Distinguishing generations of quartz and a distinct gas signature of deep high-grade Carlin-type gold mineralization using quadrupole mass spectrometry

Abstract

Carlin-type gold deposits in north-central Nevada occur in a region that was the locus of igneous and/or hydrothermal activity over time and the overprinting of silicification associated with metamorphism, base metal mineralization, barren jasperoid, and post-ore alteration hinders exploration efforts. Therefore, the potential to distinguish unrelated generations of quartz using the results of quadrupole mass spectrometer (QMS) gas analyses was evaluated for the Getchell trend, and compared to results from the Goldstrike property in the Carlin trend and a Carlin-type gold occurrence in the Huijiabao trend, China.The results of fluid inclusion petrography, QMS analyses, and interpretation of gas data using N2–Ar–He ternary diagrams and CO2/CH4 vs. N2/Ar plots indicate that different generations of quartz in the Getchell trend were deposited by hydrothermal fluids having distinct characteristics. Metamorphic quartz containing one-phase CH4 and halite-bearing liquid–water dominant vapor-poor inclusions has the highest gas content (37.6 mol%) and highest N2/Ar values (3250), which are suggestive of an evolved magmatic fluid. Pre-ore and quartz–base metal veins contain CO2-bearing inclusions. Base metal veins have N2/Ar values ≥675 and a positive correlation between H2S/Ar vs. N2/Ar that is suggestive of a magmatic fluid. Carlin-type quartz contains liquid-dominant vapor-poor inclusions and is characterized by total gas contents >10 mol% and N2/Ar values >300 that are suggestive of mixed crustal–meteoric–magmatic fluid sources. Post-ore quartz and barren jasperoid formed from mixtures of gas-poor meteoric water and crustal fluids.Samples of deep high-grade mineralization from the Goldstrike property have total gas contents up to 10.5 mol% and N2/Ar values >300. Fluid sources include crustal, meteoric, and magmatic. Gas data from the Goldstrike property, Turquoise Ridge deposit, and metamorphic quartz of the Getchell trend plot near the N2 apex of N2–Ar–He diagrams, which is indicative of a magmatic fluid end member.Carlin-type mineralization in the Huijiabao trend is unique because fluids were CH4-rich and there is a correlation between CH4 and hydrocarbons. Gas analyses commonly record <1 mol% total gas for evolved (crustal) and organic fluids. Fluid inclusions in post-ore quartz and barren jasperoid are also gas poor, but CO2 is the dominant gas specie.Together, the results show that fluid inclusion gases can be used to distinguish different generations of quartz in mining districts with complex origins.