Gem- and lithium-bearing pegmatites of the Pala district, San Diego County, California

Abstract

The Pala pegmatite district, in northwestern San Diego County, California, has been a widely known source of gem and lithium minerals. Formal mining operations began in the eighteen-seventies; but the most active period was from 1900 to 1922. By 1947 the district's total recorded mineral output was valued at about three-quarters of a million dollars. This output includes 23,480 short tons of lepidolite, 2,980 pounds of tourmaline, and 1,325 pounds of gem spodumene. Small amounts of amblygonite, beryl, feldspar, and quartz also have been mined. Deposits of lepidolite and gem minerals have been extensively worked in six mines, and many additional deposits have been prospected or mined on a small scale. The dominant rocks of the district form parts of the southern California batholith, of probable Cretaceous age. Some older rocks, chiefly schists and quartzites, occur as screens, septa, and pendants. Both these and the igneous rocks are covered in places by surficial deposits of Quaternary age. At least 400 pegmatite dikes are exposed in an area of about 13 square miles. Most of them trend northward and dip gently to moderately westward, and many are marked by broad bends in strike and dip. They are remarkably persistent, and range from small stringers to large dikes with bulges nearly 100 feet thick. In several places they occur as swarms of closely spaced, subparallel dikes. In some swarms these dikes branch and converge along their strike, and in places they form thick, composite bodies in which each member dike commonly retains its identity. The pegmatites occur mainly in gabbroic rocks, and appear to have been emplaced along a well-developed set of fractures. These fractures are independent of the primary structural features of the enclosing rocks, and transect contacts between major crystalline rock units; they may well have been subhorizontal at the time of pegmatite emplacement. Some of the pegmatites are essentially homogeneous in mineralogy and texture, but most are composed of units that plainly differ from one another in lithology. Graphic granite is the chief constituent of the outermost units, or border zones, which generally are thin, discontinuous, and fine grained. It also composes most of the adjacent, coarse-grained wall zones, which ordinarily are the thickest and most persistent of the pegmatite units. Graphic granite is particularly common in the hanging-wall parts of the dikes, but constitutes nearly the full thickness of many dikes. It appears as relict masses in the fine-grained lower parts of some dikes, but also occurs as pods and thin stringers in such fine-grained pegmatite. Discoidal masses of coarse-grained pegmatite form the innermost zones, or cores, of many dikes. Such masses are generally thin and elongate, but more pod-like cores are present in the thick bulges of a few dikes. Some cores are composed of quartz, perthite, or an aggregate of these minerals, and others consist of quartz and giant crystals of spodumene. Spodumene of gem quality occurs wholly within the cores, and represents the relatively small amount of this mineral that has escaped all hydrothermal alteration. Lithiophilite and triphylite occur in or adjacent to some quartz-spodumene cores, and commonly have been altered to manganese- and iron-phosphate minerals. Some of the cores are separated from nearby wall zones by one or more intermediate zones, which form discontinuous or complete envelopes. These units are present only in the largest dikes or in dikes with thick bulges, and generally are rich in coarse-grained perthite. Fracture-filling units are widespread, and consist chiefly of quartz, albite, biotite, fine-grained muscovite, or combinations of these minerals. Some transect, wall zones, but merge with inner zones. Others lie wholly within a single zone, and still others cut across entire pegmatite bodies. There are all gradations between simple open-space fillings and replacement bodies developed along fractures. Fracture-controlled replacement bodies are superimposed upon the zonal pattern of nearly all of the pegmatites. They are composed mainly of albite, quartz, and muscovite, and, less commonly, of lepidolite and tourmaline. Such bodies are most easily recognized where they transect wall-zone graphic granite. Similar mineral aggregates occur in the central parts of many dikes, where they generally corrode the surrounding pegmatite zone. These centrally disposed units, which commonly contain residual masses of earlier minerals, include much of the district's "pocket pegmatite," a rock type composed mainly of fine- to coarse-grained quartz, albite, orthoclase, microeline, muscovite, lepidolite, and tourmaline. Most of the minerals are euhedral. All the gem tourmaline and beryl, as well as the commercial concentrations of lepidolite, occur in so-called "pocket pegmatite." Such rocks actually contain very little open space, although some cavities are partly or completely filled with a clay through which gem crystals are scattered. Pocket pegmatite occurs in cores and immediately adjacent zones, chiefly along the footwalls or in the footwall parts of the cores. Fine-grained granitoid rocks, composed chiefly of quartz and albite, are common in the footwall parts of most dikes, and also occur locally in other parts of many dikes. Some varieties are essentially uniform in texture and structure. Others, known collectively as "line rock," are strikingly marked by alternating thin layers of garnet-rich and garnet-poor pegmatite, or of schorl-rich and schorl-poor pegmatite. Layering in some of these rocks also is caused by distinct variations in texture. The Pala dikes are believed to have been formed by crystallization of pegmatite liquid that was injected along fractures during the final stages of consolidation of the southern California batholith. The pegmatite zones appear to have developed from the walls of the dikes inward, probably by fractional crystallization and incomplete reaction with residual liquid. Many, if not all, of the relations in the central parts of the most complex pegmatites seem best explainable in terms of progressive accumulation and late-stage crystallization of mineralizing fluids, with accompanying deuteric replacement of earlier-formed minerals. In order to develop a few of the larger replacement bodies, material may have been derived from other parts of the dikes, or possibly from sources farther removed. The fine-grained pegmatite units cut across the zonal structure of some dikes, and appear to have been formed in part at the expense of graphic granite. The pocket pegmatite also is at least in part of replacement origin, and is plainly younger than some of the fine-grained pegmatite.

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