Effects of Fault Zone Permeability on Fault Controlled Fluid Circulation in Active and Fossil Hydrothermal Systems in the Great Basin

Faults play a critical role in fluid circulation along modern hot springs and in the formation of Eocene Carlin-type gold deposits in the Great Basin. Here we present a suite of numerical models that asses the conduit/barrier behavior of the faults at Beowawe and Carlin trend, Nevada. In order to match the published fluid homogenization temperature, fluid-rock isotope exchange and silica precipitation data along the Carlin trend and Beowawe geothermal system, a vertical fault permeability between 10^-11 to 10^-13 m², a fault zone anisotropy of 100 (Kz/Kx), and a permeability contrast on the order of 10³ to 10⁴ between lithostratigraphic units and faults are needed. Carlin trend the fault segments that cut the low permeability (10^-17 m²) upper plate rocks were found to have a lower vertical permeability and resulted in the formation of a loop style fluid circulation through the underlying carbonate rocks which host the gold mineralization. This resulted in significant fluid-rock isotopic exchange. On the other hand, the absence of low permeability cap rocks at Beowawe produces a single-pass hydrothermal flow cell with relatively little fluid-rock isotope exchange. Field observation of fault gauge and damage zone formations along the ore feeder faults at the Post-Betze and Gold Quarry mines supported the numerical model parameter selection.

Key words: Great basin, fault, fluid circulation, permeability, anisotropy