Epidote‐Bearing Veins in the State 2–14 Drill Hole: Implications for Hydrothermal Fluid Composition

Abstract

Epidote‐bearing veins in State 2–14 drill core from 900 to 2960 m depth were examined using backscattered electron microscopy and electron probe microanalysis to characterize the mineralogy, parageneses, texture, and composition of vein minerals. In order of decreasing abundance, minerals in epidote‐bearing veins are pyrite, calcite, K‐feldspar, quartz, anhydrite, hematite, chlorite, Fe‐Cu‐Zn sulfides, actinolite, titanite, and allanite. The downhole distribution of minerals in epidote‐bearing veins (+ pyrite and quartz) varies as a function of depth and includes: (1) calcite above ∼2000 m, (2) K‐feldspar between 1700 and 2745 m, (3) anhydrite between 2195 and 2745 m, (4) hematite ± sulfides above 2773 m, and (5) actinolite below ∼2890 m. Where present, K‐feldspar was the first mineral to precipitate in veins followed by epidote. In all other veins, epidote was the earliest vein mineral to form. Calcite, quartz, anhydrite, hematite, and sulfides were paragenetically later. Compositional zoning, common in most vein epidotes, is typically symmetric with Al‐rich cores and Fe3+ ‐rich rims. The minimum mole fraction of Ca2Fe3Si3O12(OH) (XPs) in vein epidotes decreases systematically with increasing depth from ∼0.33 at 906 m to ∼0.21 at 2900 m, and the maximum XPs at any given depth is greater than 0.33. Thermodynamic analyses of phase relations among vein‐filling minerals and aqueous solutions at depths near 1867 m and 300°C indicate that the modern reservoir fluid in the Salton Sea geothermal system is in equilibrium with calcite + hematite + quartz + epidote (XPs = 0.33) ± anhydrite. The predicted fugacity of CO2 (∼14 bars) for the modern Salton Sea brine is in close agreement with the calculated value of fCO2 for the 1867 m production fluid. Theoretical phase diagrams in the system CaO‐K2O‐Fe2O3‐Al2O3‐SiO2‐H2O‐O2‐S2‐CO2 demonstrate that the mineralogies and mineral parageneses recorded hi epidote‐bearing veins and the observed variations in Al‐Fe3+ content of vein epidotes may result from only minor changes in the fugacity of CO2, O2, and S2 of the geothermal fluid.