CHARACTERIZATION OF PAST HYDROTHERMAL FLUIDS IN THE HUMBOLDT HOUSE GEOTHERMAL AREA, PERSHING COUNTY, NEVADA: GEOCHEMICAL AND PARAGENETIC STUDIES OF CORE SAMPLES

The Humboldt House geothermal area (HHGA) may serve as a model geothermal system for the development of geothermal resources in Nevada. The HHGA is an area 200 km northeast of Reno, west of the Humboldt Range in Pershing Co., NV. HHGA has approximately 21 km² of surface geothermal features that include siliceous sinter deposits and hot shallow wells. Regional geology is dominated by the north-south trending basin and range physiographic province. High-angle faulting overprinted Mesozoic shear zones during Miocene extension. These faults provided a deeply rooted feeder system that has permitted hydrothermal circulation, producing both the geothermal area and nearby gold deposits.

A total of five wells have been drilled with three obtaining core. Temperature gradient surveys are being conducted in all five wells over a period of several months to determine steady-state reservoir temperatures and profiles. Well P32-2 was cored through surface siliceous sinter deposits to a total depth of 152 m. Initial observations include clay alteration, vein filling minerals, and silicification of lacustrine sediments. Minor pyrite was noted throughout several reduced siliciclastic zones, and major pyrite was noted along fractures and within vugs. Well P1-10 was cored just southeast of Florida Canyon Mine to a total depth of 457 m. Lithologies include two primary types: clast- or mud-supported conglomerate and silicified conglomerate. Initial observations show pervasive veining, vuggy texture and major pyrite along fractures. Also noted were veins exhibiting concentric layers of chalcedony, and internal voids partially filled with euhedral quartz crystals documenting presence of fluids that periodically formed deposits along the walls of fractures. These textures may provide evidence for several hydrothermal fluid events.

Analytical methods include: standard petrographic analysis and SEM to determine mineral phase relationships; ICP-MS for whole rock geochemistry to determine major, minor and trace elements; petrographic study of fluid inclusions, followed by laser ablation - ICP-MS; and XRD for clay identification. The results of our analyses will include determination of paragenesis and identification and geochemical characterization of major fluid events.