layered rock is not coextensive with the dikes that enclose it, although it is commonly continuous for many tens of feet, both along the strike and down the dip of most dikes.
The strikingly planar structure of most line rock is marked by gentle undulations, which give a nodular or orbicular appearance to the surfaces of masses broken nearly parallel to the general attitude of the layering. In other places, the layers form gently curving loops, which taper or become broader as traced in a direction normal to the walls of the pegmatite. There is some evidence of plastic movement in part of the line rock. Flexures that suggest a uniform direction of dragging are widespread, though not particularly abundant. Piercing folds occur locally, and in most of them line rock in which the layering has been thrown into whorls transects the structure of adjacent undisturbed layers.
Some line rock appears to grade upward into massive quartz, with a gradual diminution of garnet-rich layers and a gradual increase in the distance of their spacing. This transitional rock is very striking in appearance, because of manganese oxide stains derived from alteration of the garnet. Much of the line rock contains masses of graphic granite with very ragged edges. These masses, markedly similar to those in the more homogeneous fine-grained rocks described above, have been interpreted as residua of much more extensive pre-existing graphic granite by Schaller.  On the other hand, much graphic granite occurs in the line rock as distinctly later sills, dikes, and augen. The individual masses ordinarily range in diameter from about an inch to at least 6 inches, and the aggregates that were injected are similar to those in the less well layered, fine-grained rocks described above. Most of them are clearly sill-like, and transect the planar structure of the host rock in very few places. It seems clear, on the basis of numerous exposures throughout the district, that the typical garnet-rich line rock contains at least two generations of graphic granite.
Additional rock types are common in the pegmatite dikes of the Pala district, but most of them are transitional between types already described. Others consist of abundant fracture-controlled minerals in graphic granite, in other very coarse-grained pegmatite, or in one of the finer-grained rocks. They vary greatly in texture and mineralogy, and do not lend themselves readily to classification and systematic description.
A few pegmatite lenses and dikes of a wholly different type are within the limits of the district. They range in composition from gabbro to granodiorite, and appear to be more closely related to the exposed batholithic rocks than do the younger and more truly granitic pegmatites described above. Many of them are lenses and irregular stringers of gabbroic pegmatite. Some are very coarse, and contain crystals of calcic plagioclase and hornblende as much as 4 inches in maximum dimension. Few of these basic pegmatites are continuous, and most of them are less than a foot thick and 4 feet long.
|click to enlarge) of Hiriart Mountain. Katerina mine and Ashley ranch in left foreground, Fargo mine near center foreground, and Vanderburg mine near summit of mountain. Dips of pegmatite dikes are moderate. View is toward the north-northwest. Pacific Air Industries photoAerial view (|
Other pegmatites, much more closely related to the principal varieties described in this report, are of tonalitic to granodioritic composition. They contain coarse, white plagioclase—chiefly median to calcic oligoclase—with quartz, biotite, muscovite, garnet, and some black tourmaline but no lithium minerals. Most of these pegmatites are in the northwestern and northeastern parts of the district. Several prospect pits have been sunk in this type of rock east of McGee road and 2000 feet north of the northwest corner of Hiriart Mountain (pl. 1), and similar openings have been dug farther to the northwest, in an area underlain chiefly by granodiorite.
The pegmatite dikes of intermediate composition are much like those that are truly granitic, as far as general structure is concerned, but are not so continuous along their strike. This lack of continuity is even more characteristic of the gabbroic pegmatites.
|Section through west tunnel, North Star mine, showing tapered edge of pegmatite dike in gabbro and quartz-mica schist.|
Many of the Pala pegmatites consist of two or more units of contrasting lithology, as already shown. The distribution of these units within each dike is essentially systematic, and is related—at least in part—to the overall structure of the dike. The disposition of minerals and the pattern of textural variations also follow certain rules within each unit, although there are many irregularities of detail. Most prospectors and mine operators in the district have recognized this orderliness, and the distribution of mine workings bears testimony to their thorough exploration of contacts between line rock and overlying graphic granite in a search for pocket material.
Rock units of contrasting composition and texture have long been recognized in many pegmatites. As early as 1871, for example, T. S. Hunt  remarked upon the distinct layering of many “granitic veinstones” in Maine. References to bands, layers, lenses, ribs, segregations, shoots, streaks, and zones of massive quartz and of other minerals or mineral aggregates are common in many early reports. Some investigators were impressed by irregularities of mineral distribution in pegmatites, it is true, but others recognized these as essentially irregularities of detail. During the period 1900–35 it was repeatedly noted that cavities, concentrations of both common and unusual minerals, and concentrations of economically desirable minerals tend to occupy characteristic positions. The attention of most investigators, however, was focused more upon the mineralogy of such contrasting units than upon their structural and petrologic relations.
During more recent years, increasing emphasis has been placed upon interpretation of the internal structure of pegmatites, both as a means of determining their genesis and as an aid in planning exploration, development, and mining.  Most workable concentrations of pegmatite minerals are in rock units that differ markedly in one or more respects from adjacent barren units. This has been repeatedly demonstrated by detailed studies in many areas. On the other hand, few homogeneous, or internally structureless pegmatite bodies contain concentrations of commercially desirable minerals sufficiently rich to yield satisfactory returns in mining under present economic conditions.
Three fundamental types of pegmatite units have been recently defined and described  as follows:
- Fracture fillings are units, generally tabular, that fill fractures in consolidated pegmatite.
- Replacement bodies are units formed primarily by replacement of consolidated pegmatite. Although there are all gradations between simple fracture fillings and fracture-controlled replacement bodies, the structural control for many replacement bodies is not clear.
- Zones are successive units that ordinarily reflect the shape or structure of the enclosing pegmatite body. In lenslike or podlike pegmatites they have a concentric pattern, and in the more tabular bodies they appear as simple layers. Many zones are not completely developed, and form straight or curving lenses, troughlike or hoodlike bodies, or chains of lenses.
Contacts between adjacent pegmatite units vary considerably in distinctness. Those between units of markedly different composition or texture are commonly of knife-edge sharpness, and often may be mapped on a scale as large as 20 feet or even 10 feet to the inch, whereas some of those between mineralogically similar units are difficult to assign within narrow limits, especially where such units are intergradational or very coarse grained.
So varied and uneven are the textures of typical pegmatites that no single term, such as “pegmatitic,” suffices for most descriptions. The following size classification of pegmatitic textures, adopted by the U. S. Geological Survey, is used throughout this report.
|Term|| General grain size
(in terms of maximum
diameter of each grain)
|Very fine (includes sugary, aplitic||Less than ⅛ inch|
|Fine||⅛ inch to 1 inch|
|Medium||1 inch to 4 inches|
|Coarse||4 inches to 12 inches|
|Very coarse, or giant||Greater than 12 inches|
A grouping of pegmatite zones into four main categories has been proposed by Cameron, Jahns, McNair, and Page  as follows:
- Border zones, or outermost zones.
- Wall zones.
- Intermediate zones.
- Cores, or innermost zones.
According to this classification, pegmatites that are not homogeneous ordinarily range from simple masses with only a border zone surrounding a core to masses with a border zone, wall zone, core, and several intermediate zones. There is no theoretical limit to the possible number of intermediate zones, but few pegmatites in the Pala district contain more than three such units.
The border zones of most Pala pegmatites are thin, inconspicuous selvages. They consist generally of fine-grained graphic granite in which most of the quartz rods are less than 1 millimeter in diameter. The host perthite crystals rarely are greater than 1 inch in maximum dimension, and most of them are less than ½ inch. A few pegmatites are marked by selvages of line rock or other fine-grained material.
The border zones pass into typical coarse-grained graphic granite of the adjacent wall zones with an abrupt increase in grain size. They are in even sharper contact with the country rock, and in some pegmatites shearing has juxtaposed thin slices of gabbro and border-zone material.
In general the outermost zones are an inch or less in thickness, and most are markedly discontinuous. Indeed, relatively fine-grained selvages are absent from many dikes, in which coarse-grained graphic granite lies against the gabbroic country rock. In most pegmatites, the border zones are more widespread along the hanging wall than along the footwall contacts.
In addition to graphic granite, most of the border zones also contain small quantities of fine-grained albite, muscovite, widespread garnet, and some schorl as tiny scattered specks. In general these minerals are rather evenly disseminated, but in a few pegmatites they are arranged in layers parallel to the nearby wallrock contacts. A very regularly layered border zone is well exposed in the workings of the Pala View mine, on the south side of Queen Mountain. Here a selvage of fine-grained graphic granite, ½ to 2½ inches thick, contains small, scattered flakes of pale-green muscovite, and is marked by thin, subparallel layers rich in quartz and garnet. The garnet occurs as deep-red crystals 2 millimeters in maximum diameter. In places where individual layers are at least ¼ inch thick, this mineral forms subgraphic intergrowths in quartz. Most of the garnet-rich layers are near the inner part of the border zone along both the hanging-wall and footwall contacts of the dike.
Wall zones, which represent by far the most abundant pegmatite material in the district, are composed of graphic granite, with or without other minerals. In most pegmatites, substantial quantities of interstitial quartz are present, commonly with subordinate albite, muscovite, and schorl. The typical graphic granite is most abundant and continuous in the hanging-wall parts of the dikes. There is some evidence, first cited by Schaller,  that it was once present in the footwall parts, possibly in great abundance, and that it was subsequently replaced by line rock and other types of fine-grained, albite-rich pegmatite. On the other hand, there are many dikes in which such evidence is rare, or does not seem to be present at all, so that the concept of a graphic-granite wall zone, symmetrically developed with respect to a central plane in each dike, cannot be applied with assurance to all of the pegmatites that contain line rock.
Some pegmatites consist almost wholly of graphic granite, and appear to comprise only two zones. In them the typical coarse-grained graphic granite should be termed a core, rather than a wall zone, although it seems probable that quartz-rich core segments are present somewhere in nearly all the dikes that appear to consist wholly of graphic granite.
|Sketch of pegmatite-gabbro relations on northeast wall of main tunnel, Pala View mine.|
Most of the wall zones are continuous. They constitute almost the full thickness of many dikes, in which they are interrupted only here and there by small segments of cores or intermediate zones (fig. 19). Large parts of other dikes, however, contain well-developed cores, which typically appear as discoidal masses of very coarse-grained pegmatite. These are single masses or rows, and ordinarily occupy central positions within the dikes (fig. 20). Most are markedly elongate, as if in reflection of the containing dikes themselves, and some are as much as 50 feet long. Excellent examples of thin, but continuous cores are exposed in the White Cloud, Tourmaline King, and El Molino dikes, and in the northern part of the Stewart dike. The thicknesses of such units rarely exceed 3 feet in the dikes of regular shape, but in some of those with prominent bulges or protuberances, the innermost zones are more nearly equidimensional. Such cores and core segments are commonly 5 to 15 feet thick, but strike lengths rarely exceed 35 or 40 feet.
The thinnest cores and segments of cores consist generally of massive quartz in very coarse-grained aggregates of anhedral crystals, or of such quartz with scattered large euhedral crystals of perthite. In some of these perthite-bearing units, the quartz is the subordinate constituent. Indeed, the cores of a few pegmatites consist almost wholly of very coarse-grained blocky perthite in subhedral to anhedral aggregates. In a few others the cores are massive quartz with scattered lath-shaped crystals of spodumene.
The relations of these simple innermost units are shown diagrammatically in figures 19 and 20A. The subhedral graphic granite, with its local interstitial aggregates of quartz, perthite, and albite of much smaller grain size, typically grades into the central units of coarse, euhedral perthite crystals in massive quartz. Quartz-perthite cores of this type are similar to miarolitic cavities, in that the feldspar crystals seem to have grown into a liquid- or gas-filled cavity, the cavity having been subsequently filled with quartz crystals.
Much less common than these tabular cores are the thick, pod-like cores at or near the centers of distinct bulges in several of the pegmatite dikes. Some of these thick cores consist of large euhedral to subhedral perthite crystals with interstitial massive quartz, and grade into the adjacent graphic granite of the wall zone as described above. Others are composed of nearly pure massive quartz, of massive quartz with giant spodumene crystals, or, rarely, of massive quartz, spodumene, and large anhedral crystals or crystal aggregates of amblygonite. Because of the great mineralogic difference involved, such units are very sharply separated from the adjoining wall zones. Still other cores are separated from the wall zones by one or more intermediate zones, which appear as partial or complete envelopes around the cores. Some of the intermediate zones are so incompletely developed that they appear only as curving lenses or rows of lenses. In a few pegmatites, distinguished by long, subdued bulges, the cores and intermediate zones have the form of discontinuous layers, rather than thick pods.
The general sequence of essential-mineral assemblages from zone to zone within a single pegmatite dike is remarkably consistent throughout the district. A consistency in sequence of textures also is present, so that the zonal lithology as a whole follows a well-defined pattern. In terms of fundamental textural and mineralogic characteristics, the arrangement of zones from the walls of the Pala pegmatites inward is as follows:
- Fine-grained graphic granite.
- Coarse-grained to very coarse-grained graphic granite.
- Perthite, chiefly in aggregates of very large subhedral crystals.
