Genetic implications of regional vs. fracture-controlled fluid flow in carlin-type gold deposits, Nevada

Abstract

The ability to identify differences in the chemistry of hydrothermal solutions that formed Carlin-type gold deposits in areas that experienced regional fluid flow in proximity to Eocene igneous rocks (e.g., the Carlin trend) vs. fracture-controlled fluid flow distal to Eocene igneous rocks (e.g., the Getchell trend) was evaluated using fluid inclusion microthermometry, quadrupole mass spectrometer (QMS) gas analyses, and H-isotopes.Primary fluid inclusions were measured in eleven samples of Main-stage quartz; Late-stage quartz, realgar, and barite; and post-ore calcite interlayered with chalcedonic quartz from the Getchell and Twin Creeks mines. Fluid inclusions in Main-stage jasperoid from the NE-trending Turquoise Ridge Fault are cogenetic one-phase CO2 and liquid-rich that homogenize at 178°–237°C, with salinities of 4.3–6.2 wt% NaCl equiv. Inclusions in Late-stage quartz containing disseminated stibnite have Th of 122°–253°C and salinities of 4.2–6.7 wt% NaCl equiv. Barren realgar hosts inclusions with Th of 65°–119°C and salinities of 0–18 wt% NaCl equiv. Finally, inclusions in hydrothermal barite and post-ore calcite have salinities of 5.7–8.1 wt% NaCl equiv. and 0.9–1.1 wt% NaCl equiv., respectively, but Th overlap between 65°–134°C.Gas data include 22 samples of Late-stage realgar-stibnite-orpiment mineralization from the Getchell, Twin Creeks, and Betze mines. Fluid inclusions in samples of auriferous stibnite from the Getchell trend have total-gas contents ≤ 7.7 mol%; plot near the N2 apex (magmatic gas source) in a N2-Ar-He ternary diagram; and trend from the shallow meteoric to magmatic field of a CO2/CH4 vs. N2/Ar diagram. Barren stibnite and realgar contain gas-poor inclusions (0.01–2.2 mol% total gas), with fluid mixing for realgar indicated by the positive correlation of He-CH4 and data trending from the shallow meteoric to evolved crustal field in a CO2/CH4 vs. N2/Ar diagram.Hydrogen isotopes were measured for nine samples of Main- and Late-stage minerals from the Getchell, Twin Creeks, and Betze mines. Main-stage siliceous ore from the Getchell trend have бD of −128‰ to −98‰, realgar samples from all three mines have бD of −152‰ to −142‰, and two orpiment samples from the Getchell mine have бD of −57‰ and −44‰.Distinct differences are evident in the fluid inclusion and H-isotope data for hydrothermal systems that formed Carlin-type deposits distal vs. proximal to Eocene igneous rocks. Magmatic and crustal fluids dominated in the fracture-controlled Turquoise Ridge deposit, Getchell trend and meteoric water did not become prominent until the formation of post-ore chalcedonic quartz–calcite veins. In contrast, regional flow along the Carlin trend incorporated large amounts of meteoric water. By interpreting H-isotopes with respect to fluid inclusion data for the Carlin and Getchell trends, gold did have a magmatic source in both areas. However, meteoric water also leached gold from country rocks during regional flow to contribute to the large endowment of gold in the Carlin trend. Genetic models used in the exploration for Carlin-type deposits must therefore account for mineralization that is fracture-controlled or related to regional fluid flow.