Summary of Structural Setting and Fluid Characteristics of Low-Sulfide Au-Quartz Veins in Northwestern Nevada

Low-sulfide AuÐquartz veins in northwestern Nevada formed
during both brittle and brittle-ductile shear deformation and were
accompanied by sparse base-metal minerals. The veins are concentrated
in 12 mining districts in a 60Ð to 100Ðkm-wide, 200Ðkm-long,
linear, north-trending corridor extending from the Humboldt Range
in the south to the Pueblo Mountains in the north. The veins show a
consistent northeast strike in the central parts of the corridor,
whichÑon the basis of several kinematic indicatorsÑstrongly suggests
that this broad zone acted as a regional-scale shear zone. The
metamorphic grade of the host rocks ranges from sub-greenschist to
amphibolite facies, but most host rocks have been metamorphosed
to greenschist facies. Dating, by both 40Ar/39Ar and KÐAr methods,
and cross cutting relations indicate that age of veining and mineralization
is Late Cretaceous.
Most low-sulfide AuÐquartz veins in northwestern Nevada may
be classified into four main types of vein systems: (1) beddingand(
or) foliation-parallel veins; (2) shear-related and ladder veins;
(3) fault-related veins; and (4) fold-related and saddle reef veins.
Bedding- or foliation-parallel and fault-related veins are present in 1
to 3 sets of planar veins, while shear-related and ladder veins are
present in en echelon patterns characteristic of shear zones. Foldrelated
and saddle reef veins are restricted either to fold hinge zones
or along the fold limbs. The geometry of all these quartz veins is
related to host rock lithology, competency contrast, and degree of
deformation. The vein systems may have developed in conjunction
with shear or extensional fractures that accompanied emplacement
of veins in either brittle-ductile or ductile deformation regimes.
Quartz textures in most low-sulfide AuÐquartz veins are syntectonic
vein growth and cavity-filled bull quartz with ribbon, stylolitic,
fibrous, and laminated textures. Calcite also is intergrown
with quartz in some veins. The veins are composed predominantly
of irregular, anhedral, and recrystallized quartz crystals with local
pyrite, galena, sphalerite, and other sulfide minerals. Alteration selvages
chiefly are composed of white mica. In addition to Au, local
areas have anomalous concentrations of Ag and base metals.
Structural styles and textures strongly suggest that the veins formed
at a deep crustal level during regional deformation accompanied by
fluid flow during syn- or post-metamorphic processes.
Fluid-inclusions in vein quartz contain H2OÐCO2ÐNaCl fluids
of relatively low salinity (<10.0 wt. % NaCl equiv., median 3 to 5 wt. % NaCl equiv.) with no daughter minerals. The range of homogenization temperatures (Th) in these inclusions is from 200 to 300ûC. Stable isotope data of muscovite-quartz pairs indicate oxygen isotope equilibrium temperatures between 280 and 470ûC. The d18O, dD, and d13C data from vein materials suggest that metamorphic dehydration reactions are the most likely source of the mineralizing fluid. These low-sulfide AuÐquartz veins are interpreted, on the basis of these data, to have formed at temperatures of approximately 300 to 450ûC and at fluid pressures between 1.2 and 2.8 kb, which are consistent with temperature- and depth-estimates of the brittleductile transition and greenschist facies metamorphism in this part of Nevada. Regional distribution of low-sulfide AuÐquartz veins and these data indicate that a regional-scale, deep crustal, thermo-tectonic event with widespread fluid flow occurred during the late Mesozoic throughout much of northwestern Nevada. The fluid that formed these veins was of metamorphic origin.