HR: 10:55h
AN: T22A-03 INVITED     [Abstracts]
TI: An Ore-grade, Epithermal Silver/Base-Metal Deposit in the Southwestern Salton Sea Geothermal Field, Imperial Valley, California
AU: * Hulen, J B
EM: jhulen@egi.utah.edu
AF: Energy & Geoscience Institute, University of Utah 423 Wakara Way, Suite 300, Salt Lake City, UT 84108 United States
AU: Moore, J N
EM: jmoore@egi.utah.edu
AF: Energy & Geoscience Institute, University of Utah 423 Wakara Way, Suite 300, Salt Lake City, UT 84108 United States
AB: Eight shallow to intermediate-depth geothermal production wells in the giant high-temperature Salton Sea geothermal field (SSGF) penetrate rich, thick intercepts of epithermal vein-hosted and stratabound lead-zinc-silver mineralization. Although certainly too hot (up to 250°C) to mine at present, this substantial metallic sulfide concentration in an active geothermal system boasts [true] thicknesses and grades ranging upward to 30 m @ 2 oz/T silver, 5% zinc, and 2% lead, with lesser credits in copper, cadmium, and cobalt. At an estimated (though unconfirmed) several million metric tons, it is clearly an orebody for the future. Michael McKibben and colleagues at the University of California, Riverside long ago documented high concentrations of base metals in the Salton Sea field's hypersaline production brines, as well as trace to minor amounts of galena, sphalerite and chalcopyrite in the field's fluvial/deltaic siliciclastic reservoir rocks. On this basis, an orebody such as the one reported here was certainly anticipated, but remained elusive until now. The southwestern SSGF silver/base-metal orebody consists principally of thick, intricately banded epithermal veins with quartz, barite, fluorite, adularia, chalcedony, mixed-layer illite/smectite, and traces of hematite as the main gangue minerals. Subdidiary ores occur as dense intergranular disseminations in (atypically for the field) coarse-grained sandstones, cemented by the same gangue minerals, and with all rock-forming feldspars completely altered to adularia. Sphalerite in both type of deposits is amber to ``root-beer'' colored and intricately zoned (up to at least 100 recognizable bands) with evidence of alternating dissolution and precipitation events during crystal evolution. Fluid-inclusion microthermometry indicates that the sphalerite was precipitated from moderate (for the SSGF)-salinity brines (12-17 wt% NaCl equivalent) at temperatures comparable to those currently prevailing in the mineralization depth range (220-250°C). Comparison of the fluid-inclusion data and orebody systematics with overlying moderate-salinity production fluids and underlying hypersaline brines (23 wt.% total dissolved solids) suggests (following McKibben et al.) that fluid mixing and intermittent reheating as well as hydrogen sulfide influx were the main causes of mineralization. There are no perfect analogues for this intriguing silver/base-metal deposit in an actively spreading continental rift, but the orebodies most closely resembling it are Tertiary-age Creede-type, epithermal, moderate-salinity, low-sulfidation deposits and some of the moderate-salinity, detachment-fault-related Tertiary base-metal orebodies in the metamorphic core complexes of northwestern Arizona and southeastern California.
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1043 Fluid and melt inclusion geochemistry
DE: 3617 Alteration and weathering processes (1039)
DE: 3625 Petrography, microstructures, and textures
DE: 8105 Continental margins: divergent (1212, 8124)
SC: Tectonophysics [T]
MN: Fall Meeting 2005