HYDROTHERMAL ALTERATION OF BASIN SEDIMENTS IN AN EXTENSIONAL BASIN AND RANGE GEOTHERMAL SYSTEM: IMPLICATIONS FOR HYDROCHEMICAL PROCESSES AND FLUID SOURCE

Humboldt House geothermal area (HHGA), located in Pershing County Nevada, is an extensional Basin and Range geothermal system formed by deep-sourced hydrothermal fluids upwelling along the range bounding fault zone that laterally disperse into adjacent basin sediments. A comprehensive mineralogical and geochemical analysis has been conducted on cores from three geothermal wells in the basin to evaluate the structure and hydrochemical processes that occurred at shallow depths (<1000 feet) during the evolution of the geothermal system.

The recovered core rocks are fluvial/pluvial sediments that have undergone varying degrees of hydrothermal alteration depending on the distance from the fault zone. The mineralogical and structural evidence in the two cores nearest the fault zone indicate that reducing, acidic hydrothermal fluids spread laterally through the permeable alluvial sediments and mixed with cooler meteoric groundwater to form a boiling water table. In both cores most of the primary minerals in the sediments have undergone extensive alteration and replacement by secondary mineral phases, primarily silica, chlorite, smectite and pyrite. These two cores contain a ~350 foot section of silicic alteration capped by section of acid-sulfate alteration which appears laterally continuous between the cores. The core nearest to the fault zone exhibits evidence of boiling, with adularia replacing K-feldspars and quartz replacing bladed calcite. Pervasive authigenic barite at the top of the silicified sections suggests mixing with cooler groundwater. The chemical analysis of the silicified sections supports the hypothesis that the original source of the hydrothermal fluids was meteoric with little, if any, contribution from magmatic fluids.