Structural setting and fluid characteristics of metamorphic gold-quartz veins in northwest Nevada

Metamorphic gold-quartz veins in northwestern Nevada formed during both brittle and brittle-ductile shear deformations during a regional tectonic change from NW-SE to NNE-SSW shortening. Age data (both 40Ar/ 39Ar and K-Ar) and crosscutting relations indicate that age of mineralization was Late Cretaceous. The veins are concentrated in a 60- to 100-km-wide and 170-km-long, linear, north-trending corridor, and include three separate tectonostratigraphic terranes. The regional-scale sigmoidal pattern of the vein arrays strongly suggests that this broad zone acted as a regional-scale shear zone with dextral shear movement. The metamorphic grade in host rocks ranges from sub-greenschist to amphibolite facies, but the majority of the host rocks have been metamorphosed in greenschist metamorphic facies.; Most common structural styles of metamorphic gold-quartz veins are classified as: (1) bedding- and/or foliation parallel veins, (2) shear-related and ladder veins, (3) fault-related veins, (4) fold-related and saddle reef veins, and (5) bedding-perpendicular veins. The geometry of the veins is related to host rock lithology, competence contrast, and intensity of deformation. Structural styles and textures suggest that the veins formed at a deep crustal level through syn- or post-metamorphic processes.; Fluid inclusions in vein quartz samples characteristically contain H ₂O-CO₂-NaCl fluids with low salinity (3 to 5 wt. % NaCl eq.), consistent density (0.5–0.85g/cm3) and mole percent (11–23 mole %) of CO₂ with other contaminants (CH₄ and/or N ₂). The T_h in these inclusions ranges from 200–300°C. Stable isotope data on muscovite-quartz pairs indicate oxygen isotope equilibrium temperatures between 290 and 450°C, which is consistent with pressure corrected temperatures of formation of the fluid inclusions. The δ 18O, δD, δ13C, and δ34S data on vein materials suggest that metamorphic dehydration reactions are the most likely source of the mineralizing fluid. The metamorphic goldquartz veins are interpreted to have formed at temperatures of approximately 300–450%C, and at fluid pressures between 1.2 and 2.8 kbar, which are consistent with temperature- and depth-estimates of the brittle-ductile transition and greenschist facies metamorphism. Characteristic structural styles, textures, distribution, and geochemistry of metamorphic gold-quartz veins and these data indicate a north-trending, regional-scale, deep crustal level, thermo-tectonic event with widespread homogeneous fluid flow in northwestern Nevada during the Late Cretaceous.