GOLD-BEARING ARSENIDE, SILVER ANTIMONIDE, AND CLAY MINERALS IN PRODUCTION-WELL AND POWER-PLANT SCALES FROM THE SALTON SEA GEOTHERMAL FIELD, CALIFORNIA

The Salton Sea geothermal system of southern California is an active, high-temperature hydrothermal system located in a continental rift setting. Hot (up to 365°C), hypersaline (23-30 wt % TDS) brines produced at the Salton Sea field are rich in the valuable metals Zn, Mn, Ag, and Li, and also contain significant Fe, As, Cu, and Pb. In association with the large throughput of geothermal fluids and the production of 330 MW of electricity, a variety of metalliferous scale deposits have formed in production wells, steam-gathering facilities, and geothermal power plants. Titanium liners in the upper portions of production wells accumulate mostly amorphous iron silicate (hisingerite) and poorly crystalline, iron-rich nontronite, along with minor magnetite, sphalerite, and chalcopyrite in banded scale deposits. Scales deposited at greater depths near flashpoints (points of boiling) in steel-lined wells consist of Bi-, Cu-, and locally Au-rich loellingite (FeAs2). One such scale sample is almost all coarsely crystalline loellingite with a crudely bladed habit. Although from a deposit at most a few kg in weight, the arsenide in this scale boasts bonanza gold grades of up to 233 grams (8.2 oz) per ton.

The solid phases precipitated in the low-temperature environment of the surface facilities and power plants differ from the scale material deposited in the high-temperature well bores. Residual solids from spent power-plant brines are composed predominately of amorphous silica (opal-A and hisingerite), minor amounts of chlorite and trioctahedral saponite, and trace amounts of Cu- and Fe-arsenides (metadomeykite, westerveldite), Ag-Sb alloys (allargentum, dyscrasite), barite, and fluorite. The low-temperature environment appears to favor Ag in antimony alloys, As in Cu-arsenide, deposition of ferrous Fe in amorphous iron silicate, and divalent Fe and Mg in saponite and chlorite. In contrast, the high-temperature borehole environment favors deposition of Au, Ag and As in Fe-arsenides, and ferric Fe in nontronitic clay.