Late Cretaceous Orogenic Au-Ag Vein Deposits of Western and North-Central Nevada: Consequences of Sevier Flat-Slab Subduction, and Links with Other Cordilleran Orogenic Deposits

摘要

Late Cretaceous, shear-related, metamorphic (i.e. orogenic) Au-Ag vein deposits occur in a broad, 700-km long, north-south trending belt in western and central Nevada, partly coincident with the Luning-Fencemaker thrust belt and here called the Argentoro belt. The Argentoro includes better-known Au and/or Ag deposits at Spring Valley, Rochester, Pine Grove, Candelaria, and Mineral Ridge. The belt continues northward, becoming mostly buried by Tertiary volcanic rocks near the Oregon border, and may extend south into southeastern California and northern Mexico where similar Late Cretaceous vein deposits occur in metamorphic rocks. Total production and current reserves/resources for known Late Cretaceous vein deposits in this entire region approaches 25 Moz Au and 550 Moz Ag, with nearly 9 Moz Au and 495 Moz Ag attributable to the Nevada deposits alone. Au-Ag deposits of the Argentoro belt are all associated with: 1) mesothermal "bull quartz" veins, 2) variably metamorphosed host rocks of Mesozoic and older age, 3) spatial relation with pre-mineral granitic magmatism, 4) deeply-sourced, dilute, CO_2-rich ore fluids, 5) similar ore mineralogy and ubiquitous quartz-sericite-pyrite-carbonate alteration, and 6) gold placers. The Argentoro belt formed late in Mesozoic deformational history, and veins typically cut ductile shear fabrics and even Late Cretaceous intrusions. Regional-to district-scale controls are high-angle, N-striking faults typically exhibiting strike-slip kinematics, whereas deposit-scale controls include NW-, EW-, and NE-striking dip-slip fracture arrays internal to the N-striking master faults, and less systematic orientations related to faults, folds, etc. of the inherited geologic fabric. The strongest fracture development occurs in and around massive units such as meta-volcanic rocks, intrusions, and quartzite, and veins typically refract and pinch-out where they project in limestone, phyllite, or other less-brittle rocks. In western and central Nevada, orogenic vein deposits are concentrated near and along the Jurassic Luning-Fencemaker fold-and-thrust belt, in weakly to moderately metamorphosed domains characterized by strong deformation. These occur mainly in areas underlain by transitional to oceanic crust, parallel to and west of the inferred edge of ancestral North America. Sparse K-Ar and ~(40)Ar/~(39)Ar ages on vein micas and K-feldspar range from -59 Ma to 114 Ma, although many ages fall between -85 and 70 Ma; therefore, much younger than and unrelated to -200 to 142 Ma Luning-Fence-maker deformation. We suggest most Nevada orogenic veins formed in the Late Cretaceous, shortly after cessation of Sierra Nevada are magmatism. This timing correlates with Sevier-related flattening of the subducting Farallon oceanic plate, with mineralizing fluids possibly derived from metamorphic dehydration reactions that affected the subducting slab. Ages from Nevada reflect transition of late Sevier deformation to a brittle-ductile and, ultimately to a brittle regime as the orogen cooled and may have been partly exhumed in the latest Cretaceous. The close link of orogenic veins to placer deposits raises the intriguing question of whether Nevada orogenic deposits contributed to the rich placers along and above the Mother Lode, California, via a large, westward-flowing fluvial network in the early Tertiary. If their contribution was significant, then the scale of Late Cretaceous precious metal mineralization in Nevada is vastly underestimated.