Controls of High Grades within the Clementine Vein System in the Hollister Low-Sulfidation Epithermal Au-Ag Deposit, NV
AuthorSmith, Joshua Michael
AdvisorMuntean, John L
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The Hollister gold-silver deposit is a very well preserved low-sulfidation epithermal system associated with middle Miocene bimodal volcanism related to the Northern Nevada Rift. Over 3,600 kg of gold were mined from volcanic rock-hosted, low-grade, disseminated ore from 2 open pits between 1990 and 1992. Later drilling led to the discovery of blind, high-grade vein systems in underlying Paleozoic rocks, which from 2007 to 2013 produced 10,668 kg (343,574 oz) of gold and 65,998 kg (2,121,555 oz) of silver from ore averaging about 34 g/t Au and 200 g/t Ag. Vein 18, the most productive vein of the Clementine vein system, was studied to document patterns of hydrothermal features around the vein and high-grade shoots within the vein, which can be applied to exploration for other high-grade veins at Hollister and in other epithermal districts. Geological, mineralogical, and geochemical features, including ore and gangue paragenesis; quartz textures; vein forms and structural data; concentrations of ore-related trace elements; and fracture-controlled and pervasive alteration in the surrounding Paleozoic wall rocks were logged from drill core and mapped onto a longitudinal section along, and a cross-section across Vein 18. Vein 18 is a discrete, open-space filled vein, ranging from 0.15 - 1 meter thick, with local stockwork and breccia. Its paragenesis is characterized by 4 stages: 1) quartz-bladed calcite (replaced by quartz)-adularia-chlorite-pyrite-sphalerite-chalcopyrite-Ag selenides/sulfosalts-electrum; 2) quartz-pyrite-illite-ammonium illite-montmorillonite; 3) quartz-alunite-dickite-kaolinite-specular hematite-marcasite-siderite; and 4) quartz-pyrite veinlets related to a later hydrothermal event. Elevation exerts a strong control on the gold and silver grades, with the highest grades occurring between 120 and 230 m below the current surface, which is very close to the original paleosurface and/or paleo-water table, as suggested by numerous mercury-bearing sinter and opalite bodies in the district. Boiling was likely the mechanism for ore formation, as evidenced by the strong spatial relationship between high grades and the distribution of bladed quartz after calcite, banded quartz, and adularia. Boiling and Stage 1 ore formation occur in repetitive cycles of pyrite→base-metal sulfides→Ag-selenides→electrum that commonly occur with banded and bladed quartz, separated by periods of largely barren massive quartz. Analyses of vein intercepts indicate a Au, Ag, Se, Cu, Pb, Zn, and Sb association for the ore. Stage 2 interlayered illite-montmorillonite commonly partially or completely replace adularia. Stage 3 is steam-heated alteration, which formed widespread kaolinite and a funnel-shaped zone of alunite±dickite alteration that rooted down on Vein 18, overprinting Stages 1 and 2, likely during lowering of the paleo-water table. δ34S values of ore-stage pyrite in banded quartz veins range between 4.2 - 4.5 /, whereas those for late stage alunite are between 6.3 - 6.6 /, consistent with a steam-heated origin. Features that suggest proximity to high-grade ore shoots include: 1) The presence of banded and bladed quartz and adularia; 2) A common paragenesis of quartz textures, which indicates multiple cycles of boiling; 3) Discrete veins that provide significant development of quartz textures; 4) Illite or illite-smectite zones that can reflect original distribution of adularia; 5) Discrete zones of steam-heated alunite, which can define high H2S flux from underlying zones of upwelling, boiling fluid; and 6) Increases in Se and Cu concentrations.