Modeling multiphase non-isothermal fluid flow and reactive geochemical transport in variably saturated fractured rocks: 2. Applications to supergene copper enrichment and hydrothermal flows

Abstract

Reactive fluid flow and geochemical transport in unsaturated fractured rocks has been of increasing interest to investigators in the areas of geo- and environmental- sciences. To test geochemical hypotheses based on petrologic observation and to predict geochemical reactions that occur through a complex dynamic interplay of physical and chemical processes, we use the methods presented in a companion paper (part 1, this issue p. 16–33) to investigate two problems: (1) supergene copper enrichment in unsaturated-saturated media and (2) predicted effects of thermohydrology on geochemistry during the Drift Scale Heater Test at the Yucca Mountain potential nuclear waste repository, Nevada. Through these two examples we address the importance of the following issues on geochemical processes: (1) participation of gas phase in transport and reaction, (2) interactions between fractures and rock matrix for water and chemical constituents, (3) heat effects on fluid flow and reaction properties and processes. In the supergene enrichment system, oxygen gas diffusion from the land surface through fractured rock promotes the alteration of the primary sulfide minerals and the subsequent deposition of secondary minerals. Modeling of the large-scale heater test shows effects of fracture-matrix interaction, heat-driven vaporizing fluid flow, and CO₂ degassing on mineral alteration patterns. The two examples also serve as a demonstration of our methods for reactive transport in variably saturated fractured rocks.