GREAT BASIN METALLOGENY: INTERPLAY OF TECTONIC EVENTS, BASEMENT FAULTS, PERMEABILITY, DIVERSE FLUIDS, ROCK BUFFERS, & PHYSICOCHEMICAL TRAPS

The Archean to Present history of the Great Basin is punctuated by tectonic events that caused fluids of different origins (sea water, basinal brine, meteoric water, metamorphic water, magmatic water) to move through the crust. Consequent reactivation of basement faults localized deformation, sedimentation, magmatism, and hydrothermal activity in overlying rocks to form mineral belts that contain ore deposits of different types and ages (inheritance). Within and outside mineral belts, fluids evolved from their initial chemistry towards compositions that reflect the fO2 and fS2 buffering capacity of, and the ligands and metals present in, the rocks (±older mineralization) through which they moved. In N. Nevada, rock buffering is evidenced by fluids of diverse origins that evolved to H2S-rich compositions via reactions with carbonaceous, pyritic strata so they could transport Au repeatedly over Pz-Cz time (convergent evolution). Ore formed where metal laden fluids encountered effective physicochemical traps.

In well studied regions such as this, GIS+geoinformatic approachs to tectonics and metallogeny are beneficial because relevant geologic, geophysical, geochemical, and hydrologic information can be organized, identified, and displayed, in space and time, to evaluate relationships and inform genetic models or empirical resource potential estimates. Understanding of metallogenic events is enhanced via numerical modeling of fluid flow (in 2- and 3-D) and metal scavenging, transport, and deposition along flow paths of interest. Modeling requires hydrogeologic reconstructions, at regional and district scales, integration of thermal, chemical, and isotopic data to constrain fluid flow paths, and facilitates hypothesis testing. Each approach leads to the identification of critical data gaps that prompt new investigations. The USGS is using these approaches to understand relationships between key tectonic events, hydrothermal fluid flow, and ore genesis in the Great Basin.