The Sal ton Sea Geothermal Brines Michael A. McKibben Department of Earth Sciences University of California Riverside, CA 92521 The sediment-hosted Salton Sea geothermal system (SSGS) contains some of the most saline, metal-rich hydrothermal fluids known from an active system (White et al., 1963; Skinner et al., 1967; Helgeson, 1968). Recent extensive scientific and commercial drilling into this system has provided a wealth of new samples and data relevant to understanding this unique hydrothermal system and its metalliferous brines (Elders and Sass, 1988; McKibben et al., 1988a, 1988b; 1988; McKibben and Eldridge, 1989; McKibben and Williams, 1989; Williams and McKibben, 1989; McKibben et al., 1990). Only a brief overview of the SSGS is presented here. Reservoir Topology The saline brine reservoir of the SSGS appears to have a domal upper surface defined by both the 2600C isotherm and a sharp fluid interface where salinities increase rapidly with depth from <12 % TDS to >15 % TDS (Figure 1). This interface occurs at a depth of only 500 .m in the central part of the field (Figure 2) and appears to be density-stabilized (non-convective) by a balance between salinity and temperature (Williams, 1988; Williams and McKibben, 1989). The underlying reservoir of hypersaline brine has a temperature- salinity gradient that permits convection, and the brine's stable isotopic homogeneity supports this possibility. The lower limits to the brine reservoir, if any, are not known; saline brine has been encountered at depths greater than 4 km in many locations in the northern Trough (Figure 3). Currently there is no economic incentive for the geothermal companies to drill to produce brine from depths greater than 2-3 km. A model for the brine reservoir and its evolution has been proposed (Williams and McKibben, 1989). In this model, rift basinal brines that have accumulated in the closed Salton Sea basin have been magmatically heated above spreading center fragments to generate an internally-convecting brine diapir that has advected to within 0.5 km of the Trough surface. According to this model, the geothermal companies are currently drilling into the top of this brine diapir. Brine Chemistry The hot (up to 3650C), hypersaline Na-Ca-K-Cl brines contain up to 26 % TDS and are rich in Fe, Mn, Zn, Pb and other metals (Table 1). On a molal rather than weight basis, the dominant constituents in these brines areNa, Ca, K, Cl, B and NH 3 • Interestingly, there are 127 Downloaded from http://pubs.geoscienceworld.org/segweb/books/book/1829/chapter-pdf/3928088/9781934969656_ch14.pdf by University of California-Riverside user on 19 August 2022