Geology and trace element geochemistry of the Hollister Gold Deposit, Ivanhoe District, Elko County, Nevada

Hollister is a hot spring type disseminated epithermal gold deposit located in north-central Nevada and within the northwestern extension of the Carlin gold trend. Stratigraphy of the deposit consists of an Ordovician sedimentary basement and a Tertiary (mostly early-to-middle Miocene) bimodal volcanic/volcaniclastic cover. Prominent structural features include a pre-Tertiary paleotopographic ridge and some northeast- and northwest-striking faults. Paleo hot-spring sinters are widely distributed and underlain by a zone of intense silicification and acid-sulfate alteration. Gold mineralization in the deposit is hosted predominantly by Tertiary volcanic rocks and to a lesser extent by the Ordovician Valmy Formation. Alteration closely associated with mineralization includes silicification and argillization. Gold is present as electrum, and is closely associated with pyrite, marcasite and trace amounts of sphalerite, freibergite, pyrrhotite, and chalcopyrite. K-Ar dating of alunite gives an age of 17.9 $\pm$ 0.8 Ma. Secondary two-phase (L + V) fluid inclusions in quartz of the mineralized Valmy orthoquartzite have an average homogenization temperature of 230.1$\sp\circ$C and salinity of 10.2 wt.%NaCl equivalent. Statistical analysis of trace element data shows that the original epithermal fluids were supersaturated with Au, As, Sb, Tl, and Hg and undersaturated with Ag, Zn, Mn, Li, and Ba. During the supergene oxidation process, Au, As, Sb, Tl, Hg, Mn, Li, and Ba were generally stable, but Ag was easily leached. The Ag/Au ratio, therefore, is a good measure of the degree of oxidation for this type of deposit. A gold mineralization index (GMI) was established as follows: GMI = 4.45Au + 0.00373As + 0.0127Sb + 0.909Tl + 0.185Hg. This index is distributed more widely and regularly than gold alone and any other single trace element, and forms a halo above gold orebodies; it has good promise as an exploration tool. Thermodynamic calculation shows that the epithermal fluid changed from an early high $f\sb{o\sb2}$ and low pH system accompanying the formation of the alunite/pyrite assemblage to a low $f\sb{o\sb2}$ and middle pH system at the main gold deposition stage. Gold was transported predominantly as hydrosulfide complexes such as Au(HS)$\sb{\bar 2},$ cooling, oxidation, and boiling of the epithermal fluid are the major gold depositional mechanisms