The geology of the Copper Chief Prospect, Mineral County, Nevada

The Copper Chief Prospect is on the south slope of the Gabbs Valley Range, Mineral County, Nevada. It is a scheelite-sulfide deposit associated with skarn- type alteration.

Shales and limestones of the Triassic Luning Formation have been intruded by sills, dikes, and stocks of quartz monzodiorite or granodiorite of Cretaceous (?) age. Reactions between the intrusions, their host rocks, and the associated fluids have resulted in the formation of endoskarns, exoskarns, and contact metamorphic assemblages.

Unaltered samples of the intrusions from the Copper Chief Prospect are porphyritic and contain phenocrysts of plagioclase feldspar in a groundmass of quartz and potassium feldspar wIth minor amounts of hornblende and biotite and trace quantities of apatite, sphene, and zircon. The intrusions were altered to endoskarns in the high-temperature stages of hydrothermal alteration. Endoskarns of low alkalinity are characterized by epidote, calcite, and crystals of clinopyroxene that have pseudomorphically replaced hornblende. Those of high alkalinity (periskarns) are characterized by an extreme enrichment in potassium feldspar, which may also be accompanied by wollastonite, garnet, epidote, and crystals of clinopyroxene that replace primary hornblende. Other types of alteration imposed upon the intrusive rocks includes 1) quartz- sericite, and 2) calcite-iron oxide.

Thermal contact metamorphism of the host rocks caused by heat released during the emplacement and crystallization of the intrusions has converted shale to hornfels and limestones to marble, In contrast hydrothermal contact metasomatism of the carbonate host rocks has formed alteration assemblages characterized by wollastonite, garnet, and clinopyroxene-garnet. Garnet exoskarns are the most abundant product of caic-silicate metasomatism at the Copper Chief Prospect. They range from grossularite to andradite in composition but most garnetites have intermediate compositions (grandites).

Alteration of the intrusions and their hosts occurred under the temperature and pressure conditions of the upper hornblende to lower pyroxene hornfels facies (600-800¬į C, 100-3000 bars) of contact metamorphism and contact metasomatism.

The ore minerals exhibit a preferred distribution with respect to the alteration assemblages and controls of deposition. Scheelite is localized in clinopyroxene- rich garnetites, whereas the sulfides (ähalcopyrite, pyrite, and chalcocite) are most abundant in andradite garnetite, marble, and veins of quartz.

Regional uplift and erosion followed emplacement of' the intrusions and deposition of the caic-silicates and ore-bearing minerals. Volcanic activity of middle to late-Tertiary age resulted in the deposition of lava flows and tuffs. Late-Tertiary displacement on the Walker Lane system of faults has cut both the volcanic rocks and the underlying local basement of sedimentary, Intrusive, and associated metamorphic rocks.