California Gold Production Summary

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California Gold Production Summary

The bedrock in the Mother Lode counties consists of steeply dipping, northwest-trending belts of phyllite, schist, slate, and greenstone, intruded locally by small bodies of peridotite and granodiorite (Knopf, 1929, p. 8-9). The oldest rock unit in the area, the Calaveras Formation of Carboniferous age, is composed chiefly of black phyllite with minor quartzite, limestone, and chert. Green amphibolite schists are interbedded with the Calaveras Formation and are believed to be of equivalent age (Knopf, 1929, p. 10). Overlying the Calaveras Formation is the Mariposa Slate, of Jurassic age. This unit, which is considerably less metamorphosed than the Calaveras, consists of black clay slates and graywacke with small local bodies of conglomerate, sericite schist, and limestone (Knopf, 1929, p. 12). Greenstones are intimately interlayered with the black slate. The rocks have been invaded by intru-sives of several ages. The Calaveras Formation is cut by metadiorites, and both formations are intruded by lenses of serpentine, which was originally peridotite, and by dikes and masses of hornblendite, gabbro, granodiorite, and albitite porphyry. Potassium-argon age determinations on minerals of some of these granitic bodies give evidence of two sep rate orogenies in Mesozoic time one in Late Jurassic and one in Late Cretaceous (Curtis and others, 1958, p. 5-10). All these rocks are overlain by patches and sinuous deposits of interbedded stream gravels and rhyolite, andesite, and basalt flows.

The Mother Lode gold deposits probably were formed during the final stages of the intrusion of the Sierra Nevada batholith (Knopf, 1929, p. 48).

The gold deposits of the Mother Lode are associated with a zone of reverse faulting that is parallel in general to the northwesterly trend of the Calaveras and Mariposa Formations but locally cuts all rock types of both formations. Ore bodies are of two general types - quartz veins and mineralized country rock (Knopf, 1929, p. 23).

The quartz veins are large tabular masses of quartz that strike northwest and dip northeast. Though they appear to be locally conformable with the country rock at the surface, the veins cut across various units of the country rock along the strike and down the dip. Individual veins, as much as 50 feet thick and a few thousand feet long, are localized in systems of parallel or subparallel lenses with blunt ends, some of which fray out into stringers. The vein mineralogy is simple. Milky quartz, the predominant veinfilling, is characteristically ribboned, different layers having been deposited at different times. A small amount of sulfides, mostly pyrite, accompanies the quartz (Knopf, 1929, p. 27). Gold occurs in the free state, commonly in steeply pitching shoots where the veins bulge or at vein junctions and in stringer lodes. The gold is interstitial with the quartz and the sulfides.

The ore bodies in country rock are of diverse types, but the mineralized greenstone, known as gray ore, and mineralized schists are the most productive. The mineralized greenstone is composed of ankerite, sericite, albite, quartz, and 3 to 4 percent pyrite and arsenopyrite (Knopf, 1929, p. 33). It is interlaced with veinlets of quartz, ankerite, and albite. Gold is intergrown with the sulfides or is interstitial with quartz. The mineralized schist ore bodies are composed chiefly of ankerite and subordinate sericite, pyrite, quartz, and albite (Knopf, 1929, p. 34). Free gold is associated with pyrite.

Western Mining History

California Gold Production Summary

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