GEOCHEMICAL AND GEOCHRONOLOGIC CONSTRAINTS FOR EVOLUTION OF THE EPITHERMAL AU-AG VEINS OF THE NORTHERN GREAT BASIN: NEW DATA FROM MIDAS, NEVADA

Many epithermal ‘bonanza ores’ of gold and silver are preserved in the Northern Nevada Rift (NNR) and Great Basin of northern Nevada and the adjacent Owyhee Mts of Idaho. Previous models for these deposits have typically invoked magmas as heat sources, which drove large-scale fluid convection in the upper crust, with attendant leaching of precious metals from large volumes of crust and precipitation in vein systems. Recent studies of deposits near the center of the NNR and in the Owyhees have provided two major findings: 1) the isotopic composition of lead alloyed with gold in many of the deposits is relatively primitive and similar to that of the most primitive basalts of the regionally extensive, mid-Miocene Columbia River Basalt (CRB) province, and distinct from the regional upper crustal host lithologies; and, 2) the timing of ore formation (from ⁴⁰Ar/³⁹Ar dating of adularia) is close to the crystallization of the basalts in this province (ca. 16.5 — 15.5 Ma). We infer that the precious metals in these deposits originated from the same mantle source as the primitive CRB lavas. The veins of the Ken Snyder mine of the Gold Circle district are considered to have formed ca. 15.4 Ma (Leavitt et al., 2004) through large-scale crustal convection associated with rhyolite dome emplacement. We find that laser ⁴⁰Ar/³⁹Ar plateau ages of basalt flows and dikes hosting this ore are variable and suggest multiple stages of formation; considering plateau defined by more than 90% of the gas released, we obtained ages for two different basalts of 16.32±0.17 and 15.508±0.066 Ma (2σ precision). We suggest that these basalts are broadly coeval with the early CRB and Steens lavas associated with the inception of the Yellowstone hotspot. The lead isotopic composition of basalts at Midas, lead alloyed with electrum in the ore, and lead of the regional metasedimentary rocks are similar and also relatively radiogenic, which can be interpreted to support a model for large-scale crustal convection. However, the lithosphere at Midas is thick in comparison with deposits to the west that are also underlain by accreted terranes characterized by more primitive lead. We suggest the early basalts and the bonanza-type ores of the region originated from the same mantle source but experienced increasing lithospheric contamination with increasing distance to the east.