Prolific geothermal well Magmamax-6B (M-6B), in the southwestern Salton Sea geothermal field, yields initially 290-298oC hypersaline brine from an interval of hydrothermally altered mudstone and sandstone disrupted by veinlets and breccias mineralized with specular hematite and anhydrite. About half the well’s total production is from one such breccia zone that coincides with an abrupt, 8°C reversal in the static-temperature profile. Converging textural, petrologic, chemical, and thermodynamic evidence suggests that: (1) the M-6B breccias were formed in a strike-slip fault zone as a result of hydraulic implosion abetted by explosion of overpressured, high-temperature pore fluids; (2) the hematite and anhydrite cementing these breccias were precipitated from a downflowing brine originally cooler than the 300°C reservoir rock which the fluid eventually invaded; (3) the downflowing brine may have slightly cooled and mixed with a counterflow of hot brine already ascending the fault zone; (4)
the M-6B temperature reversal may be a further consequence of the above process; (5) the reversal nonetheless is only a minor local perturbation in a system that is still thermally prograde; (6) the observed brecciation and mineralization as well as the inferred brine downflow are geologically recent phenomena; (7) sandstones in the reservoir interval are productive only where comparatively shallow and just incipiently altered; and (8) a late-stage assemblage of anhydrite and hematite in this geothermal system may signal favorably rejuvenated porosity and permeability.