Sulfide Mineral Precipitation from Hydrothermal Fluids

Abstract

Sulfide minerals precipitate from hydrothermal solution where their constituent metals and sulfide are more stable residing in the solid than in aqueous solution. An understanding of how and why sulfide minerals precipitate means understanding what chemical species hold metals and sulfide in solution, and what fundamental chemical and physical processes may liberate those reactants to precipitate in solids–processes such as pH increase, chemical reduction, dilution or cooling. In the natural setting, the fundamental chemical and physical drivers of sulfide precipitation are a response to a wide range of processes. For example, pH increases where acidic hydrothermal fluids react with feldspars or carbonates in wall rocks, or where a hydrothermal fluid boils out CO2 to the gas phase. The pH also increases where a brine is diluted by fresh water while maintaining equilibrium with a silicate wall-rock assemblage. Cooling follows from isoenthalpic boiling, which is caused by pressure drop as a fluid ascends in a hydrothermal system. Cooling also results from dilution by cold water or from heat conduction into cold wall rocks. Chemical reduction occurs where an oxidized fluid reacts with carbon or methane or ferrous iron. In this chapter, we explore the fundamental chemical and physical processes that drive sulfide mineral dissolution and precipitation as well as a limited set of more complex processes that control the fundamental drivers. The basic goal is to understand the chemistry of the simplest processes so as to improve our ability to deduce what actually happens in the natural world. Our basic approach is to run thermodynamic calculations of whole-system models of the processes causing sulfide precipitation and dissolution—models that take into account simultaneous equilibria involving assemblages of minerals at chemical equilibrium among themselves and with hundreds of aqueous species. We then interpret the models in terms of fundamental effects of pH …