- Very coarse anhedral quartz (massive quartz) with scattered large euhedral crystals of perthite.
- Very coarse anhedral quartz (massive quartz).
- Very coarse anhedral quartz (massive quartz) with large subhedral crystals of amblygonite and euhedral crystals of spodumene.
- Very coarse anhedral quartz (massive quartz) with large euhedral crystals of spodumene.
- Very coarse anhedral quartz (massive quartz).
All of these units are known to occur together in only three pegmatites in the district, the Stewart, Pala Chief, and Vanderburg-Katerina. Most of the dikes contain only three or four of the units, generally Nos. 1, 2, and 4; 1, 2, and 5; or 1, 2, 4, and 5. Many of those on Chief and Hiriart Mountains contain No. 7 as well. Units Nos. 3 and 6 are in the thickest dikes only, and No. 6 is rare. Regardless of the number of such units present in a given pegmatite, however, their order conforms to the general sequence outlined above.
Albite and muscovite are irregularly distributed in most of the units and are locally abundant. The zones contain many other minerals also, but these minerals are either minor accessory constituents or appear to have been introduced after the host zones were formed. If all these minerals were taken into consideration in an analysis of the zones, they would complicate but not alter the basic sequence.
|Large crystals of perthite, with long quartz rods (above level of small opening) overlie a very coarse-grained aggregate of perthite and interstitial quartz, muscovite, and albite. Large crystal of pale yellowish-green beryl is crossed by shadow of hammer handle. Katerina mine.|
Most of the fracture fillings consist of quartz, albite, biotite, fine-grained muscovite, or combinations of these minerals, with or without minor accessory constituents. They range in thickness from knife edges to nearly 10 inches, and are most abundant in the outer zones of the pegmatites. Many transect the graphic granite of the wall zones, but merge with inner zones, particularly those very rich in quartz. The exposed parts of others lie wholly within single zones. Still others cut across entire pegmatite bodies, and are plainly younger than all the zones nearby.
Three general types of fracture structures appear to have guided mineral-depositing solutions in the pegmatites. One type, parallel to the pegmatite walls, ordinarily consists of individual subparallel fractures that are spaced ½ inch to at least 2 feet apart. Many of them are so regular in their development that they give the pegmatite a sheeted appearance. A second type, consisting of through-going fractures, is somewhat less common. Although these fractures also are regular in their distribution, few of them are very closely spaced. A third type of fracture, irregular but abundant and widespread, includes openings along cleavage directions in feldspar and other minerals, and also less regular openings along boundaries between adjacent mineral grains or through the grains themselves. Some of these fractures contain biotite blades that transect both quartz rods and surrounding host feldspar in the graphic granite of many pegmatites.
The minerals in most fracture fillings have corroded the fracture walls, and there are all gradations between simple open-space fillings and fracture-controlled replacement bodies. In general, the more complex the mineralogy of the fracture-related masses, the more they appear to have been formed by replacement of pre-existing pegmatite.
Some of the fracture fillings are composite, and evidently are the result of repeated fissuring and filling with new material. These are not abundant, but are in numerous pegmatites, notably the White Cloud, Tourmaline Queen, Stewart, Douglass, Ocean View, Little Chief, and several dikes on Hiriart Mountain. The layering is commonly emphasized by alternations of milky and clear quartz, or clear and smoky quartz, or, rarely, of quartz and other minerals.
Replacement bodies, many of which are obviously fracture-controlled, are present in nearly all the pegmatites. They are younger than the rock that forms the enclosing zones and their structural pattern is plainly superimposed upon the essentially concentric or layer-like arrangement of the zones. They are composed chiefly of fine- to coarse-grained albite, quartz, and muscovite. Lepidolite and tourmaline also are widespread, but are much less abundant. Numerous accessory species are present in some of the pegmatites, but rarely in more than very small quantities. They include beryl, bismuth minerals, clay minerals, columbite-tantalite, cookeite, manganotantalite, monazite, stibiotantalite, sulfide minerals, topaz, zeolites, and zinnwaldite. Other accessory minerals appear to have been formed earlier, and may well be indigenous to the zonal units of the pegmatites. Apatite, cassiterite, lithiophilite, triphylite, and some beryl, garnet, and schorl are typical examples of these.
The simplest replacement bodies are fracture-related units that have corroded the fracture walls. Perhaps most easily interpreted are those in the hanging-wall zones of graphic granite, in which they are generally parallel to the pegmatite-country rock contacts. They range from 1 millimeter to several inches in thickness, and commonly extend for several tens of feet along the strike. Groups of these units form anastomosing networks where developed along a single set of fractures, and more reticulate networks where their distribution was controlled by two sets of fractures.
On the west face of the south cut of the Stewart mine, excellent examples of fracture-related replacement bodies are near the hanging wall of the pegmatite. Here numerous thinly tabular masses of fine- to medium-grained quartz-albite pegmatite with abundant muscovite and green tourmaline clearly were formed along subparallel fractures in coarse-grained graphic granite. These units cut across individual crystals of perthite, and also across quartz rods and groups of such rods in the graphic granite. Thinner and more widely spaced replacement bodies are exposed in the main cut of the Tourmaline Queen mine. They are composed mainly of quartz and muscovite, with local concentrations of albite, tourmaline, and lepidolite.
Comb structure, with platy to pencil-like crystals oriented normal to the walls, is fairly common in the most tabular fracture fillings. The interiors of these units are marked in many places by sharply terminated crystals of quartz, cleavelandite, muscovite, lepidolite, and totirmaline. Individual crystals are rather small, especially where they are in the outer parts of the host pegmatites. Some of the replacement units contain crude layers, distinguished mainly by variations in quartz, muscovite, or tourmaline content. Some of this layering seems best ascribed to repeated Assuring and introduction of additional material. Other layers evidently were formed by diffusion processes, as they appear to be superimposed upon a coarser textural pattern. Many of these replacement bodies are layered only around a few scattered voids where crystals of tourmaline and lepidolite are more abundant than elsewhere.
Fracture-controlled replacement bodies are not confined to the hanging-wall parts of the dikes, but are also in line rock and other fine-grained granitoid units that commonly form the footwall parts. Most of them are essentially concordant with the planar structure in the line rock, or with the pegmatite-wall rock contacts or contacts between zonal units within the pegmatites. Many of these concordant units are as much as 50 feet long, but most do not exceed 6 feet in maximum dimension. Like those in the hanging-wall parts of the dikes, they commonly branch and join. Elsewhere they are connected by markedly discordant masses of similar material, so that the whole represents ladder veins or larger, reticulate masses. In a few pegmatites, notably the Naylor, replacement units occupy positions in the crests and troughs of warps and tighter folds in line rock. They resemble small phacoliths, with saddlelike or troughlike form.
There are all gradations between fracture-controlled replacement bodies that form series of parallel layers and those that form intricate stockworks. The variations are particularly well illustrated by tabular masses of cleavelandite in very coarse-grained aggregates of quartz. In the White Cloud, Pala Chief, and several other pegmatites, such cleavelandite aggregates were developed along closely spaced parallel fractures in milky to dark-gray quartz, and the two minerals form a strikingly layered rock known throughout the district as zebra rock, banded rock, or stripe rock. Elsewhere, as in the Stewart and Katerina pegmatites, the albite forms reticulate or irregularly ramifying veinlets that transect individual crystals in the quartz aggregate. All stages of replacement can be recognized, and in some places only remnants of quartz, representing the cores of original fracture blocks, attest the former existence of a massive quartz unit. Similar relations are characteristic of much lepidolite and muscovite formed in quartz.
|Large graphic-granite mass surrounded by much finer-grained aggregate of quartz, muscovite, perthite, and albite.|
Mineral aggregates similar to those that form replacement units in the outer parts of the pegmatites are more abundant in the central parts of many dikes, but their origin is not so plain. Almost without exception, they appear to be younger than the rock that surrounds them, and in most places they clearly corrode this rock. Whether or not most of them were formed wholly at the expense of this rock is not readily demonstrated, however, for residual masses of earlier pegmatite are rare in their central parts. Nevertheless, these mineral aggregates are here provisionally termed replacement units (possibly in large part of deuteric origin), because of differences in age, texture, and structure between them and the typical zonal units.
These younger units include much of the pocket pegmatite in the district. They range in form from the thinly tabular tourmaline-quartz pockets of such pegmatites as the Fargo and Tourmaline Queen to thickly ellipsoidal lepidolite-rich masses like those in the Stewart pegmatite. Thicknesses range from slightly less than 1 millimeter to about 30 feet, lengths from about 1 inch to as much as 200 feet. Typically these masses are discoid in general shape, but they are so irregular in detail that they appear in most exposures as blobs or splotches. This lack of clear-cut form is further emphasized by the widespread tendency of such units to grade outward into complex systems of fracture-fillings and fracture-guided replacement masses.
Whereas the smallest and most nearly tabular replacement units are widespread in their distribution, the largest and more bulbous ones ordinarily are restricted to the central parts of the host dikes. More specifically, masses of this pocket pegmatite occur in cores and immediately adjacent zones, chiefly along the footwalls or in the footwall parts of the cores and core segments. It does not occur simply along contacts between line rock and overlying graphic granite, as commonly alleged, although it must be admitted that the tops of the main masses of line rock are near the footwalls of the cores and core segments in many of the dikes. Such pegmatite thus is ordinarily most abundant in the central parts of bulges in single dikes or junctions of two or more merging dikes. It is also localized in the nearly flat, terracelike parts of several dikes, as pointed out by Donnelly.  To what extent such correlation with gently-dipping parts of the dikes can be generally applied in the district is not yet known.
The structure of the fine-grained granitoid rocks common in the footwall parts of most Pala pegmatites is quite distinct from that of the zones, fracture fillings, and replacement units described above. The masses of fine-grained, albite-rich rock commonly occupy the lower, or footwall, one-fourth to one-half of pegmatite dikes in which the remainder of the rock is graphic granite or graphic granite with small segments of very coarse-grained pegmatite. Some of these masses are composed of line rock and others wholly of aplitic rock without planar structure, but most of them contain both rock types. Indeed, there are all gradations between them, both along and across the strike. As shown in figure 25C, the line rock is commonly separated from the footwall contact of many dikes by 6 inches to nearly 10 feet of aplitic rock that is not layered.
These albite-rich rocks are not confined to the lower parts of the pegmatite dikes, although they are most abundant there. They occur also in the upper parts and even along a few hanging-wall contacts, where they form poorly defined tabular masses (fig. 25D). Where such masses are present in the hanging-wall zones of graphic granite, they impart a crudely layered appearance to weathered surfaces of the pegmatites. Line rock occurs within the cores of a few dikes and appears to be in part superimposed upon the very coarse-grained pegmatite that typically forms these innermost units. Thus it is markedly quartzose where best developed, and commonly grades into much coarser pegmatite containing only a few garnet- or albite-rich layers. In many places it appears to have encroached upon the footwall parts of cores and core segments (fig. 25C).
Where two subparallel pegmatites branch or join, the distribution of line rock generally is like that indicated in figure 25E [Note: In the original text, this reads “8E,” a non-existent figure]. The line rock in the lower dike is continuous, whereas that in the upper one fades into the graphic granite or other zonal rock at some distance from the point of junction. This distance ranges from a few inches to many tens of feet, but ordinarily is 10 feet to 30 feet. These relations are characteristic also of the aplitic rocks that lack planar structure.
The structural relations of line rock and associated types in the more bulbous pegmatites are not so clear, owing to the rarity of critical exposures. In general, however, the fine-grained units occupy the lower, or footwall, parts of the dikes, and appear to have encroached upon the inner zones. As exposed in the underground workings of the Stewart, Pala Chief, and Katerina mines, the aplitic rocks are much more continuous than the nearby cores and intermediate zones, both along the strike and down the dip.
Not only are the fine-grained granitoid rocks quite distinct texturally from the other rock types in the pegmatites, but the thin and regular layering in the line rock is an unusual feature. Moreover, these rocks form tabular units that are not symmetrically disposed with respect to the walls of the enclosing dikes, either broadly or in detail. This is in sharp contrast to the relations of the zones, most of which at least tend to be symmetrical.
There seems to be a widespread impression among those who have read reports on the Pala pegmatites that the bulk of the material in these dikes is of replacement origin. This is not correct. In few of the dikes, for example, do the coarse-grained replacement units appear to constitute more than 1 percent of the total pegmatite material present. Even if the various types of aplitic, albite-rich pegmatite in the district were to be interpreted as having been formed at the expense of pre-existing pegmatite, the sum of replacement material still would be less than the amount of graphic granite and other rock types indigenous to the earlier-formed units. In one way or another, this has been pointed out—although admittedly not emphasized—by most previous investigators in the district. In his classic paper on the genesis of the pegmatites, Schaller  indicates, for example, that graphic granite is virtually the sole constituent of some dikes and is abundant in most others, and that the graphic-granite pegmatites “have been affected but very little by replacing solutions….” This statement is made despite the fact that he considers little material but the microcline in the graphic granite to have been formed by processes other than replacement.
