Abstract

Several Tertiary volcanic-hosted precious metal deposits occur in the Mojave block in southern California which is bounded on the north by the Garlock fault and on the southwest by the San Andreas fault. The Middle Buttes volcanic dome complex hosts several low-grade gold and silver orebodies and is comprised of Miocene-age coalescing lava domes, flows, vent breccias, and pyroclastic deposits lying unconformably on Cretaceous basement of the Sierra Nevada batholith. The volcanic rocks range in composition from andesite through rhyolite and are cut by numerous faults.The Shumake deposit, over 5 million tons of ore with an average grade of 0.043 oz per ton (1.5 ppm) Au and 0.4 oz per ton (14 ppm) Ag, is centered around two large quartz veins and the intervening altered rhyolites. The large veins are characterized by pyrite, arsenopyrite, and silver sulfosalts, with minor base metal sulfides and gold. Gold in the altered rhyolite occurs with stockwork quartz veinlets and an oxidation assemblage of iron oxides, scorodite, and red kaolinite. Alteration of the rhyolites is best characterized by sericite, composed of fine-grained illite and muscovite, and stockwork quartz veinlets with associated adularia. Fluid inclusions indicate that the stockwork quartz formed around 210 degrees C. The quartz, sericite, and minor adularia assemblage is underlain by propylitic alteration and overlain in one part of the deposit by alunite and white kaolinite alteration. These main-stage alterations were followed by an oxidation assemblage which is interpreted to be related to collapse of the hydrothermal system. The deposit is dominated by anomalous abundances of Au, Ag, Hg, As, and Sb. Spatial geochemical variations suggest that the two massive quartz veins were dominant components of the hydrothermal plumbing system.The paleohydrologic system at the Shumake deposit is analogous to the adularia-sericite type of volcanic-hosted epithermal precious metal deposits. Faults and fractures provided the pathways for hydrothermal fluid movement. Vein textures suggest episodic mineralization that can be explained by seismic pumping of hydrothermal fluids. Since a close temporal relationship exists between volcanism and ore deposition, magmas probably provided heat to the hydrothermal system. Thus, for the Shumake deposit, faulting provided both the pathways and driving mechanism for the hydrothermal system while heat was provided by magmas.

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