Concentrated saline brine tapped by a deep well drilled for geothermal power near the Salton Sea, California, deposited metal-rich siliceous scale at the rate of 2 to 3 tons per month. The iron-rich opaline scale contains an average of 20% Cu and up to 6% Ag present in bornite, digenite, chalcopyrite, a new dense polymorph of chalcocite, stromeyerite, and native silver.The brines are in equilibrium with an assemblage of sulfide minerals in the reservoir rocks. Sphalerite containing 16.6 mol % FeS is in equilibrium with the brine in which the activity of sulfur (a (sub s 2 ) ) is estimated at 10 (super -10.2) at 325 degrees C. This is in good agreement with the projected high-temperature data of Barton and Toulmin (2) for the system Fe-Zn-S. Other sulfides identified in the reservoir rocks are pyrite, pyrrhotite, chalcopyrite, and galena.The heavy metals in solution are apparently derived from the sediments of the brine reservoir, being released from the silicate minerals in which they occur in trace amounts, as metamorphism of the sediment proceeds. Although the total of the heavy metals in solution greatly exceeds the sulfur, on a molal basis, the brine is saturated with respect to sulfide components and addition of more sulfide ion would only cause precipitation of sulfides.