Many of the Pala pegmatites are sub-parallel dikes that join with or branch from one another. As shown in plate 1, they are particularly common on Hiriart Mountain. Many do not actually join, but are separated by screens or partitions of wallrock 6 inches or less in thickness. Other juxtaposed dikes are in actual contact, either locally or for considerable distances along their strike, or dip, or in both directions (fig. 34). Their respective identities are preserved over the entire areas of contact, as shown by their individual internal structures, and by the presence here and, there of country-rock films between them. Still other dikes appear to mix, or merge with one another, generally at distances of several tens of feet from the points of junction.
The composite nature of many dikes is reflected by repetitions of line rock or other units of aplitic rock within them, and by the slivers and seams of wallrock separating the individual elements of the dikes. Repetition of line rock and associated types is by no means a reliable criterion, however, as these units are in several zones in some pegmatites that plainly are not composite. Much more certain is the evidence of repeated zoning, such as the occurrence of cores or other inner zones at more than one consistent horizon within a given dike. Many of these dikes, as traced along the strike, split into two or more separate dikes, in each of which is preserved the internal structure of the corresponding part of the composite.
|Diagrammatic sketch showing typical relation between wall zone rich in graphic granite (outside the dashed line) and core segment of quartz and euhedral perthite; simply zoned pegmatite dike.|
Large composite dikes in the district include the Stewart, Pala Chief, San Pedro, Vanderburg, Katerina, and El Molino. In the vicinity of the Gem Star mine, the Stewart dike consists of at least three subparallel branches, or “splits,” with intervening septa of gabbro. It is composite also where exposed in the main quarry of the Stewart mine. On the west wall of this quarry a layer of graphic granite 5 to 9 feet thick is overlain by a 5-foot zone of massive quartz with giant euhedral perthite crystals. It is underlain by a very thick zone rich in coarse, blocky perthite, and this unit in turn is underlain by rock similar to that overlying the graphic granite. A contact between two juxtaposed pegmatites is present within the graphic-granite mass, and careful inspection of the quarry face indicates the position of this contact along a thin layer of much finer-grained graphic granite.
Line rock also is repeated at three different stratigraphic positions within the El Molino pegmatite in the vicinity of the main mine workings. Here two thin dikes lie above a much thicker one, and are separated from it in a down-dip direction by a distinct wedge of granodiorite. Along the outcrop, however, all three dikes are in direct contact for almost the full length of the mine area.
Another, very widespread type of composite pegmatite body consists of sills and dikes of graphic granite and other perthite-rich pegmatite in much larger host masses of fine-grained, albite-rich pegmatite. They are particularly abundant on Hiriart Mountain. The masses of younger pegmatite range from thin stringers to sills as much as 8½ feet thick. These younger elements of the composite masses are remarkably continuous along their strike. Some are single sills, others in groups or even swarms of subparallel sills. Most of them are simple aggregates of graphic granite, commonly with bladed biotite and muscovite, albite, and some garnet. Others, especially the larger ones, have symmetrical internal structure, with wall zones of graphic granite and inner zones of other very coarse-grained pegmatite. In several composite dikes, notably the Fargo on Hiriart Mountain, mining has been successfully carried on in such zoned masses, which are surrounded by earlier fine-grained granitoid rock.
In a sense, the composite dikes are merely those in which fracture fillings of later pegmatite are present. Some of these fillings plainly have corroded the fracture walls. The distinction between fracture fillings that are basically minor parts of a single dike, on the one hand, and those that are separate intrusive elements in a composite dike, on the other, is necessarily an arbitrary one, as there are all gradations between the two extremes.
|Euhedral crystals of perthite in massive quartz, El Molino mine. Note the narrow cavities at right-hand margin and lower right corner of large perthite crystal.|
It is not the purpose of this report to discuss in full detail the minerals of the Pala pegmatites, nor to annotate the voluminous literature on this subject. Descriptions of some of the minerals have been recorded from time to time,  and much additional information obtained by Waldemar T. Schaller awaits publication.
The minerals noted and recorded from the Pala pegmatites are listed in table 3. Most abundant and widespread among them are albite, biotite, garnet, lepidolite, microcline and orthoclase (generally perthitic), muscovite, quartz, spodumene, and tourmaline. The most common of the minor constituents are amblygonite, beryl, clay minerals, columbite-tantalite, and lithophilite and associated phosphate species. The relative abundance of the various minerals indicates that the pegmatites are truly granitic in composition, and that many are unusually rich in lithium. Other elements present in noteworthy quantities are beryllium, columbium and tantalum, manganese, and phosphorus, and in lesser quantities bismuth, fluorine, iron, caesium, and rubidium. Antimony, copper, and tin are rarer constituents of the pegmatites.
|Coarse perthite and quartz. Perthite crystals are fringed with aggregates of muscovite and albite. Schorl crystals are large.|
The principal minerals of the pegmatites, quartz and potash feldspar, are supplemented in most parts of the district by several other species. Muscovite, albite, and garnet are present in all the pegmatites, and generally are widely scattered through each dike. The black variety of tourmaline, schorl, also is widespread. The gem varieties of tourmaline, though less common, are present in many dikes. Most of the pink tourmaline in the district is on Queen Mountain, where it is associated with blue and green varieties. Gem tourmaline is much less common farther east in the district, where most of it is green or yellow green.
Spodumene is abundant in a few dikes, and is a much less common constituent of others. It seems to be rare or absent in most of the dikes west of the Stewart on Queen Mountain, but is exceptionally abundant in the Stewart dike. Large quantities of this mineral are present also in the Pala Chief and Vanderburg-Katerina pegmatites. In general, spodumene makes up a higher proportion of the dikes in the central and eastern parts of the district than in the western part. It is very rare in the pegmatites on Pala Mountain, south of the San Luis Rey River. The same generalizations apply to the known distribution of the clear, gem variety of spodumene, and also to amblygonite and other lithium phosphate species.
Lepidolite is a characteristic associate of both spodumene and tourmaline, but is more widely distributed than either of these minerals. It is common in the pegmatites on Queen Mountain, where most of the gem tourmaline mines have been developed, and is found also in the spodumene-bearing pegmatites of Chief Mountain and Hiriart Mountain. Beryl also is widespread, but seems to be more abundant in the central and eastern parts of the district than elsewhere. Common beryl is locally abundant in the Stewart dike.
The minor accessory constituents are sporadic, but in general are more abundant in the complex, multi-zoned pegmatites than elsewhere. The greatest variety and bulk of accessory mineral material have been obtained from such pegmatites as the Stewart, Pala Chief, and Vanderburg-Katerina. The Tourmaline King, Tourmaline Queen, Anita, Senpe, and El Molino pegmatites also have yielded notable quantities of the relatively rare minerals.
|List of minerals in pegmatites of the Pala district (observed by present writers except where otherwise noted).|
|Albite||X [see legend]||X||X|
|Hematite and goethite||x||x||x|
|* Very rare.
x Not common.
xx Common, or abundant in at least one pegmatite.
X Abundant and widespread.
--- Not observed.
a Reported by Clifford Krondel in Mineralogy of the oxides and carbonates of bismuth: Am. Mineralogist, vol. 28, pp. 532–533, 1943.
b Reported by Adolph Pabst in Minerals of California: California State Div. Mines, Bull 113. p. 229, 1938.
c Reported by W. T. Schaller in Bismutb ochers from San Diego County, California: Am. Chem. Soc. Jour., vol. 33, pp. 162–166, 1911.
Microcline and Orthoclase. Microcline, the most abundant mineral in the Pala pegmatites, occurs as white, gray, and tan crystals that range from about ¼ inch to 8 feet in maximum dimension. These crystals are anhedral to subhedral in coarse-grained aggregates of graphic granite or in finer-grained quartz-perthite-muscovite-albite pegmatite, and euhedral in most very coarse grained varieties of pegmatite. The lighter-colored crystals are especially common in the outer parts of the pegmatites. In general they occupy the full thickness of only those dikes that contain no inner units of very coarse-grained pegmatite or pocket pegmatite. The gray varieties, many of them of very dark shade, are typical of cores and intermediate zones that consist of very coarse-grained pegmatite other than graphic granite. They are most abundant in those dikes that have a very complex internal structure.
|Coarse-grained quartz-spodumene pegmatite exposed in wall of low room, Pala Chief mine.|
Nearly all of the microcline contains perthitic intergrowths of albite and albite-oligoclase as subparallel blebs and plates ordinarily less than 1 millimeter thick and 1 centimeter long. Some very thin plates, however, are much longer than this. In general the largest plagioclase individuals are in the coarsest host crystals of microcline, and hence are most common in the inner zones of the pegmatites. The plagioclase of the perthite also shows a systematic variation in composition, becoming progressively more sodic from the walls of the pegmatite inward.
Orthoclase and microcline are widespread constituents of the pocket zones in the pegmatites, where they generally form large, equant crystals with well-developed faces. They are known among miners and mineral collectors in the district as pocket spar crystals. They are ½ inch to at least 15 inches in diameter, and their faces are characteristically flat and meet in sharp lines. Many of the crystals are transparent, and others are white to lightgray. All are coarsely perthitic. The crystal faces are commonly corroded to depths of ¼ inch, and the intergrown plagioclase, evidently much more resistant to corrosion than the host potash feldspar, stands out on such surfaces as narrow, sharp ridges. Many of these corroded surfaces are marked bv thin films of iron oxide.
The orthoclase is intimately associated with perthite, and is more abundant than the perthite in the pocket pegmatite of some dikes. In general, however, the orthoclase constitutes only a small fraction of the total potash feldspar within the dikes as a whole. Thin-sections of most orthoclase crystals show irregular patches with the typical gridiron twinning of microcline. In testing the theory that inversion of orthoclase to microcline is commonly caused by strains set up during the grinding of a thin-section, Donnelly  prepared several sections by various means, but found no significant variations in the proportion of triclinic feldspar. On the other hand, he did find the typical gridiron twinning of microcline to be more common in thin-sections than in crushed fragments obtained from the same specimen.
Many of the pocket spar crystals of perthitic orthoclase are coated with glassy, transparent, nonperthitic microcline, and the two potash feldspars  are in essential crystallographic continuity. Most of these coatings are less than ¼ inch thick. Like the host crystals of orthoclase, many are deeply corroded.
Quartz. Quartz is in virtually all of the pegmatite units, and evidently was formed during all general stages of pegmatite development. It is present as spindles, rods, and other elongate masses in graphic granite, and is a major constituent of line rock, of other fine-grained albite-rich types, and of the fine- to coarse-grained aggregates of albite, muscovite, tourmaline, and other minerals. It forms large groups of very coarse, anhedral crystals in coarse-grained pegmatite of various types. It fills fractures, forming veinlike masses in potash feldspar and other minerals, and, finally, it occurs in pocket pegmatite as crystals with well-developed faces.
The quartz is milky white to light-gray, but both smoky, and clear, colorless varieties are in the interior parts of many dikes. Its coarsest form is as anhedral crystals 6 inches to at least 6 feet in diameter, and in some pockets as well-formed prismatic crystals that range in length from an inch to as much as 3 feet. Many of them are distinctly smoky. The anhedral crystals are distinguished by rhombohedral cleavage and coarse, lamellar twinning. Individual lamellae are 0.5 millimeter to 10 millimeters thick. Much of this quartz is separated into crystallographic zones by groups of growth lines and roughly planar aggregates of finely divided impurities. A few appear to be phantom crystals without the outer crystal faces.
Albite. Albite is very abundant in the pegmatites, both as fine-grained, sugary, crystalline aggregates (Ab90-97) and as parallel or radiating groups of coarse cleavelandite crystals (Ab94-99). The fine-grained variety of albite is lustrous and white, and is most abundant in line rock and related aplitic units. It is common also in fine- to coarse-grained aggregates of quartz, muscovite, and schorl that are interstitial to graphic granite or to pegmatite composed of coarse euhedral perthite in massive quartz.
Much sugary albite occurs along fractures and cleavage cracks in potash feldspar, and pseudomorphs of albite after such feldspar are not uncommon. Some pseudomorphs of sugary albite appear to have been formed after graphic granite, through selective replacement of potash feldspar by plagioclase. Most of the quartz rods are residua, and show their original orientation and much of their original shape. A great deal of attention has been devoted to this type of evidence by W. T. Schaller, who has used it to demonstrate large-scale albitization of graphic granite.
Fine-grained albite occurs also as perthitic spindles and plates in coarse microcline and orthoclase crystals.
Cleavelandite is generally present in cores and other inner units of the pegmatites, and is locally very abundant. Most of it is white, but pale apple green and some bluish varieties are not uncommon. The platy crystals are ¼ inch to as much as 3 inches long, ⅛ inch to 2½ inches wide, and 1/32 to ⅛ inch thick. Many of them are curved or warped, and others are flat, with broadly curving ends. The mineral is characteristically twinned, and shows very thin lamellae.
All varieties of the plagioclase generally form aggregates that, where relatively free from other minerals, crumble when struck with a hammer.
