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

Abstract

Geochemical evolution in hydrothermal fractured rock systems occurs through a complex interplay of multiphase fluid and heat flow and chemical transport processes.Building on previous work, we present here simulations of reactive hydrothermal flow that include (1) detailed fracture‐matrix interaction for fluid, heat, and chemical constituents, (2) gas‐phase participation in multiphase fluid flow and geochemical reactions, (3) the kinetics of fluid‐rock chemical interaction, and (4) heat effects on thermophysical and chemical properties and processes. The present study uses, as an example, water and gas chemistry data as well as caprock mineral composition from the hydrothermal system in Long Valley Caldera (LVC), California. The flow system studied is intended to capture realistic features of fractured magmatic hydrothermal systems. The purpose of this “numerical experiment” is to gain useful insight into process mechanisms such as fracture‐matrix interaction, liquid‐gas phase partitioning, and conditions and parameters controlling water‐gas‐rock interactions in a hydrothermal setting. Simulation results indicate that almost all CO2 is transported through the fracture. Cooling and condensation results in an elevated CO2 partial pressure. The CO2 is the dominant gas‐phase constituent close to the land surface. Close to the heat source, 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 meteoric water enhances mineral dissolution and precipitation effects. The rock alteration pattern is sensitive to reaction kinetics. The predicted alteration of primary rock minerals and the development of secondary mineral assemblages are generally consistent with field observations in the LVC. The observed sequence of argillic alteration in the LVC consists of an upper zone with smectite and kaolinite (in the lower‐temperature region), a lower illite zone, and an intermediate mixed illite and smectite zone. The sequence is reasonably well reproduced in the numerical simulation. In addition, calcite and chlorite precipitation in the hot region coincides with the observations. Using the reactive geochemical transport model, we have successfully simulated spatial distribution of the three argillic alteration zones.