Characterization of Gold and Related Mineralization at the North Bullion Carlin System, Railroad Project, a Nevada Carlin-~type Gold Prospect

摘要

A study of the North Bullion Carlin system, a Nevada Carlin-type gold prospect located on trend with the large Carlin-type gold deposits (CTGD) of the Carlin Trend, identified characteristics of ore and alteration mineralization, mineral paragenesis, and pyrite and whole rock geochemistry that are similar to characteristics that typify Carlin-type gold deposits. The North Bullion Carlin system is hosted in Devonian shelf carbonates and the Mississippian overlap assemblage, host rocks that are different from typical CTGD shelf-slope carbonate assemblage host rocks. The mineralogy and ore-stage gold-bearing pyrite rims at North Bullion are not as well developed as they are in typical CTGD, and alteration and geochemical relationships are subtle. Yet the North Bullion Carlin system exhibits many of the features observed in large deposits.Observations demonstrate that North Bullion Carlin system is a CTGD, potentially on the periphery of a larger system. Gold mineralization at the North Bullion Carlin system is controlled by structures, which include the North Bullion Fault Zone and the Massif Fault, and stratigraphic controls, which include the lithologic contacts between the Devonian Devils Gate Collapse Breccia, Mississippian Webb Formation, and Mississippian Chainman Formation. The uppermost horizon of gold mineralization is located at a lithological contact between limestone and mudstone within the Webb Formation, and grade decreases away from this contact in the mudstone. The lowermost horizon of gold mineralization is located within the Devonian Devil’s Gate collapse breccia and the multi-lithic tectonic breccia along the North Bullion Fault Zone and Massif fault. Gold mineralization does not extend above the uppermost dacite dikes and sills within the Mississippian Chainman Formation, suggesting the dacite dikes and sills provided a local seal to the deposit.Cross-cutting relationships, textures, and geochemistry indicated the presence of four generations of pyrite. Pre-ore stage (POS) pyrites formed during diagenesis of the Devonian Devil’s Gate Formation, Mississippian Webb Formation, and Mississippian Chainman Formation. Pre-ore stage pyrites contain Fe, S, Co, Ni, Pb, Ag, and Sn, with variable Bi, Ti, and Zn. Ore-stage 1 (OS 1) pyrites are Au-bearing hydrothermal pyrites within the 38.2 Ma Tertiary porphyritic dacite dikes. Electron probe microanalyzer and LA-ICP-MS quantified Fe, S, Co, and Pb, with variable Bi and Ti, and Au in OS 1 pyrites, but did not detect significant As, Tl, Hb, or Sb. Ore-stage 2 (OS 2) pyrites are more typical Carlin-type Au-bearing pyrites that form partial sub-micrometer rims on POS and OS 1 pyrites and OS 2 microcrystals. Based on electron probe microanalyzer and LA-ICP-MS analyses, OS 2 pyrites contain Au and variable As, Cu, Hg, Sb, and Tl. Late-ore-stage (LOS) pyrites cross-cut sedimentary host rock minerals and late-ore-stage drusy quartz. Late-ore-stage pyrites contain Fe, S, Pb, and Ag, with variable Bi, Sn, and Zn, and locally contain Au, As and Sb.Ore-stage 1 pyrites within Tertiary porphyritic dacite dikes are associated with quartz, illite, and kaolinite. Ore-stage 2 pyrites that rim POS pyrites in sedimentary host rocks are associated with jasperoid, illite, and carbon; OS 2 pyrites that rim OS 1 pyrites in dacite dikes are associated with quartz, illite, and kaolinite.Late-ore-stage minerals within sedimentary host rocks include secondary quartz, late-ore-stage pyrite, realgar, and calcite. Post-ore-stage minerals within sedimentary host rocks include barite, stephanite, kaolinite, halloysite, and oxide minerals. High-grade silver intervals at North Bullion Carlin system (≤ ~ 20 oz/t) result from the presence of the mineral stephanite (Ag5SbS4) that precipitated after Carlin-type gold mineralization, is unrelated to Carlin-type mineralization, and is likely related to one of the post-dacite igneous intrusive events.A compilation of statistical analyses including classification support vector machine, non-metric multi-dimensional scaling model, Spearman rank correlation matrix, and Mann-Whitney U-Test on whole rock geochemistry on 5’drill core samples reveal the key elements related to gold and deposit scale trends. These statistical analyses and models illustrate that in samples with a low gold grade (0.1 ppm Au) the elements that best correlate with Au, with decreasing correlation, are Hg, Sb, Tl, and/or As. In samples with a moderate gold grade (1.0 ppm Au) the elements that best correlate with Au, with decreasing correlation, are Tl, Hg, and As. Gold correlates positively with Fe, S, Pb, Co, Ni, Cu, Al, and K identifying these elements as good indicators predicting the presence of gold. Gold correlates negatively with elements in carbonate host rocks (Ba, Ca, Mg, Mn, and Sr), and these elements indicate of the absence of gold. Silver, Mo, and Zn do not correlate with Au.At the deposit scale the North Bullion Carlin system exhibits characteristics of ore, alteration, and mineral paragenesis that are typical of Carlin-type gold deposits, but the mineralization is subtle and not as well developed as in large, high-grade deposits. At the micro-scale the ore pyrites, and whole rock geochemistry are similar to typical Carlin-type gold deposits. The Carlin-type pathfinder trace elements, As, Hg, Sb, and Tl, best correlate with gold in ore-stage mineralization. Statistical analyses of whole rock geochemistry indicate that Ba, Mn, Al, P, Cr, and V are the elements that provide the best vector to ore and could identify the periphery of gold mineralization.