The coarse albite is present chiefly along fractures in quartz, perthite, spodumene, and other relatively early minerals; along contacts between such minerals, especially spodumene and quartz; as irregular aggregates with quartz and muscovite; and as cavity linings in pocket pegmatite. Where it is distributed along fractures or mineral contacts, individual crystals tend to lie with their broad surfaces perpendicular to these planar features. Elsewhere they form sheaflike or rosettelike aggregates. Beautifully formed groups of coarse cleavelandite blades commonly fringe crystals of quartz, potash feldspar, spodumene, and muscovite in the pocket-rich parts of the pegmatites.
|Matrix specimen from richest gem-bearing part of dike, Pala Chief mine. Short, bladelike crystal of spodumene (upper right) in cleavelandite and quartz. Large crystals and aggregates of lepidohte flakes at left. Photo by courtesy of Monta J. Moore.|
Micas. Muscovite is present in all the pegmatites, and is a common constituent of most of them. It is scattered through the line rock and related granulitic types as white to very pale green flakes and foils that rarely exceed 1 millimeter in diameter. Similar thin crystals occur locally in graphic granite and other coarse-grained, perthite-rich pegmatite. Individual crystal plates range in diameter from 0.2 millimeter to 3 millimeters. Much coarser plates are present in aggregates of quartz, albite, and tourmaline that are interstitial to large crystals or crystal groups of graphic granite, perthite, or coarse, anhedral quartz with perthite crystals.
The inner parts of many pegmatites contain very coarse-grained muscovite, which is characteristically associated with albite and quartz. The crystals, or books, are well formed in pocket pegmatite, and attain diameters of several inches with thicknesses of as much as an inch. Average dimensions, however, are approximately ½ inch and ⅛ inch, respectively. The mineral characteristically ranges from yellowish green to dull, pale to deep green, and is generally marked by ruling and herringbone structure. Some books are so severely marred by fine, closely spaced crenulations that they appear silvery white and opaque. Many contain inclusions of green to very dark blue-green tourmaline, and some are fringed with lepidolite. The lithia mica ordinarily is crystallographically continuous with muscovite, so that the two minerals form zoned crystals with pink margins and green centers. Some have intermediate zones that are yellow to white.
Muscovite occurs also in the inner pegmatite units as nearly pure aggregates of ⅛-inch to ½-inch plates. Most of these aggregates are 3 inches to about 6 feet in maximum dimension. Locally they contain quartz, albite, and scattered prisms of green or black tourmaline. Some of these aggregates fill fractures in other minerals, especially in coarse, anhedral quartz.
Tabular to equidimensional masses of extremely fine-grained muscovite occur in a few pegmatites. Some of these masses are distinctly podlike. The mica is very pale green, and is generally so fine-grained that it has a waxy appearance. The aggregates, which are rarely greater than 4 inches in maximum dimension, are most abundant along fractures or at fracture intersections.
Biotite is a common, but quantitatively minor mineral in the outer parts of nearly all the pegmatites. It is typically associated with muscovite, both in graphic granite and in the fine-grained rocks that are so abundant in the footwall parts of the dikes. Individual plates and blades of this mineral generally range from ¼ inch to 4 inches in diameter or length. All are very thin, and most of them form mere “skims” in the host rock. Some are slightly altered, and are stained with iron oxides.
The biotite occurs as isolated crystals, as dense aggregates of small foils, and as subparallel to radiating groups or sprays of broader and thinner crystals. In many places individual crystals transect the quartz rods in graphic granite and also boundaries between mineral grains, and clearly were formed along fractures. Elsewhere they are intergrown with other minerals as if they had crystallized with them. Much of the bladed biotite near the hanging-wall contacts of the dikes is oriented normal or nearly normal to these contacts. Possibly much of this biotite was developed as a result of reaction between pegmatite solutions and the gabbroic country rock.
Lepidolite is one of the most characteristic pocket minerals. Where best developed, it forms compact aggregates of thick flakes and plates 2 millimeters or less in diameter. These aggregates are present as lenses and pods less than a foot thick in most pegmatites, but in others, notably the Stewart, they are 10 to 15 feet thick and many tens of feet long. They range in color from pale rose to deep lilac or even purple. The coarsest flakes ordinarily are lightest in color, whereas most of the purplish aggregates are extremely fine-grained, with a waxy appearance.
Although most masses of fine-grained lepidolite are in the innermost parts of the pegmatites, some markedly tabular aggregates of this mica and albite, quartz, and tourmaline fill fractures in the graphic granite and coarse-grained quartz-perthite units of several pegmatites. Excellent exposures are in the hanging-wall parts of the Tourmaline King, Tourmaline Queen, and Stewart dikes.
|Fine-grained granitoid rocks. Boulder with exposed face nearly perpendicular to garnet-rich layering. Note variations in thickness, sharpness, and spacing of layers. South side of Hiriart Mountain.|
Coarse-grained lepidolite occurs only in the typical pocket pegmatite, where it is most commonly associated with muscovite, albite, quartz, and lithia tourmaline. Most individual crystals are ¼ inch to 2 inches in diameter, with thicknesses of 1 inch to ⅛ inches. Many are tightly intergrown in a manner similar to the much smaller individuals in the compact aggregates described above. In contrast, some aggregates of lepidolite and cleavelandite are markedly cellular, with numerous irregular voids ⅛ inch to ⅜ inch in diameter. Elsewhere these two minerals occur as well-formed crystals that line clay-filled cavities. Plates of lepidolite commonly are attached to earlier crystals or crystal aggregates of cleavelandite and quartz. The lithia mica forms the outer parts also of composite muscovite-lepidolite crystals, as already noted.
Garnet. Of the several kinds of garnet in the pegmatites, an iron-bearing representative of the manganese-aluminum variety, spessartite, is the most widespread. It forms dodecahedral and trapezohedral crystals that range in diameter from 0.1 millimeter to as much as 3 millimeters. Most crystals are salmon pink to brownish-red in color. They are particularly abundant in the line rock, but are also widely scattered through the other types of fine-grained, albite-rich pegmatite, and through graphic granite and other coarse-grained perthite-rich units. Their distribution appears to be regular only in the line rock. Many of these garnets are slightly altered, and are coated with stains of manganese oxide; where the alteration has been unusually intense, large masses of line rock are stained.
|Fine-grained granitoid rocks. Boulder with exposed face nearly perpendicular to garnet-rich layering. Unusual patterns formed by undulations in layers. South side of Hiriart Mountain.|
Spessartite is present also in the innermost pegmatite units as well-formed crystals as much as an inch in diameter. Some of these crystals are clear and free from cracks and other imperfections. Most of them, however, are marred by fractures and alteration, in general to a greater degree than the smaller crystals near the walls of the dikes. Most of the crystals with diameters greater than half an inch are so severely fractured that they crumble readily to a brown grit. In contrast, some pocket garnet occurs as flattened inclusions in muscovite, and is commonly pure and unaltered. These crystals are too small to be of use as gem material.
Grossularite (essonite), the calcium-aluminum garnet, is much less abundant in the Pala pegmatites than in those of the Ramona and other southern California districts. A manganesian variety occurs sparingly in the pocket-bearing parts of the Tourmaline King, Stewart, Douglass, and several dikes on Hiriart Mountain, chiefly as well-faced crystals 1/16 inch to ¼ inch in diameter. These crystals are dodecahedrons and trapezohedrons of exceptionally attractive honey-yellow to orange-brown color. Most of them are attached to crystals of quartz or cleavelandite.
A manganese-bearing variety of andradite, the calcium-iron garnet, is a rare constituent of the border zones of several of the pegmatites adjacent to gabbroic country rock, and the mineral may have been derived in part from digested wallrock material. This garnet forms very small crystals of deep-red color, and is generally associated with biotite.
Tourmaline. Many varieties of tourmaline are present in the pegmatites. Schorl, the deep black iron tourmaline, is the most abundant, forming both the characteristically striated prisms and columnar aggregates, or bundles of rodlike crystals. The mineral appears lustrous and black in hand specimen, blue to deep violet under the microscope. Some of the crystals are shattered and somewhat chalky, apparently in part because of alteration. Many are fractured, especially in a direction parallel to the base, and some of these fractures have been healed with quartz, albite, or aggregates of these minerals and mica. A few broken crystals of schorl have been recemented with finer-grained black tourmaline.
The schorl is most abundant in the upper parts of a given pegmatite, and the crystals generally increase in number and coarseness from the hanging-wall contact downward toward the horizon of the pocket zone. Exceptionally rich concentrations occur immediately above pockets in many dikes, and very coarse, well-formed crystals are present in most of the pockets themselves. The footwall parts of the pegmatites also contain this mineral chiefly as numerous small crystals that are rather uniformly scattered through the fine-grained, aplitic units. Such tourmaline-bearing granulite forms much of the lowermost part of the Stewart dike.
The schorl is scattered as pencil-like crystals through the perthite-rich pegmatite of the wall zones and outer intermediate zones of many dikes, and in places forms swarms of such crystals. Elsewhere the crystals are nearly parallel, and appear to have developed normal to contacts with the wallrock, to contacts between minerals, or to fractures. The schorl occurs also as sprays and rosettes, some of very large size. Several crude rosettes in the Stewart dike are 7 feet in diameter, and somewhat similar large radial aggregates are found in the Tourmaline King, Tourmaline Queen, and Pala Chief pegmatites. Most of the schorl is associated with muscovite, albite, and quartz, and commonly forms fine- to medium-grained aggregates that are interstitial to much coarser crystals or crystal groups of graphic granite, perthite, or, rarely, anhedral quartz. Graphic intergrowths of quartz and schorl are abundant in places, but rarely form masses greater than 6 inches in maximum dimension.
|Sharply layered line rock flanked on left by massive quartz with euhedral perthite and on right by flne-grained rock without prominent layering. Garnet-rich layers are irregular. South side of Hiriart Mountain.|
Other varieties of tourmaline in the Pala pegmatites comprise the medium- to deep-blue indieolite, the pink to pale-red rubellite, the green emeralite, and the colorless achroite. Nearly all intermediate colors, tints, and shades occur also, with only the brown types not well represented. Two or more colors are commonly present in a single crystal, and either are disposed in random fashion or are arranged systematically. Some bicolored and multicolored crystals of prismatic habit are characterized by a concentric, or layerlike zoning with respect to their long axes. Deep-blue to black cores with single, double, or triple rims that are colorless, green, blue, or pink are most common, but many other combinations occur. “Watermelon tourmaline” is one attractive type in which an outer rim of green surrounds a colorless layer, and both these layers enclose a pink core. Some pink crystals of exceptional quality are veneered with a thin black or very dark-blue rim.
A different type of color zoning characterizes other crystals, which contain two or more contrasting layers that lie perpendicular or nearly perpendicular to their long axes. Many of these crystals are green on one end and pink on the other, and many others grade from black to green or pink, or even from black to colorless. Nearly all combinations are known, and some crystals show five or more alternations of colors in their length. More than one type of color zoning in a single crystal is not uncommon, especially where schorl is involved. The end of the crystal nearest the pegmatite wall is ordinarily black; the color changes to pink, green, or other alkali-bearing types as the crystal is traced toward its other end. The black material commonly persists farther in this direction in the interior of the crystal than in its outer parts, so that part of the crystal contains a black core and a lighter-colored rim. It should be emphasized that there are numerous exceptions to this generalization, and that many pink-cored crystals, for example, have deep blue or even black rims.
The colors in some zoned crystals are very sharply bounded from one another, whereas in others they appear to merge very gradually. In most of them, however, the colors intergrade within distances of 2 millimeters or less.
The crystals are rather consistently prismatic, and occur as isolated individuals, as columnar composites, as parallel or radiating groups, and as jackstrawlike aggregates. Some blue and pink aggregates of very fine-grained tourmaline form irregular masses in the pocket bearing parts of many pegmatites. They appear to be feltlike, and are very pure. Some crystals are so closely spaced that they form the principal constituent of the rock, but most of them represent less than 5 percent of the pegmatite unit in question. Individuals vary considerably in size, ranging from tiny needles to single prisms 6 inches in diameter and at least 4 feet long. Most of them are less than an inch in diameter and 4 inches long.
The crystals are commonly fresh and lustrous, and many are clear. The chief flaws are fractures, cavities, and inclusions, many of which are so small and closely spaced that they appear as a cloudiness or milkiness in the mineral. Sonic larger fractures are clearly healed with quartz or finer-grained aggregates of other tourmaline. Much of the tourmaline is opaque or otherwise of poor quality because of alteration, rather than the presence of mechanical imperfections. This alteration is most pronounced in the pink and colorless varieties, in which it causes a progressive decrease in hardness and toughness, loss of luster and transparency, and an increasingly clayey appearance. All stages of this alteration are well shown in the Stewart dike, especially in the underground workings of the Stewart and Gem Star mines.
