On fluid flow and mineral alteration in fractured caprock of magmatic hydrothermal systems

Geochemical evolution in hydrothermal fractured rock systems occurs through a complex interplay of multiphase fluid and heat how and chemical transpor t processes. Building on previous work, we present here simulations of reac tive hydrothermal flow that include (1) detailed fracture-matrix interactio n for fluid, heat, and chemical constituents, (2) gas-phase participation i n multiphase fluid flow and geochemical reactions, (3) the kinetics of flui d-rock chemical interaction, and (4) heat effects on thermophysical and che mical properties and processes. The present study uses, as an example, wate r and gas chemistry data as well as caprock mineral composition from the hy drothermal system in Long Valley Caldera (LVC), California. The flow system studied is intended to capture realistic features of fractured magmatic hy drothermal systems. The purpose of this "numerical experiment" is to gain u seful insight into process mechanisms such as fracture-matrix interaction, liquid-gas phase partitioning, and conditions and parameters controlling wa ter-gas-rock interactions in a hydrothermal setting. Simulation results ind icate that almost all CO2 is transported through the fracture. Cooling and condensation results in an elevated CO2 partial pressure. The CO2 is the do minant gas-phase constituent close to the land surface. Close to the heat s ource, dissolution dominates over precipitation. Away from the heat source precipitation dominates because chemical constituents, transported from the bottom, precipitate in a lower-temperature environment. Mixing with cold m eteoric water enhances mineral dissolution and precipitation effects. The r ock alteration pattern is sensitive to reaction kinetics. The predicted alt eration of primary rock minerals and the development of secondary mineral a ssemblages are generally consistent with field observations in the LVC. The observed sequence of argillic alteration in the LVC consists of an upper z one with smectite and kaolinite (in the lower-temperature region), a lower illite zone, and an intermediate mixed illite and smectite zone. The sequen ce is reasonably well reproduced in the numerical simulation. In addition, calcite and chlorite precipitation in the hot region coincides with the obs ervations. Using the reactive geochemical transport model, we have successf ully simulated spatial distribution of the three argillic alteration zones.