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Fluid Inclusions and Origin of Carlin-style mineralization at the Cove Deposit, Nevada
AuthorShapley, Sarah Rose
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Carlin-style gold deposits are the leading gold source in the United States. Carlin-style mineralization is known for “invisible gold,” referring to the location of gold as Au+ incorporated into the crystal lattice of arsenian pyrite. The fine-grained nature of ore assemblages as well as lack of associated quartz veins and workable fluid inclusions (FIs) make it difficult to determine the conditions of mineralization, including the temperature, pressure, salinity, chemistry, and therefore the origin of the ore-forming hydrothermal fluids. Although there is an emerging consensus for magmatic inputs of fluid and gold to Carlin-style deposits, there is still significant research that could be undertaken. The Cove deposit, located in the McCoy mining district of north-central Nevada, grades from deep base metal vein type mineralization (BMVT) to Carlin-style gold mineralization. Both types have been previously dated to the Eocene, forming 38.0 ± 1.2 and 32.2 ± .1 Ma based on a zircon date from a weakly altered dike and an 40Ar/39Ar date from an unaltered overlying tuff. Previous research at Cove provides the geologic framework, including identifying magmatic signatures for sulfur isotopes of BMVT mineralization. The close spatial and temporal association of BMVT and Carlin-style mineralization at Cove affords a unique opportunity to investigate the ore-forming fluids and the origin of Carlin-style deposits through the analysis of FIs. Two dominant quartz generations each hosting a primary FI type were identified in this study. Euhedral quartz forming early in BMVT formation at Cove (Q1) contains lobate two-phase liquid rich and clathrate bearing FIs (Lw-c). Lw-c FIs have 20-40 vol.% vapor bubbles and homogenization temperatures of 273-371°C (avg. 335°C) with a standard deviation of 25°C. The salinities are low, with an average of ≈3.5 wt.% NaCl eq. (range 2.24-4.80 wt.%) and a standard deviation of only 0.48 wt.%. Gas contents consist of 7.81 average mol% of CO2 and 0.91 average mol% of CH4. A later generation of subhedral quartz (Q2) hosts liquid rich lobate to irregular two-phase aqueous FIs (Lw). Lw inclusions have comparable average salinities to Lw-c FIs of 3.4 wt.% eq. NaCl but a wider range (1.4-9.3 wt.%) leading to a larger standard deviation of 1.2 wt.%. Temperature of homogenization are 150-350°C (avg. 242.7°C) with a larger standard deviation than Lw-c inclusions of 42.2°C. Gas contents were frequently below detection using Raman spectrometry, but where measured average 9.4 mol% CO2 and 0.79 mol% CH4. Although the FIs are higher temperature that those seen in Carlin-style mineralization, they contain similar salinities and gas contents. Element concentrations from LA-ICP-MS analysis of FIs show enrichments in Carlin suite elements: Au, Ag, Sb, As, along with minor Tl and Te. These results suggest that higher temperature fluids forming BMVT mineralization, when cooled, have the potential to form distal Carlin-style mineralization. Late secondary hypersaline brines and coeval vapor assemblages postdating Lw-c and Lw inclusions indicate phase separation occurred at Cove and signify a change in conditions at Cove. The brines carry similar element enrichments as low salinity FIs at Cove, with slight gold enrichments in the vapor phase. These similarities to Carlin-style mineralization place Cove at an intersection between magmatic and Carlin-style systems. Current research (1) uses FIs to examine the ore fluid composition for the BMVT veins to determine if it is a possible source of the hydrothermal fluid that resulted in deposition of Carlin-style gold mineralization, (2) identifies ?18O zonation in carbonates correlating with mineralization as a result of meteoric infiltration at Cove, and (3) identifies variations between the ?34S isotope values associated with base metal veins and the ?34S isotope values associated with Carlin-style mineralization at Cove.