A little of the schorl and nearly all of the other tourmaline occur in pocket pegmatite. They are most abundant in the central parts of the dikes, where they commonly form handsome crystals. They occur also in fracture fillings and fracture-controlled replacement bodies, especially in the upper parts of the thickest dikes. The rubellite is associated with quartz, cleavelandite, pocket perthite, muscovite, and especially with lepidolite. In the Stewart dike crystals of nearly fresh to thoroughly altered pink tourmaline form rosettes in fine-grained lepidolite, and this attractive lepidolite-tourmaline rock has been displayed in museums the world over.
|Line rock (top) adjacent to very coarse-grained aggregate of perthite and graphic granite. Note cleavage reflections from large crystals. At bottom is fine-grained quartz-albite-perthite-muscovite pegmatite with less sharply developed layering. South side of Hiriart Mountain.|
The emeralite, or green tourmaline, is generally associated with quartz, cleavelandite, and other pocket minerals, and in many places occurs as inclusions in coarse books of muscovite. It is the most common alkali tourmaline in the fracture-filling and replacement units in perthite-rich pegmatite. The blue and colorless varieties are generally present in the pocket-bearing parts of the pegmatites, and are most abundant in and adjacent to concentrations of lepidolite.
Spodumene. Most of the spodumene in the Pala pegmatites has formed as very coarse, lath-shaped crystals. In general they are white to gray, and are opaque. Where fresh, they have a nearly pearly luster, but in most exposures they are sufficiently altered to appear dull and even earthy. Individual crystals are as much as 14 inches wide and 9 feet long, but the average dimensions are more nearly 3 inches and 2 feet, respectively. Few are thicker than 2 inches, and in most of them this dimension is less than 1 inch. They are deeply striated in a direction parallel to their elongation. Many crystals are twinned, with the twin planes parallel to their flat sides.
The coarse variety of spodumene almost everywhere occurs with quartz that is also very coarse grained, and aggregates of these two minerals are especially well developed in the Stewart, Pala Chief, and Vanderburg-Katerina dikes. Common associates are cleavelandite, muscovite, and lepidolite. Many of the spodumene crystals are thoroughly altered to white, gray, tan, or pink clay pseudomorphs. Much of the clay is halloysite, but some is montmorillonite and other species. There are all stages of alteration between nearly fresh spodumene and clay masses with no residuum of the original mineral.
A very small proportion of the spodumene is unaltered, and appears as attractive transparent crystals and crystal fragments. Much of this material is the pale pink to deep bluish-lilac variety kunzite, much is the colorless to yellowish triphane, and a little is the green hiddenite. Some of it is color-zoned, generally with lilac centers and colorless to green rims that are thicker at the ends of the crystals than along their sides. Most of the clear spodumene occurs as deeply etched cleavage fragments marked by striations and by triangular pits. These features have been described in detail by Schaller.  A little of the clear material forms complete or nearly complete crystals which are typically lathlike in the deposits on Chief Mountain, but are shorter and thicker in many of the deposits on Hiriart Mountain.
The kunzite, hiddenite, and triphane are plainly varieties of spodumene that escaped the alteration described above. Many specimens recently obtained from the Pala Chief and Katerina mines show clear spodumene as cleavage-bounded remnants within individual host crystals of partly altered spodumene. All these remnants have the same crystallographic orientation within a given host crystal, and there can be no doubt that they represent those parts of the crystal that were not altered.
Most of the clear spodumene fragments are less than 2 inches long, but there are some noteworthy exceptions. A few crystals of gem quality, recovered from the Pala Chief, Vanderburg, and Katerina mines, were at least 15 inches long and weighed 16 to 27 ounces. Several of them yielded flawless cut stones weighing 75 to 250 carats.
The gem spodumene occurs within the cores of the pegmatites, generally in a very coarse-grained quartzspodumene unit. The unaltered or only partly altered crystals are most common along the margins of these innermost zones in at least two pegmatites, but in most others the quartz-spodumene unit is so thin that such details of distribution are of little practical significance. Most of the crystals, whether partly altered or not, are embedded in the quartz, or, locally, in aggregates of quartz, cleavelandite, and micas. The fragments of clear material are characteristically surrounded by white to deep-pink clay, some of which is stained black by manganese oxides. Most of the clay appears to have been derived from adjacent spodumene rather than from feldspars or other aluminum-bearing minerals.
Beryllium Minerals. Several kinds of beryl (beryllium aluminum silicate) are present in the Pala pegmatites. Perhaps best known are the alkali-rich varieties in the pocket pegmatite of such dikes as the Tourmaline King, Tourmaline Queen, Pala Chief, Senpe, and Vanderburg-Katerina. They are the white to colorless goshenite and the pale-rose to peach-colored morganite. The crystals are equant to tabular, with sharply defined faces. They range from ⅛ inch to 6 inches in maximum dimension, with an average of 24 inches or slightly less. Many crystals are fairly simple tablets with small prism and broad basal faces, but most are marked by numerous modifying faces, some of which do not differ greatly in orientation from the base. Groups of complex crystals in parallel growth are common, and where the mineral is clear these aggregates are especially attractive. The white to pinkish beryl is generally associated with cleavelandite, muscovite, lepidolite, and quartz, and in places with spodumene and tourmaline.
Pale blue-green, moderately deep-blue, yellow-green, and golden beryl occurs in the quartz-rich cores of several pegmatites, chiefly as equant to prismatic crystals 3 inches or less in diameter. They are commonly well-formed, with sharply defined prism, pyramid, and basal faces, but some crystals are rough and subhedral. Most of them are milky or otherwise marred by inclusions and structural imperfections, but a few are clear. The subhedral crystals commonly appear sugary, owing to numerous closely spaced fractures. Some of the smaller, well-formed crystals are markedly chatoyant, and yield excellent cat’s eye stones when cut.
This core beryl is not particularly abundant in any pegmatite, but is locally common in the central parts of the Stewart, El Molino, and several other dikes. It is generally associated with albite, and in places with muscovite. In the Katerina, El Molino, Pala Chief, and San Pedro mines it occurs with pinkish beryl, and a few crystals contain greenish cores and pinkish rims. The distribution of the colors suggests true crystallographic zoning.
Irregular masses of gray, yellow-green, and pale-green beryl, generally without obvious crystal form, are locally abundant in very coarse-grained units of the Stewart, San Pedro, and Vanderburg-Katerina pegmatites. In general this type of beryl is nearer the walls of the dikes than either of the types already described, and it is typically a constituent of quartz-blocky perthite intermediate zones. The crystals are as much as 14 inches in maximum dimension, although most are less than 5 inches. They are opaque, and ordinarily are marred by numerous fractures. Many of those that are gray or greenish gray are not easily distinguished from some types of quartz and feldspar in the pegmatites, although they have a characteristic greasy luster.
Still another variety of beryl is also anhedral, and occurs with graphic granite, muscovite, and locally with albite in the outermost parts of several pegmatites, notably the Stewart, Pala Chief, and El Molino. It is not abundant, although it is so difficult to recognize that it may well have escaped attention at many places. It forms irregular masses of white to medium-gray color which rarely exceed an inch in maximum dimension. So far as can be ascertained on the basis of refractive-index determinations and a few chemical analyses, this type of beryl contains the highest proportion of BeO and the lowest proportion of alkalies, as compared with other types in the pegmatites. The pocket beryl, in contrast, has the highest indices of refraction and contains the highest proportion of alkalies, notably sodium and caesium. The beryl formed in the cores and intermediate zones is intermediate in composition between these two extremes. The decrease in beryllium-oxide content of beryl from the walls of the dikes inward is compatible with the findings of geologists working in several pegmatite districts elsewhere in the United States.
Bavenite, a hydrous beryllium-calcium-aluminum silicate, forms clusters of small, radiating prismatic crystals, and is a very rare pocket constituent of the Pala Chief pegmatite. The crystals are white to colorless, and resemble those described from the Mesa Grande district by Schaller and Fairchild.  It is sporadically distributed on the sides and ends of beryl crystals, and also surrounds small ragged masses of beryl. Evidently it was derived from the beryl by alteration.
|Euhedral crystals of perthite in massive quartz, El Molino mine. Note the narrow cavities at right-hand margin and lower right corner of large perthite crystal.|
Small, tabular crystals of white to light-gray bertrandite, a hydrous beryllium silicate, show similar relations with respect to beryl. The bertrandite occurs in the Pala Chief and Katerina mines. In at least one specimen from the east end of the Pala Chief open cut, an aggregate of the crystals is a pseudomorph after a very small tablet of beryl, and elsewhere similar aggregates line cavities in corroded crystals of white to pink beryl. Most of the bertrandite crystals are less than 1 millimeter long.
Phenakite, beryllium silicate, is a rare constituent of the Vanderburg-Katerina dikes. It forms both flat, colorless crystals with sharply defined faces, and subhedral masses that are distinctly milky. It is associated with very small crystals of white to pale-blue topaz, and both minerals are attached to the exposed edges of large cleavelandite aggregates in the pocket pegmatite. None of the phenakite crystals exceeds ½ inch in maximum dimension. Three tiny crystals of bertrandite were in a small cavity in one phenakite-cleavelandite specimen from the main workings of the Katerina mine.
Helvite, a silicate-sulfide of beryllium, manganese, iron, and zinc, is an exceedingly rare constituent of the pegmatite in the Gem Star and Katerina mines. It forms small, honey-colored tetrahedral crystals that are less than 1 millimeter in diameter. They occur on the surfaces of cleavelandite and spodumene crystals, and in general are typical of the pocket-bearing parts of the dikes.
Bismuth Minerals. Several bismuth minerals are present in the quartz-rich cores of pegmatites in all parts of the district, but constitute an almost negligible part of the pegmatite material as a whole. In the Stewart mine, an irregular mass of native bismuth weighing more than 100 pounds was encountered in the underground workings not far from the West adit. The principal associated minerals were quartz, spodumene, and amblygonite. According to Kunz,  the bismuth occurred as long, irregular crystals, and as platy crystalline masses as much as 12 or 15 millimeters long. One crystal an inch long was reported. Minor quantities of native bismuth occur also in the innermost zones of the Tourmaline King, Pala Chief, and Vanderburg-Katerina pegmatites, chiefly in association with quartz, albite, and lepidolite. It generally forms small scales and foils, which are typically pinkish to silvery on freshly broken surfaces.
Fibrous gray bismuthinite, bismuth sulfide, is associated with bismuth in the Stewart and Katerina mines, and with bismutite in the Pala Chief, Margarita, and El Molino workings. The bismuth and bismutite probably have been formed at least in part by alteration of the bismuthinite.
Bismite (bismuth oxide), pucherite (bismuth vanadate), and possibly a bismuth hydroxide form earthy, gray to yellowish-orange coatings on fractures in quartz, bismuth, and bismuthinite, especially in the Tourmaline King, Tourmaline Queen, Stewart, Pala Chief, and Vanderburg- Katerina pegmatites. Where individually recognizable the pucherite forms minute platy to needlelike crystals of gray to yellow-brown color. The bismite in some places has formed very small, tabular crystals, but in general occurs as a crystalline powder.
Bismutite, a bismuth carbonate, is probably the most widespread of the bismuth-bearing minerals. It fills fractures in bismuth and bismuthinite, and evidently was formed by the oxidation of these minerals. A few masses of bismutite contain cores of unaltered gray bismuthinite. The carbonate ranges in color from gray through canary yellow to pale orange yellow, and its luster is characteristically dull. It forms thin smears, veinlets, and some nodular masses as much as 2 inches in diameter, and is particularly widespread in the quartz-rich pegmatite of the Katerina mine.
Beyerite, a calcium-bismuth carbonate first described by Frondel,  forms gray-green to yellow-gray earthy coatings on fracture surfaces in the quartz of the Stewart and Katerina pegmatites. In many specimens it is intimately mixed with bismutite, and the two minerals are not readily distinguishable.
Clay Minerals. Clay minerals occur in all the Pala pegmatites, and appear to have been developed in two distinctly different ways. Minerals of the kaolinite group, formed by the weathering of feldspars, are widespread in the near-surface parts of the dikes. They coat feldspar crystals and fractures within them, and also fill larger, more continuous fractures that are traceable for several feet or even tens of feet along their strike. Some of this supergene clay is stained with iron oxides that presumably were derived from weathering of mafic minerals in the overlying gabbroic rocks. Much of this iron-stained clay has been mistaken by mineral collectors and even by some miners for the hypogene pocket clay that characteristically encloses the gem minerals in the pegmatites.
Several species of clay minerals in the pegmatites appear to be much older, and to have formed under hypogene conditions. This mode of origin is shown by the independence of their distribution with respect to the present surface, to postulated older erosion surfaces, to weathered and unweathered parts of the dikes, and to post-dike fractures. Moreover, they are associated with unaltered pyrite and other sulfide minerals, and consistently occur with tourmaline, cleavelandite, and other typical pocket minerals. These clays include representatives of both the montmorillonite and kaolinite groups, and have been discussed briefly by Ross and Hendricks.  Individual species thus far noted comprise endellite, halloysite, kaolinite, and montmorillonite.
To most of the miners in the district these minerals are known collectively as pocket clay. They are most abundant in the pocket-bearing parts of the dikes, where they commonly form the matrix in which the gem crystals occur. They are earthy to waxy where pure, but in most places are distinctly gritty because of numerous small, angular fragments of quartz and other minerals. They range in color from white and light-gray through yellowish and pinkish to raspberry red and bright reddish-brown. Most of the clay is distinctly pinkish, and some is marked by irregular white splotches. In places the clay minerals are stained along irregular fractures by manganese oxides.
