Hot (up to 365?C), metalliferous brines produced in the Salton Sea geothermal system are rich in Fe, Mn, Zn, Cu and other metals. Scales deposited from these brines form in the upper portions of production wells and in surface plumbing, and are dominated by an amorphous iron silicate phase similar to hisingerite [Fe(OH)3* SiO2], along with minor chalcopyrite and other sulfides. Scales deposited on downhole steel liners near the flashpoint consist largely of magnetite and loellingite (FeAs2) and may contain up to 40 wt % As, 2 wt % Bi, 1 wt % U, 0.1 wt % Au, as well as high concentrations of other heavy metals.

The composition of the scales varies with the brine chemistry and pH conditions at different locations within the geothermal field. Brines produced from the southwest portion of the field contain about 23 wt % TDS, and the production scales are dominated by a poorly crystalline smectitic clay. Based on the translucent orange-brown to olive-green color, desiccation cracks, botryoidal to fibrous texture, electron microprobe and X-ray diffraction analyses, the iron-rich smectite mineral is nontronite that likely crystallized from a gel-like state. Opaque bands in the scale are composed of minor amounts of magnetite. Light-colored crusts consist of banded Ca-Fe-Mn carbonates with patches of sphalerite.

Production fluids from the north part of the field contain about 30 wt % TDS and have deposited scales that are composed of alternating bands of green clay (nontronite) and black metallic arsenides (loellingite), along with trace amounts of halite, barite, fluorite, and calcite. Whole-rock chemistry of one of the clay-rich scales indicates Fe2O3 at 31 wt %, MgO at 6 wt %, Na2O at 2 wt %, SiO2 at 38 wt %, and LOI (loss on ignition) of 13 wt % (for a total of 95 wt %). Trace elemental analysis of this scale indicates between 40 and 100 ppm Ba, Sr, Rb, and V; between 30 and 85 ppm Co, Cu, and Ni; 23 ppm Cs; 53 ppm Ga; and greater amounts of Mn and Zn, at 2410 ppm and 320 ppm, respectively.

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