Geochemical modeling of hydrothermal fluids beneath the southern moat of Long Valley caldera, California

The southern moat of Long Valley caldera, California, contains an active hydrothermal system which has been studied for its geothermal potential and for its importance in predicting future volcanic activity within the resurgent caldera. This research attempts to test the hypothesis, using the reaction path model EQ3/6, that hydrothermal fluids flow east from the Shady Rest well to the Casa Diablo thermal area in a continuous aquifer hosted in permeable, hydrothermally altered tuffaceous units.; Thermal fluids are near neutral at reservoir temperatures (∼200°C). Calcite, quartz, pyrite, and smectite are saturated in the fluids prior to boiling, mixing, or reaction with wallrock. The chemistry of Casa Diablo well MBP-4 is most closely modeled by mixing of 83% Shady Rest well fluid with 17% nonthermal groundwater and reaction of ∼2.6 cm3 of altered Bishop Tuff per liter fluid. Mass balance calculations indicate possible input of volcanogenic H₂S and CO₂.