Much of the white and pinkish clay occurs as pseudomorphs after spodumene. Also derived from spodumene are dense, butter-colored types of endellite and halloysite that are known locally as “turkey-fat clay.” Pale to deep pink clay minerals also were formed by the alteration of tourmaline, and abundant pink to brownish clays by the alteration of feldspars. Not only do they form pseudomorphs after these minerals where alteration has been complete, but they occur in fractures and shear zones that transect partly altered crystals, as well as large masses of the pegmatite itself. These clays also line cavities formed by the selective corrosion and solution of the quartz rods in graphic granite.
Columbium-Tantalum Minerals. Members of the columbite-tantalite series (iron-manganese columbate-tantalate) are widespread minor constituents of the pegmatites, and are locally abundant in the central parts of the Stewart and Vanderburg-Katerina dikes. Most common is ferrocolumbite, in which the iron:manganese ratio is greater than 4:1 and the columbium:tantalum ratio is greater than 3:1. This mineral forms tabular crystals, generally no more than ¼ inch thick and 1 inch by 1 inch in plan. The principal faces of most of the crystals are flat, but some very thin and broad crystals are markedly curved. Many of these platy individuals are 1⁄16 inch or less in thickness and as much as 4 inches in maximum dimension. They commonly occur in radiating groups. A few crystals are more equant, and appear as subhedral to euhedral “chunks” ½ inch to 6 inches in diameter, with an average of about an inch.
|Idealized sections of pegmatite dikes in the Pala district, showing typical relations of intermediate zones and cores.|
The columbite is dull black on crystal faces, but has a bright submetallic luster on freshly broken surfaces. In contrast to this is manganotantalite, a closely associated species found in the Katerina and very sparingly in several other mines. This mineral forms rather thickly tabular crystals with slightly brownish outer surfaces and a distinctly resinous, splintery appearance on freshly broken surfaces. It is very rich in manganese, and has a high ratio of tantalum to columbium.
Both the columbite and the manganotantalite are in the cores and other inner units of the pegmatites They are characteristically associated with quartz, less commonly with cleavelandite and sugary albite. Many of them are coated with fine flakes and scales of yellowish muscovite.
|Quartz-spodumene pegmatite in wall and back of large room, Stewart mine. This unit is overlain by massive quartz (dark, at top), and is underlain by quartz-cleavelandite-perthite-muscovite pegmatite (at top and below level of man’s head). Massive lepidolite ore at lower right. [Note: This caption appears as in the original text, but appears to refer to Fig. 22, and vice versa]|
Two crystals of stibiotantalite, antimony tantalate-columbate, were observed in the Katerina mine, where they were in close association with coarse-grained cleavelandite, quartz, orthoclase, and colorless to pinkish beryl. The crystals were near several clusters of manganotantalite tablets, which they resembled in color and luster. In contrast, however, the antimony mineral showed a perfect cleavage. Numerous other brownish crystals of tantalum-bearing minerals were tested for antimony, but all were found to be ordinary manganotantalite.
Small crystals of pale honey-yellow to very dark microlite, essentially a tantalate of calcium, are present in pocket aggregates of quartz, lepidolite, muscovite, albite, and tourmaline, but are known from only two pegmatites, the Tourmaline King and the Stewart. The crystals are octahedral and dodecahedral, with maximum dimensions that rarely exceed 1⁄16 inch.
Lithia Micas. In addition to muscovite, biotite, and lepidolite the pegmatites also contain cookeite, hydrous lithium-aluminum silicate. It is a rather widespread pocket species, and ordinarily forms a coating on crystals and crystal aggregates of quartz, lepidolite, spodumene, albite, and orthoclase. It forms white, buff-colored, and very pale pink aggregates of small plates and flakes. It is most common in some of the gem-bearing parts of the Tourmaline King, Tourmaline Queen, Stewart, Pala Chief, and Vanderburg-Katerina dikes.
Zinnwaldite, the lithium-iron mica, is very sparingly present in several pegmatites as dark-gray to deep reddish brown crystals and flakes. It resembles phlogopite in having a bronzelike luster. Most of the crystals are stubby prisms ¼ inch in maximum length. They are only in the pocket-bearing parts of the pegmatites, where they are most commonly associated with quartz, cleavelandite, and beryl.
Phosphate Minerals. Amblygonite, lithium-aluminum fluo-phosphate, is a common constituent of the spodumene-bearing pegmatites, especially those on Hiriart Mountain and in the south part of the Stewart dike. Ordinarily it forms subhedral to anhedral crystals ½ inch to at least 18 inches in diameter. Larger individuals commonly appear as subrounded or nodular masses. Many crystals are single, but others are grouped in ovoid to discoidal aggregates several feet or even tens of feet in diameter. The largest of these aggregates, encountered in the underground workings of the Stewart mine, was nearly 40 feet long, 2 to 15 feet wide, and 16 feet in maximum thickness. This was exceptional, however, and few of the other aggregates in the district exceed 3 feet in maximum dimension.
Most of the amblygonite is white and bluish white, and has a typical pearly luster. Cleavage surfaces of the coarse crystals are broadly curved, and in many places are markedly uneven in detail. Most of the masses are cut by irregular networks of fractures and zones of shattering that are 1/32 inch to ¼ inch wide. The broken material is “healed” with fine-grained, sugary amblygonite of pale green to pale blue color. Both individual crystals and the larger crystal aggregates typically occur in massive quartz, but are also associated with albite, lepidolite, muscovite, and spodumene. In a few dikes the amblygonite is in readily distinguishable zones of very coarse-grained pegmatite, composed of quartz and some spodumene. These units characteristically occur along the outer margins of quartz cores, or of quartz-spodumene cores.
Associated with the spodumene and amblygonite in the inner parts of many pegmatites are lithiophilite, lithium-manganese phosphate, and triphylite, lithium-iron phosphate. These minerals form equant to thickly tabular crystals with rough but well-defined faces. They occur both singly and in clusters of as many as a dozen crystals. Most of them are so stained by manganese oxides that they appear as black blotches on the walls of the mine workings, and in this respect they resemble the spessartite garnet so common in many other pegmatite districts.
Individual crystals are ¼ inch to 17 inches in maximum dimension, with an average of about 4 inches in the pegmatites where they are most abundant. Where fresh, the triphylite is bluish gray, and the much more abundant lithiophilite ranges from flesh colored through pinkish tan to light reddish-brown. Unaltered and only partly altered lithiophilite is fairly abundant, especially in the Stewart dike, but nearly all the triphylite in the pegmatites has been converted to other, secondary phosphate minerals.
The lithiophilite and triphylite are most common in the outer parts of the quartz-spodumene zones and quartz-spodumene-amblygonite zones, and in the inner parts of adjacent perthite-bearing zones of very coarse grain size. They are well exposed in the backs of several drifts and stopes in the Stewart mine, where they occur chiefly in coarse quartz-perthite pegmatite. Although this particular unit lies 5 feet to 20 feet or more above the lepidolite-rich rock that was mined, successive rock falls in some of the larger openings during the past two decades have revealed the presence of these phosphate minerals in greater quantities than were apparent during the periods of active operations. Lithiophilite is far more abundant than triphylite in the pegmatites on Queen and Hiriart Mountains, but triphylite may be dominant in the Pala Chief and in at least one other dike on Chief Mountain. Both minerals occur sparingly as small crystals in the central parts of a few dikes that do not appear to contain spodumene oi amblygonite. Chief among these dikes are the Tourmaline King, White Cloud, and Tourmaline Queen.
Of particular interest to mineralogists is a group of rare manganese and iron phosphate minerals in several of the pegmatites, notably the Stewart, Pala Chief, and Vanderburg-Katerina. Most of these minerals were formed directly or indirectly from lithiophilite and triphylite, and pseudomorphs, fracture-filling relations, and other evidence of their secondary origin are widespread. Progressive alteration of the two primary minerals first yielded sicklerite, iron-manganese-lithium phosphate, and then, accompanied by loss of lithium, yielded purpurite and heterosite, iron-manganese phosphates. Hydration of the lithiophilite and triphylite, and locally of the sicklerite, resulted in development of stewartite, hureaulite, and palaite  (hydrous manganese phosphates), salmonsite (hydrous iron-manganese phosphate), and strengite (hydrous iron phosphate). At least one dark-brown to black iron-manganese phosphate mineral also is present, but it has not as yet been specifically identified.
Many crystals of lithiophilite and triphylite have been altered concentrically, so that cores of residual primary material are surrounded by successive layers of the minerals derived from them. Similar relations have been described from other phosphate-bearing pegmatites, notably the Varuträsk pegmatite near Boliden, Sweden,  and several pegmatites in the Black Hills region of South Dakota.  The unaltered masses of lithiophilite and triphylite are commonly surrounded by a layer of dark reddish-brown sicklerite that ranges in thickness from a knife edge to nearly an inch. Cleavage surfaces of these minerals are continuous. In places, however, the two minerals are separated by stringers and thin lenses of buff-colored, pale amber, or reddish-brown hureaulite, which forms finely crystalline aggregates that are veined and corroded by the sicklerite. Fringing the relatively dark-colored sicklerite layer in most crystals is a somewhat thicker layer of buff to yellowish-brown salmonsite. This mineral forms cleavable masses with rather dull luster, and is typically transected by thin and continuous veinlets of finely crystalline, white to flesh-colored palaite (hureaulite). These veinlets transect also the sicklerite, lithiophilite, and triphylite. Most of the hureaulite is evidently an alteration product of lithiophilite.
Aggregates of blue, lilac, rose-red, and purple strengite,  purpurite, and heterosite are scattered through the salmonsite layers and in the dark, oxide-stained material beyond. Individual crystals are very small, but the aggregates themselves commonly are one-quarter inch to three-quarter inch in maximum dimension. Most of them are ovoid, but some are thinly tabular, and fill cracks in the salmonsite, sicklerite, and earlier phosphate minerals. They are themselves transected by stringers of palaite (hureaulite). Stewartite is present in all the other minerals, chiefly as canary-yellow films and aggregates of tiny crystals. Relatively coarse fibers, some as much as 0.5 mm. long, occur in the layers and pods of purpurite and strengite, and are locally abundant. The mineral is most abundant, however, where it occurs in lithiophilite as fracture fillings. It is intimately intermingled with other species, especially hureaulite, lithiophilite, and salmonsite, and is one of the most widespread of the secondary phosphate minerals.
Triplite, a fluo-phosphate of iron and manganese, occurs sparingly in the Stewart and Vanderburg-Katerina pegmatites, where it is associated with lithiophilite and triphylite. Like these minerals, it forms crystals with rough faces. The crystals are ¼ inch to 5 inches in diameter, with an average of about an inch. The mineral occurs also as tabular, fracture-filling masses within quartz, and rarely within coarse quartz-spodumene or quartz-perthite varieties of pegmatite. It is deep-tan to dark reddish-brown on freshly broken surfaces, and has a resinous luster. Cleavage is fairly well developed. Most crystals are heavily stained with manganese oxides, and many are coated with a blue-gray film of very finely crystalline vivianite, hydrous ferrous phosphate. The vivianite appears to have been derived from the triplite crystals.
Monazite, a phosphate of the rare-earth minerals, is widespread but not at all abundant in the pegmatites. It occurs as small, equidimensional grains, and as tabular crystals of cinnamon-brown color, chiefly in line rock and other albite-rich units. Well-formed crystals ½ inch in maximum diameter are scattered very sparingly through cleavelandite-bearing pocket pegmatite in the Katerina mine. Most of the crystals, however, are subhedral to anhedral, and are much smaller.
Small crystals of pale-pink, violet, and purple apatite, essentially a calcium phosphate, are rare constituents of the pockets, and also occur in albite-bearing parts of perthite-rich pegmatite in several of the dikes on Queen and Hiriart Mountains. Some short, well-formed prismatic crystals in the Pala View mine are ¼ inch to 1½ inches long, but in most places they are less than ¼ inch long. Many of the apatite crystals are tabular, and are flattened parallel to the base.
Sulfide Minerals. Sulfide minerals occur sporadically in several pegmatites. They are present chiefly along fractures in cores and other inner zones. Some are disseminated as small crystals, mainly in quartz. Many of the sulfide minerals in the near-surface parts of the pegmatites have been oxidized, and only stains of iron oxide or other secondary products testify to their former presence. The sulfide species are probably much more abundant at depth.
Arsenopyrite, iron sulfarsenide, forms fine-grained aggregates of silvery gray color, mainly in quartz and albite of the inner zones. Bismuthinite, already mentioned in the discussion of bismuth minerals, is most abundant in the Stewart and Katerina mines, but occurs in all parts of the district. Bornite, copper-iron sulfide, forms typically iridescent stains on fracture surfaces in the Tourmaline Queen, Stewart, Douglass, and San Pedro pegmatites, and especially in the El Molino pegmatite. It occurs also as small, rounded masses in quartz and albite. Chalcocite, copper sulfide, forms sooty black masses the size of a pea in the quartz-rich inner parts of the El Molino and Stewart pegmatites. It is typically associated with thin films of malachite and chrysocolla.
