REACTION PATH MODELING OF THE BEOWAWE GEOTHERMAL FIELD, NEVADA: IMPLICATIONS FOR THE FORMATION OF EPITHERMAL AU-AG MINERALIZATION

The Beowawe geothermal field is an active fault-controlled hydrothermal system that is one of the hottest in the Basin & Range province. Despite geothermal gradients that can exceed 60° C/km, the system appears to be amagmatic based on the absence of volcanic rocks younger than 10 Ma, low ³He/⁴He ratios, and the fluids’ meteoric stable isotope signature. Beowawe also contains characteristics of low sulfidation epithermal Au-Ag deposits, including extensive sinter deposits, banded chalcedony veins with adularia, bladed calcite, slotted quartz after calcite, pyrite, and discharged fluid with low chlorinity and near neutral to basic pH. Beowawe is weakly mineralized with Au and Ag to depths of at least 100 m, with maximum concentrations of 560 ppb and 15 ppm, respectively.

Reaction path modeling was used to test the geochemical behavior of a hydrothermal fluid as it ascended ~3 km along a fault from a hypothetical carbonate reservoir to the surface. The results show that a hydrothermal fluid that had equilibrated with carbonate and that reacted successively with shaley siltstone, diabase, quartzite, andesite, and dacite could produce much of the alteration assemblage observed in the field. The sulfur and chloride concentrations of this fluid, as determined from deep wellbore samples, were probably too low to allow high concentrations of Au or Ag to be in solution, which were only 0.15 and 5 ppb respectively at the start of the reaction path and decreased thereafter. Nonetheless, economic levels of Au-Ag mineralization could develop along the flow path due primarily to cooling, provided that enough fluid pulses traversed the fault. However, the large amount of silica predicted to precipitate along the flow path would occlude porosity and thus diminish permeability to the point that flow would cease long before ore grade mineralization was reached, unless porosity and permeability could have been increased through subsequent fracturing events. Unlike more heavily mineralized epithermal systems, Beowawe has not undergone much hydrothermal brecciation. This suggests that the locus of fluid activity has migrated over time to more permeable parts of the fault and dispersing the mineralization, rather than remaining stationary and concentrating mineralization through repeated overpressuring and fracturing events.