Summary:
The Mineralization at Ruby Hill is characterized by intrusion-related distal-disseminated, carbonate replacement, and skarn deposits that have been overprinted by younger Carlin-type gold mineralization.
The carbonate replacement mineralization is similar to other polymetallic (Pb-Zn-Ag ± Au) deposits found worldwide that are spatially associated with Cretaceous age intrusive units (Cox and Singer, 1987; Megaw et al.,1988; Plumlee et al., 1995; Titley 1993 cited in Hammarstrom, 2002; and Kamona, 2011). The carbonate replacement mineralization consists of massive to semi-massive pyrite, galena, sphalerite, and other sulfides typically with sharp boundaries into barren marble. Locally, mineralization is oxidized into gossanous bodies. Fluids are sourced from intrusions, with metals in bisulfide complexes at temperatures of 250°-500°C, with the depositional mechanism typically being a pH change that results in rapid deposition of metals (Beinlich et al., 2019).
Ore grades of gold and silver with elevated concentrations of zinc, lead, and copper and are found in the Mineral Point Trend. This mineralization is attributed to the earlier Cretaceous age of mineralization and is found predominately in the Hamburg dolomite. Ore fluids were likely similar to Carlin-type fluids and resulted in the formation of collapse breccias and an associated geochemical signature including arsenic, antimony, thallium, and mercury. Distal-disseminated deposits share many similarities to Carlin-type deposits as the hydrothermal fluids are analogous. However, distal disseminated deposits typically occur within 5 km of an intrusion, have an association with base metals, and show a zonation pattern outward from the intrusive source. Examples include Lone Tree, Cove, and Star Pointer (Nevada), Mercur and Barneys Canyon (Utah), Jeronimo (Chile), Bau (Malaysia), Mesel (Indonesia), and Zarshuran (Iran) (Hill, 2016).
Gold-silver mineralization at Mineral Point is dominantly oxide in nature with small, but higher-grade refractory material (Loranger, 2013). Mineralization is predominantly hosted within the Hamburg Dolomite and consists of decarbonatized dolomite and breccias composed of silicified and oxidized clasts of dolomite in a fine grained dolomite and silica matrix. Locally breccias are gossanous where a higher percentage of original pyrite existed. Higher grade breccia zones are cut by late, multistage quartz veins (Loranger, 2013). Mineralization also occurs along the upper contact into the overlying silicic altered Dunderberg Shale which hosts oxide and sulfide minerals.
The main mineralized zone at Mineral Point is roughly elliptical in shape, NNW-trending, and is approximately 10,000 ft in length, 2,400 ft wide, and approximately 500 ft thick. The mineralization extends from approximately 240 to 1,400 ft below surface.