Pyrite is widespread as cubes ⅛ inch or less across, and as fine-grained crystalline aggregates about an inch in maximum dimension. It occurs in quartz veinlets and in quartz-rich inner zones, where it forms yellowish- to greenish-gray aggregates. Most of the mineral, however, is partly oxidized where now exposed, and hence is dull brown in color. Pyrite occurs also in book muscovite as waferlike inclusions with square outline. Most of them are 3⁄16 inch in maximum dimension.
Molybdenite, molybdenum sulfide, forms tiny flattened crystals in the quartz of very coarse-grained pegmatite that forms the inner zones of the Pala Chief, San Pedro, Vanderburg-Katerina, and El Molino dikes.
Zeolites. The zeolite minerals heulandite, laumontitc, and stilbite are widespread but minor constituents of fracture fillings and of pockets and other replacement units in the pegmatites. All are hydrous sodium-calcium-aluminum silicates, and are most commonly associated with the clay minerals of the inner pegmatite units. Heulandite forms buff to light-brown tabular crystals 2 mm. in maximum dimension. It generally occurs with stilbite, which forms lighter-colored aggregates of small, platy crystals. The laumontite is present as rosettes and sprays of white, thinly columnar crystals, few of which are longer than 1 mm. This mineral is characteristically associated with stilbite and clay minerals, and in places appears to have been formed by the alteration of stilbite.
Rare Accessory Minerals. Andalusite, aluminum silicate, forms masses as much as 3 inches in diameter in several pegmatites exposed in the north parts of Queen and Chief Mountains. The mineral is pale pinkish to flesh colored where fresh, and has a characteristic greasy to dull pearly luster. It occurs in the outer parts of the pegmatites, and may have been formed as a result of action between pegmatite solutions and aluminum-rich wallrock.
Cassiterite, tin oxide, is a very rare constituent of several pegmatites exposed on Hiriart Mountain. It forms crystals ⅛ inch or less in diameter in the pocket-bearing parts of these dikes, where it is associated with coarse cleavelandite and small crystals of topaz. These crystals are a deep reddish-brown. Most of them are formed on cleavelandite crystals that line small, irregular cavities.
Epidote is locally abundant near the walls of several pegmatites, and is associated with garnet and biotite. It is pale to deep grassy-green in color, and forms stubby to rodlike crystals that are highly fractured. It may have been derived in part from wallrock material. Loellingite, iron diarsenide, forms massive crystalline platy aggregates, many of which are curved, and occur in groups of layerlike shells, some of which are interlayered with quartz. The mineral is silvery where fresh.
Loellingite, iron diarsenide, forms massive crystalline platy aggregates, many of which are curved, and occur in groups of layerlike shells, some of which are interlayered with quartz. The mineral is silvery where fresh. Loellingite is mainly in the cores and inner parts of adjacent intermediate zones, and commonly is in or near masses of lithium-manganese-iron phosphate minerals.
Petalite, lithium-aluminum silicate, forms white cleavable masses with pearly luster in the Stewart, Vanderburg-Katerina, and Anita pegmatites. Few of the crystals exceed an inch in maximum dimension. They occur chiefly with spodumene, but are associated also with albite and lepidolite, and in general are rather rare.
Pollucite, hydrous caesium-aluminum silicate, has been reported from the Pala district, but probably is very rare. In general this mineral closely resembles quartz, and is found in colorless to milky aggregates that are much fractured.
Several varieties of spinel are rare constituents of the dikes. Small octahedrons of blue and deep-green gahnite, zinc aluminate are along fractures in the quartz of the innermost pegmatite units in the Tourmaline King and Vanderburg mines. In some places they are associated with columbite-tantalite. The mineral occurs sparingly in line rock, especially in the garnet-rich types on Hiriart Mountain.
Hercynite, iron aluminate, forms rare tiny black to very deep blue crystals in the outer zones of several pegmatites; some crystals of slightly lighter color occur in the inner zones, where they are associated with cassiterite. Pleonaste, magnesium-iron aluminate, forms small, lustrous octahedrons of very dark green to black color. Like the hercynite, it is chiefly in the outer zones of the pegmatites. Both these minerals may well be the product of reactions between pegmatite solutions and wallrock.
Some minerals in the pegmatites are not accessory constituents, according to the strictest definitions, but have been derived from one or more other minerals by alteration. Among these alteration minerals are sericite, manganite and psilomelane, hematite and goethite, opal, and chalcedony. In general these minerals coat crystals and grains of older minerals, and also fill numerous fractures and cleavage cracks in them.
Paragenetic Sequence of Minerals
Despite numerous irregularities of detail, the paragenetic sequence of minerals in the Pala pegmatites seems to be nearly uniform throughout the district. Earliest to form were the minerals indigenous to the outer zones. They comprise very abundant perthite and quartz, sparse garnet and beryl, and small but widespread quantities of muscovite, biotite, and schorl. The last three minerals are common also along fracture surfaces, and so may be in part younger than the others. Such species as garnet and biotite may well have been derived in places through reaction between pegmatite solutions and country-rock material, elsewhere by crystallization from “uncontaminated” pegmatite solutions.
As the minerals formed, they were corroded by solutions with which they were no longer in equilibrium, and were veined and surrounded by minerals deposited from these solutions. Many were fractured, and the fractures were filled with other minerals. Corrosion and replacement of earlier-formed constituents were widespread and locally very significant processes, and in places resulted in obliteration of numerous crystals of the earlier minerals. Such processes, and particularly the corrosion, took place over an appreciable range of time, beginning not long after the original crystallization of a given mineral. This was especially true during the formation of the inner pegmatite zones, when several minerals in addition to those not characteristic of the outer zones were formed.
During development of many inner zones, perthite, albite, and schorl were formed in abundance. Much spodumene was developed in some pegmatites, not uncommonly in association with amblygonite and other lithium-bearing phosphate minerals. Parts of some inner zones plainly were extended as apophyses into outer units, particularly during the later stages of pegmatite formation. With subsequent development of typical pocket-bearing pegmatite, the minerals of the inner zones were joined by numerous accessory species, including several that appear to have been formed in very small quantities. Aggregates of these minerals fill fractures in earlier zonal units, and also form replacement masses in these units. They reflect to an even greater degree than before the continued corrosion and replacement of earlier minerals by later constituents, and perhaps are products formed by deuteric processes late in the history of the dikes.
The fine-grained granitoid units, with their relatively simple mineralogy, cut across parts of the zones, but seem to be older than the typical pocket-bearing material. They appear to be superimposed on the pattern of some zonal units, both in a broad way and in detail, but are themselves transected by aggregates of typical pocket minerals. The age relations of these rocks are complicated in most places by the widespread later graphic granite and other mineral aggregates like those in the early-formed pegmatite units. Many of the dikes that contain appreciable quantities of the fine-grained granitoid rocks are clearly composites, with masses of graphic granite that cut both the fine-grained material and older graphic granite. Quartz, albite, muscovite, and garnet are the most abundant constituents of the fine-grained rocks, and with them in many places are schorl, biotite, and scattered rare accessorv constituents.
The general age relations of the principal minerals in the pegmatites are summarized in table 4. The range of hypogene mineral development has been somewhat arbitrarily divided into four principal categories, or stages. The first encompasses a part of the primary consolidation of the dikes, and includes formation of border zones and wall zones. In some dikes it includes also the development of minerals in part from assimilated wallrock material. The second stage involves near-completion of primary consolidation, with progressive increase in the amount of reaction between crystallized pegmatite material and residual solutions. During this stage the inner zones of the dikes were formed, and some fracture fillings and replacements bodies were developed in the outer zones.
The pocket-bearing varieties of pegmatite were formed later, in part at the expense of pre-existing pegmatite of the inner zones. Although most aggregates of pocket material are in the central parts of the dikes, some are fracture-related layers, lenses, and networks in the outer units. The fine-grained albite-rich rocks that are common in the footwall parts of the dikes also contain pocket aggregates.
|General age-abundance relations of principal minerals in the Pala pegmatites.|
|Development of outer zones||Development of inner zones and formation of contemporary fracture fillings and replacement units in outer zones||Formation of line rock and associated finegrained types||Formation of pockets and contemporary fracture fillings|
|Microcline||Very abundant||Very abundant||---||Sparse|
|Quartz||Abundant||Very abundant||Very abundant||Very abundant|
|Albite||Common||Abundant||Very abundant||Very abundant|
|Bismuth minerals [b]||---||Sparse||---||Sparse|
|Lithium-phosphate minerals [b]||---||Common||---||---|
|a The four stages are listed from left to right in general order of decreasing age, but there is much overlap and a few rock types constitute exceptions to the order. Some varieties of pocket-bearing pegmatite, for example, can be classed with the inner zones, in terms of occurrence and probable genesis. The relative age of line rock is known with certainty in few pegmatites.
b Represented in part by alteration products.
The generalizations noted above, though not intended to be complete, are based upon detailed field and microscopic analyses of intermineral relations. Three general criteria for establishing age differences were used. They involve the occurrence of a mineral: (1) as a pseudomorph, where the former existence and identity of the earlier mineral can be clearly established by means of residual material, crystal form, cleavage patterns, or some other characteristic; (2) as a filling of fractures or cleavage cracks in an earlier mineral or mineral aggregate; or (3) consistently in a pegmatite unit whose age relations are known.
The criteria of irregular mineral boundaries and the occurrence of euhedral crystals of one mineral in or against crystals of another have not proved very useful in the present investigations, as they generally permit two or more reasonable but contradictory interpretations. Of somewhat greater value are such features as the occurrence of a given mineral (a) along contacts between other minerals, (b) as inclusions oriented along cleavage directions in the host mineral, (c) as embayments or other forms that indicate the corrosion of an earlier mineral, or (d) in or consistently with another mineral whose age relations are known. These relations are not wholly diagnostic, and must be interpreted with care. In general, however, they have been helpful when used in combination with one another and with the criteria noted above.
|Graphic granite underlain by very coarse-grained perthitequartz pegmatite, El Molino mine. Separating these two units is 18 inches of finer-grained pegmatite composed of abundant masses of graphic granite in a matrix of quartz, albite, and muscovite.|
Origin of the Pegmatites
The Pala pegmatites do not appear to be closely related either in time or in space, to any nearby igneous rocks now exposed. They are truly granitic in composition, whereas all the large masses of intrusive rocks in the area range from gabbro to granodiorite. Moreover, the pegmatites are distinctly younger than all these rocks, and even transect aplitic dikes that are related to the granodiorite. On the other hand, the pegmatites are of the same general age as these batholithic rocks, as all are late Mesozoic and are overlain by sedimentary rocks of Upper Cretaceous age. Despite the apparent lack of close genetic relation at the present surface, the pegmatites are thought to represent a closing stage in the consolidation of the southern California batholith, and therefore to constitute late injections of still fluid material into already crystallized units of this huge composite mass. Probably the dikes represent end products of prolonged differentiation of the batholithic magma. A well-defined sequence that ranges from early basic rocks to later granodioritic rocks is present, but there appears to be a gap in the exposed record between the granodioritic rocks and the granitic rocks represented by the pegmatites.
The pegmatite dikes in the Pala district were emplaced probably along a set of subparallel fractures, as indicated by their attitude and by the existence in the district of numerous unfilled fractures with the same general orientation. The dikes appear to have been injected as liquid material and to have crystallized essentially as simple fracture fillings, rather than to have been formed, wholly or in appreciable part, by replacement of the country rock. This is attested by several features. The walls of the dikes are remarkably straight, and can be traced across contacts between different types of country rock without change in attitude. The dikes themselves show no changes in composition, thickness, or internal structure that can be correlated with changes in the wallrock lithology. Moreover, the pegmatite fluid that was first injected must have pushed aside the country rock, as shown by irregular distortion and tight crumpling of schist and other relatively thinly foliated types of wallrock in several mines. Finally, the disposition and structure of the pegmatite zones indicates that these units developed from the walls of the dikes inward, a sequence that is not compatible with development through replacement of country rock.
The postemplacement history of the pegmatites is much less readily determined, not so much for lack of evidence as for an abundance of evidence that appears to be partly in conflict when applied to a single hypothesis of origin. Schaller,  who devoted much attention to this general question, suggested that the dikes were once simple injections of magma that yielded an orthoclase rock, that at some later time they were solid graphic granite composed essentially of only microcline and quartz, and that all the other minerals now present were introduced still later and are the result of replacement processes. Certainly the widespread evidence of both corrosion and replacement of some minerals by others, particularly in the inner parts of the pegmatites, is striking testimony to a rather complex history of development.
