Fluid evolution during Cretaceous and Eocene igneous–Hydrothermal events in the Getchell trend, Nevada

Abstract

The characteristics and evolution of fluids during the 115–75 Ma and 42–38 Ma igneous–hydrothermal events in the Getchell trend, Nevada were studied using fluid inclusion microthermometry, quadrupole mass spectrometer (QMS) gas analyses, and O–H isotopes. Cretaceous fluids produced by metamorphic devolatilization of country rocks are represented by an assemblage of one-phase CH4, one-phase CO2, halite-bearing, vapor-rich (>50% vapor), and liquid-rich (<20% vapor) inclusions in gray–transparent quartz veins. When QMS gas data are plotted in a CO2/CH4 vs. N2/Ar diagram and paired with бD values of –91.2‰ to –116.9‰, organic and evolved magmatic fluid sources are indicated. Metamorphism and mineralization that occurred prior to 95 Ma is characterized by pyrrhotite, compared with pyrite for mineralization ≤92 Ma.Cretaceous igneous–hydrothermal fluids (92–75 Ma) were CO2 dominant (≤12 mol%), CH4 poor (≤0.6 mol%), lower salinity (2.7–9.3 wt% NaCl equiv.), and have a large range in бD values (–92.1‰ to –154‰). Samples of milky quartz host three-phase CO2, one-phase CO2, and liquid-rich inclusions. Early fluids had a magmatic source based on бD values of –75.9‰ to –91.2‰ and QMS gas data that plot in the magmatic field of a CO2/CH4 vs. N2/Ar diagram. Whereas increasing amounts of meteoric water during the formation of barren and arsenopyrite-rich quartz are indicated by QMS gas data that transition into the shallow meteoric field of a CO2/CH4 vs. N2/Ar diagram and бD values of –143.1‰ to –153.8‰. Cretaceous mineralization concluded with the emplacement of ~75 Ma breccia pipes that most likely had a magmatic origin based on elevated salinities (≤15.7 wt% NaCl equiv.), style of mineralization, and a бD value of –94.6‰.The Eocene hydrothermal fluids that produced Carlin-type mineralization in the Twin Creeks deposit underwent a similar evolution. Samples of Main ore-stage jasperoid that host low-salinity (2.6–6.7 wt% NaCl equiv.) liquid-dominant fluid inclusions have бD and б18OH2O values of –108.4‰ to –206.9‰ and 7.8–14.2‰, respectively. These data plot to form a mixing trend between end-member organic and metamorphic waters in a бD vs. б18OH2O diagram. Yet near the end of Main ore-stage mineralization, the hydrothermal system transitioned from N2-rich (≤13.8 mol%) organic and metamorphic fluids to magmatic fluids (бD –93.6‰) with CO2 as the dominant gas specie. Additional evidence to support organic/metamorphic fluids is provided by growth banding in Late ore-stage orpiment defined by zones of carbonaceous material.These results show how the formation of Carlin-type deposits in the Getchell trend had a complex relationship with distal Eocene igneous activity and that genetic models need to account for magmatic, metamorphic, and evolved basinal fluids in the hydrothermal system.