Abstract

The Salton Sea geothermal system is an area of active metamorphism in a Plio-Pleistocene deltaic sedimentary sequence. Within it, hypersaline brines containing up to 250,000 ppm total dissolved solids and temperatures up to 365 degrees C are causing incipient sulfide and oxide mineralization. We have made a detailed analysis of ore mineralization in two boreholes drilled to depths of up to 2,500 m in this system. Major ore minerals observed are, in order of decreasing abundance, pyrite, hematite, sphalerite, chalcopyrite, pyrrhotite, marcasite, and galena. The associated nonore minerals are mainly calcite, quartz, epidote, anhydrite, adularia, and chlorite. Ore mineralization can be divided into three main types. Diagenetic sulfide mineralization, occurring at depths less than 760 m and temperatures less than 250 degrees C, consists of fine-grained stratiform iron sulfides occurring as cement in sandstone and as disseminations and bands in shale. Metamorphic sulfide mineralization, occurring at depths greater than 760 m and temperatures greater than 250 degrees C, includes the progressive development of porphyroblastic pyrite with increasing depth and temperature, its replacement by other Fe-Cu-Zn-Pb sulfides at depths greater than 1,220 m, and its decomposition into skeletal aggregates at depths greater than 1,525 m, concurrent with hornfelsic recrystallization of the host sediments. Vein and related pore-filling ore mineralization is of two contrasting types: hematite-silicate-sulfide + or - sulfate and sulfide-carbonate + or - silicate. The first type occurs in open porous veins in single vertical zones at least 220 m thick in each borehole. The second type occurs in veins that are generally sealed and occur in relatively thin, scattered vertical intervals in both boreholes. Thermodynamic analysis indicates that the modern reservoir brines are near equilibrium with the hematite-silicate-sulfide + or - sulfate vein assemblage at 300 degrees C, with pH = 5.4 and log a (sub O 2 ) = -30. Reactions between hematite, pyrite, and iron-bearing silicates (chlorite, epidote) probably control the redox state of the present brines. Calculated metal-chloride complex solubilities for Fe, Zn, Pb, and Cu agree well with brine analyses. Hematite, pyrite, and chalcopyrite are supersaturated; galena and sphalerite are undersaturated in the modern brines.The phase relations of the presently sealed sulfide-carbonate + or - silicate veins require that earlier fluids, if formed at the same temperature and pH, had to be more reduced and sulfur rich than the present slightly oxidized, sulfur-poor brines. Contrasting fluid redox states may be caused by a late lateral or fault-controlled influx of more oxidized surficial waters. Lack of acid alteration implies boiling has not occurred to produce the oxidized fluids. Early formed diagenetic iron sulfides are commonly replaced by later Cu-Pb-Zn sulfides and iron oxides and enveloped and resorbed by veins, acting as nuclei and sources of S and Fe for later mineralization. Precipitation probably occurs when seismic or hydraulic fracturing allows the metal-rich, sulfur-poor brines to interact with earlier pyrite sulfur in the host rocks. Thus the geothermal system as presently explored is best classified as an incipient stratabound sulfide deposit that is being overprinted by hydrothermal and metamorphic processes.

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