As pointed out by Schaller,  the graphic granite is the oldest rock unit now present in the pegmatites, and at least its feldspathic part probably crystallized directly from the injected pegmatite magma. Not only the graphic granite, but also the younger zonal constituents—chiefly coarse-grained perthite, quartz, spodumene, and lithium phosphate minerals—were formed probably in this way. The layering or concentricity of the zones of graphic granite and other very coarse grained pegmatite may be the result of fractional crystallization from the walls of the dikes inward. Such an origin for zones in pegmatites has been discussed at length by Cameron, Jahns, McNair, and Page.  Fracture-filling apophyses projecting from inner zones into zones nearer the pegmatite walls establish clearly the age relations of the zones. The reverse relation has not been observed. As traced inward from the walls of the dikes, the zones show consistent changes in mineralogy and texture, changes that apply even to variations in the properties of single mineral species, such as beryl and albite. The mineralogic and textural sequences are consistent from dike to dike throughout the district, although few dikes show all members of the sequence.
There is no evidence that the various units of graphic granite or other very coarse-grained pegmatite have formed at the expense of any pre-existing pegmatite. Careful and extended search has failed to reveal even traces of material that might be assigned to an earlier sequence of pegmatite units. Although such evidence is negative, it is so abundant that it cannot be ignored, particularly in view of the evidence for the replacement origin of much pegmatite matter other than that of the zones.
The pocket-bearing pegmatite evidently was derived in part at the expense of pre-existing zonal material, as shown by widespread pseudomorphism and fracture-guided corrosion. Quartz and potash feldspar were strongly corroded, and in places the quartz rods were selectively dissolved out of graphic granite. It is difficult to determine with confidence whether the pocket minerals were formed by solutions at least partly from sources outside the dikes, or whether they are the final products of pegmatite consolidation, either in place or derived from other parts of the dikes. Certain it is that the progressive accumulation and late stage crystallization of mineralizing fluids during consolidation of the dikes could explain many, if not all of the relations in the central parts of the complex pegmatites.
In most of the dikes the proportion of material formed at the expense of other pegmatite is not large. In a few, however, the volume of this type of material is great, and it seems likely that the material was formed partly from solutions derived from other places in the dike, or even from sources farther removed. Excellent examples are the large, lepidolite-rich masses in the Stewart pegmatite. These masses contain abundant and widespread residua of quartz, spodumene, and amblygonite that represent quartz-spodumene and massive quartz zones that once were much more extensive. The lepidolite-rich pegmatite locally transgresses zonal boundaries, and in detail the lepidolite plainly was formed at the expense of older mineral crystals much larger than their present remnants.
Whatever explanation is suggested for the origin of the pocket pegmatite and other material that is at least in part of replacement origin, the structural relations of these units are fairly plain. Both occur chiefly in the central parts of the dikes, where they are typically much less regular in shape and attitude than the zones. Some of these units, however, are nearer the dike walls, where they are generally controlled in their distribution by one or more sets of fractures.
Perhaps the most puzzling of all the pegmatite units, so far as genesis is concerned, are the fine-grained granitoid rocks. The line rock, for example, has been interpreted by Waring  as an early product of simple crystallization from a “hydrous magma,” by Merriam  as the product of rhythmic replacement in an early, probably primary aplite, and by Schaller  as a much later result of replacement of pre-existing graphic granite by soda-rich solutions. The fine-grained, albite-rich rocks are clearly younger than some graphic granite, but are older than the stringers, lenses, and small pods of later graphic granite in them. Wherever the line rock or associated aplitic material is in contact with the main masses of graphic granite that constitute much of a given dike, and the age relations of the rocks can be determined, the graphic granite is the older of the two. On the other hand, the pocket pegmatite is distinctly younger than some aplitic units, as offshoots from masses of such pegmatite transect these finer-grained rocks.
The fine-grained granitoid rocks form masses whose shape and distribution generally do not conform to the structure of the pegmatite zones. The zonal structure is the older, and the fine-grained units appear to have been superimposed upon it, particularly in its footwall parts; they cut across the ends of the concentric units in the bulges of some irregular dikes, and are not oriented in accord with the zonal structure of many other dikes. These features, together with the occurrences of graphic granite residual in the line rock and associated fine-grained types, may mean that many, if not all, of these rocks were derived from graphic granite by replacement processes. Schaller  has accumulated widespread evidence for the existence in these rocks of not only graphic granite residua, but unreplaced aggregates of quartz rods in a matrix of sugary albite, quartz, and muscovite. Evidence thus far obtained during the present investigations does not seem to warrant definite conclusions, but if Schaller’s views are correct, the fine-grained granitoid rocks must have been formed prior to the development of the pocket pegmatite in the dikes, presumably by solutions introduced from sources at considerable distances from the areas of replacement.
To be continued with “Economic Features of the Pegmatite Minerals”…
|Idealized section of pegmatite dikes in the Pala district, showing typical relations of line rock and associated fine-grained rocks.|
|Section through adit, El Molino mine, showing down-dip splitting of composite dike.|
Plates 1 and 2 are available in this PDF.
To return to text, click on the reference number.
42. Although graphic granite is a rock, rather than a mineral, the term "crystal" is used in this report as a convenient means of referring to the host perthite individual.
43. McLaughlin, T. G., Pegmatite dikes of the Bridger Mountains, Wyoming: Am. Mineralogist, vol. 25, pp. 62–63, 1940.
44. Schaller, W. T., The genesis of lithium pegmatites: Am. Jour. Sci., 5th ser., vol. 10, pp. 271–275, 1925.
45. Hunt, T. S., Notes on granitic rocks: Am. Jour. Sci., 3rd ser., vol. 1, pp. 82–89, 182–191, 1871.
46. See, for example:
Smith, W. C., and Page, L. R., Tin-bearing pegmatites of the Tinton district. Lawrence County, South Dakota: U. S. Geol. Survey Bull. 922-T, 35 pp., 1941.
Olson, J. C., Mica-bearing pegmatites of New Hampshire: U. S. Geol. Survey Bull. 931–P, pp. 373–376, 1942.
Bannerman, H. M., Structural and economic features of some New Hampshire pegmatites: New Hampshire State Planning and Dev. Comm., Min. Res. Survey, Part VII, 22 pp., 1943.
Page, L. R., Hanley, J. B., and Heinrich, E. Wm., Structural and mineralogical features of beryl pegmatites (abstract): Econ. Geology, vol. 38, pp. 86–87, 1943.
Cameron, E. N., Larrabee, D. M., McNair, A. H., and Stewart, G. W., Characteristics of some New England mica-bearing pegmatites (abstract): Econ. Geology, vol. 39, p. 89, 1944.
Jahns, R. H., and Wright, L. A., The Harding beryllium-tantalum- lithium pegmatites, Taos County, New Mexico (abstract): Econ. Geology, vol. 39, pp. 96–97, 1944.
Olson, J. C., Parker, J. M. III, and Page, J. J., Mica distribution in western North Carolina pegmatites (abstract): Econ. Geology, vol. 39, p. 101, 1944.
de Almeida, S. C., Johnston, W. D., Jr., Leonardoes, O. H., and Scorza, E. P., The beryl-tantalite-cassiterite pegmatites of Paraiba and Rio Grande do Norte, northeastern Brazil: Econ. Geology, vol. 39, pp. 206–223, 1944.
Olson, J. C., Economic geology of the Spruce Pine pegmatite district, North Carolina: North Carolina Dept. Conservation and Development, Div. Min. Res., Bull. 43, 1944.
Johnston, W. D., Jr., Beryl-tantalite pegmatites of northeastern Brazil: Geol. Soc. America Bull., vol. 56, pp. 1015–1070, 1945.
Cameron, E. N., Larrabee, D. M., McNair, A. H., Page, J. J., Shainin, V. E., and Stewart, G. W., Structural and economic characteristics of New England mica deposits: Econ. Geology, vol. 40, pp. 369– 393, 1945.
Fisher, D. J., Preliminary report on the mineralogy of some pegmatites near Custer: South Dakota State Geol. Survey Rept. of Investigations No. 50, 89 pp., 1945.
Jahns, R. H., Mica deposits of the Petaca district, Rio Arriba County, New Mexico: New Mexico School of Mines, State Bur. Mines and Min. Res., Bull. 25, 1946.
Cameron, E. N., and Shainin, V. E., The beryl resources of Connecticut: Econ. Geology, vol. 42, pp. 353–367, 1947.
Pecora, W. T., Klepper, M. R., and Larrabee, D. M., Mica-bearing pegmatites in Minas Gerais, Brazil (abstract): Washington Acad. Sci. Jour., vol. 37, pp. 370–371, 1947.
Cameron, E. N., Jahns, R. H., McNair, A. H., and Page, L. R., The internal structure of granitic pegmatites: Econ. Geology, Mon. 2, 1949.
47. Cameron, E. N., Jahns, R. H., McNair, A. H., and Page, L. R., op. cit., pp. 14–96, 1949.
48. Cameron, E. N., Jahns, R. H., McNair, A. H., and Page, L. R., op. cit., pp. 20–21, 1949.
49. Schaller, W. T., The genesis of lithium pegmatites: Am. Jour. Sci., 5th ser., vol. 10, pp. 273–276, 1925.
50. Donnelly, M. G., The lithia pegmatites of Pala and Mesa Grande, San Diego County, California: California Inst. Technology, unpublished Ph.D. thesis, p. 56, 1935.
51. Schaller, W. T., op. cit., pp. 271–277, esp. p. 275, 1925.
52. See for example:
Kunz, G. F., Gems, jewelers’ materials, and ornamental stones of California: California State Min. Bur. Bull. 37, pp. 46–101, 1905.
Murdoch, Joseph, Crystallography of hureaulite: Am. Mineralogist, vol. 28, pp. 19–24, 1943.
Schaller, W. T., Spodumene from San Diego County, California: California Univ. Dept. Geol. Sci. Bull., vol. 3, pp. 265–275, 1903.
Schaller, W. T., Notes on some California minerals: Am. Jour. Sci., 4th ser., vol. 17, pp. 191–194, 1904.
Schaller, W. T., Mineralogical notes: U. S. Geol. Survey Bull. 262, pp. 121–122, 139–140, 143–144, 1905.
Schaller, W. T., Bismuth ochers from San Diego County, California: Am. Chem. Soc. Jour., vol. 33, pp. 162–166, 1911.
Schaller, W. T., Notes on purpurite and heterosite: U. S. Geol. Survey Bull. 490, pp. 72–79, 1911.
Schaller, W. T., New manganese phosphates from the gem tourmaline field of southern California: Washington Acad. Sci., Jour., vol. 2, pp. 143–145, 1912.
53. Donnelly, M. G., The lithia pegmatites of Pala and Mesa Grande, San Diego County, California: California Inst. of Technology, unpublished Ph.D. thesis, pp. 68–70, 1935.
54. Throughout these studies, orthoclase and microcline were distinguished under the microscope on the basis of extinction with respect to the (001) and (010) cleavage traces.
55. Schaller, W. T., Spodumene from San Diego County, California: California Univ., Dept. Geol. Sci., Bull., vol. 3, pp. 265–275, 1903.
56. Schaller, W. T., and Fairchild, J. G., Bavenite, a beryllium mineral, pseudomorphou.s after beryl, from California: Am. Mineralogist, vol. 17, pp. 409–422, 1932.
57. Kunz, G. F., Native bismuth and bismite from Pala, California: Am. Jour. Sci., 4th ser., vol. 16, pp. 398–399, 1903.
58. Frondel, Clifford, Mineralogy of the oxides and carbonates of bismuth: Am. Mineralogist, vol. 28, pp. 532–533, 1943.
59. Ross, C. S., and Hendricks, S. B., Minerals of the montmorillonite group: U. S. Geol. Survey Prof. Paper 205-B, pp. 25–28, 34, 69–70, 1945.
60. According to W. T. Schaller (personal communication), x-ray studies during recent years have demonstrated that palaite and hureaulite are identical.
61. Quensel, P., Minerals of the Varuträsk pegmatite. 1. The lithium-manganese phosphates: Geol. foren. Stockholm Förh., vol. 59, pp. 77–96, 1937.
62. Fisher, D. J., Preliminary report on the mineralogy of some pegmatites near Custer: South Dakota Geol. Survey Rept. Inv. No. 50, esp. pp. 43–47, 1945.
63. Some of the material previously identified as strengite may well be phosphosiderite. See Murdoch, Joseph, Minerals of California—Supplement No. 1 to Bulletin 136: California Jour. Mines and Geology, vol. 45, p. 529, 1949.
64. Schaller, W. T., The genesis of lithium pegmatites: Am. Jour. Sci., 5th ser., vol. 10, p. 276, 1925.
65. Schaller, W. T., op. cit., p. 276, 1925.
66. Cameron, E. N., Jahns, R. H., McNair, A. H., and Page, L. R., The internal structure of granitic pegmatites: Econ. Geology, Mon. 1, pp. 97–105, 1949.
67. Waring, O. A., The pegmatvte veins of Pala, San Diego County: Am. Geologist, vol. 35, p. 366, 1905.
68. Merriam, Richard, Igneous and metamorphic rocks of the southwestern part of the Ramona quadrangle, San Diego County, California: Geol. Soc. America Bull., vol. 57. pp. 242–243, 1946.
69. Schaller, W. T., op. cit., pp. 274–277, 1925.
70. Schaller, W. T., op. cit., pp. 274–275, 1925.
Schaller, W. T., personal communications, 1943–1948.