Geology and Discovery History of the Gravel Creek Silver-Gold Deposit, Elko County, Nevada

摘要

The Gravel Creek low-sulfidation, epithermal, silver-gold deposit is located 15 km south of Mountain City and 1.5 km northeast of the historic Wood Gulch gold mine in northern Elko County, Nevada. Precious metal mineralization is localized across and above the unconformity between Mississippian-Permian Schoonover Formation metased-imentary rocks and overlying Tertiary volcanic and volcaniclastic sedimentary rocks. Tertiary stratigraphy overlying the Paleozoic basement includes Eocene rhyolite welded ash-flow tuff, grading upward into non-welded crystal-pumice tuff, which is overlain by a fluvial volcaniclastic sedimentary rock unit including interbedded pebble conglomerate, tuffaceous sandstone, carbonaceous shale, coal, and locally a capping andesite flow. Surface exposures are of capping Miocene flow-dome Jarbidge Rhyolite (16.4±0.4 Ma). Two styles of precious-metal mineralization are distinguished. The most significant Ag-Au mineralization is stratabound within the 20-degree east-dipping section of permeable ash-flow tuff and volcaniclastic sedimentary rock. Alteration consists of zones of pervasive silicification surrounding stockwork quartz-adularia-pyrite veining, overlain by zones of intense sericite-pyrite alteration. The stratabound mineralization style passes upward into near-vertical stockwork quartz-sulfide vein and vein-let mineralization within more competent overlying flow-dome rhyolite. The presence of a preserved silica sinter apron extending nearly 2 kilometers downslope from the central deposit area suggests that the hydrothermal system discharged as hot-springs near to the present topographic surface. Alteration and precious metal veins in Jarbidge Rhyolite indicate a Miocene or younger age of mineralization. Strongly mineralized rock is characterized by multiphase angular hydrothermal breccia with quartz-adularia matrix. Mineralogy of the silicified breccia includes quartz, pyrite, bladed marcasite, low-iron sphalerite and native gold. Dominant alteration minerals are quartz, sericite, adularia and pyrite. Mineralization has enrichment of the element suite Au-Ag-As-Sb-S, with an average Ag:Au ratio of about 30:1. The discovery history included recognition of subtle rock alteration during reconnaissance mapping, detection of anomalous Ag and Au concentrations in rock samples, encouraging Ag and Au intercepts from a 2008 shallow drill test, and imagination of an unconventional exploration model, leading to deep discovery drilling in 2013. Significant intercepts from 2013 drilling included drill hole WG-377 with 10.7 m averaging 10.98 ppm Au and 243.8 m averaging 42.21 ppm Ag; and drill hole WG-379 with 9.1m averaging 41.15 ppm Au and 251.5 m averaging 11.87 ppm Ag. Exploration work during 2014 included a suite of geophysical surveys, reverse-circulation and diamond core drilling. This is a new discovery from which we have yet much to learn.