Jean S. Cline, University of Nevada, Las Vegas, Department of Geoscience, 4505 Maryland Parkway, Box 454010, Las Vegas, Nevada, 89154-4010 (firstname.lastname@example.org)
Albert H. Hofstra, Robert O. Rye, and Gary P. Landis, U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, Colorado, 80225
The gold deposits at Getchell are localized within an anastomosing fault zone that cuts mildly metamorphosed Paleozoic sedimentary rocks and the margin of a Cretaceous granodiorite stock. The gold deposits are thought to be much younger than the intrusive. Gold occurs as sub-micron sized particles in arsenic-rich zones of pyrite and marcasite. The gold-bearing iron-sulfides are disseminated in altered host rocks and occur as crusts in veins. Euhedral, open-space-filling minerals exhibit four stages as follows. Most of the pre-ore quartz-pyrite veins are related to the Cretaceous intrusion. Main-ore stage minerals include near-synchronous orpiment-quartz-fluorite-pyrite-marcasite-gold. This assemblage was followed by late-ore stage realgar and calcite. Euhedral realgar crystals are generally rimmed or enclosed by coarse, anhedral, vug-filling calcite. Minor gold mineralization or remobilization of gold accompanies late-ore realgar and calcite. Post-ore calcite forms coarse, clear, euhedral crystals.
Fluid inclusion gas compositions (Table 1) and homogenization temperatures were determined by QMS and microthermometry. Pre-ore quartz contains a variety of fluids present in one-phase vapor, two-phase liquid-rich, and three-phase, halite-bearing inclusions. Trapped fluids are dominated by H2O, CO2, or CH4, with lesser N2 and H2S. Three-phase inclusions contain as much as 33 wt.% equivalent NaCl and homogenize at 190-240oC. Ore-stage fluids trapped by quartz, fluorite, and orpiment are generally H2O-dominant, moderately saline, and contain significant CO2 and H2S. Primary, two-phase inclusions delineating growth zones in fluorite contain ~4 wt.% NaCl equivalent and homogenize at 150-160oC. Large, secondary, three-phase inclusions in fluorite contain abundant CO2 vapor, lesser CO2 liquid and H2O, and have low salinities. Gas analyses show that minor N2, trace hydrocarbons (HC), and trace H2S are present. Final homogenization temperatures range from ~ 210 to 260oC. Inclusions in ore-stage quartz are generally small, two-phase, and liquid-rich; rare vapor-rich inclusions are present. Salinities of both primary and secondary, liquid-rich inclusions range from 5.5 to 6.0 wt.% NaCl equivalent; homogenization temperatures generally vary from 180 to 220oC. The formation of clathrate indicates that CO2 is present in some liquid-rich inclusions. Recognizable CO2 melting in rare vapor-rich inclusions shows that CO2 is the dominant component. Gas analyses confirm the presence of CO2 in ore-stage quartz; additionally, non-detectable to minor N2 and HC, and ubiquitous, trace H2S are present. Late-ore realgar contains predominantly secondary, single-phase inclusions. Gas analyses show inclusions to be H2O-dominant with minor CO2, non-detectable to minor HC, and trace H2S and N2. Inclusions in late-ore calcite generally exhibit inconsistent liquid/vapor ratios or appear empty, indicating inclusion necking. Inclusions are H2O-dominant with minor to moderate CO2, minor HC, and non-detectable to minor N2 and H2S. Post-ore calcite inclusions are H2O-dominant with minor CO2.
Table 1. Inclusion compositions from QMS analyses (mole percent).
To elucidate the source and exchange history of water in the hydrothermal fluids, the dDH2O and d18OH2O composition of the fluids were determined for each stage of mineralization (Figure 1). The dDH2O and d18OH2O values of fluids from pre-ore quartz lie between the magmatic and metamorphic fields and the meteoric water line (MWL). Fluids from ore stage minerals extend from high dDH2O and d18OH2O values in the vicinity of the magmatic and metamorphic water fields to low values that approach the MWL. Late-ore and post-ore stage fluids generally have lower isotope values than main-ore stage fluids. Inclusion fluids in main-ore stage quartz, fluorite, and orpiment have the highest dDH2O (-50 to -97o/oo) and d18OH2O (+12 to 0o/oo) values. Inclusion fluids in late-ore realgar have low dDH2O (-133 to -151o/oo) and d18OH2O (-13.6 to -17.0o/oo) values that plot near the MWL. Inclusion fluids in late-ore-stage calcite exhibit a wide range of dDH2O (-74 to -125o/oo) and d18OH2O (14 to 2o/oo) values.
The high dDH2O and d18OH2O values of main-ore stage fluids provide clear evidence for a deep-sourced ore fluid at Getchell that is either metamorphic or magmatic, or both. The wide range of isotope values can be explained by mixing between the deep-sourced fluid and variably exchanged meteoric water. The deep-sourced fluid was moderately saline and contained appreciable CO2 and H2S. CO2 concentrations diminish in late-ore and post-ore stage fluids, paralleling the trend toward lower isotope values. dDH2O and d18OH2O values of realgar fluids are distinctly less than those from other minerals. As the inclusions in realgar are largely secondary in origin and contain only a single phase, the low values may represent cool meteoric water that collapsed into the system subsequent to ore formation.
The results suggest that fluid mixing and reaction with host rocks were important processes during the evolution of the hydrothermal system at Getchell. Deeply sourced metamorphic or magmatic ore fluids moved upward through fractures in the Getchell fault zone. Reactions between the acidic CO2- and H2S-rich ore fluids and the host rocks resulted in carbonate dissolution, volume loss, and increased porosity and permeability. Fluid/rock reaction further caused argillization of silicate minerals and sulfidation of host rock iron, leading to gold deposition. Precipitation of quartz, fluorite, orpiment, realgar, and gold in open spaces likely resulted from cooling and dilution of ore fluids with indigenous meteoric ground water. Cooling also resulted in silicification of carbonates to form jasperoid. Calcite may have precipitated in response to phase separation, pH increase as the fluids were neutralized by reaction with the host rocks, or heating of ground water that collapsed into the system.
Fig. 1. dD and d18O compositions of water in hydrothermal fluids at Getchell.