THE IMPORTANCE OF THE AQUEOUS FERRIC/FERROUS REDOX BUFFER TO IOCG AND OTHER HEMATITE-RICH HYDROTHERMAL DEPOSITS

The coexistence of aqueous ferric and ferrous iron is a well-known redox buffer in low-temperature, acidic waters. However, due to a lack of experimental data, this buffer has been overlooked by hydrothermal geochemists. Recent experiments by the author have confirmed that aqueous ferric chloride (FeCl₄^-) is highly stable at T > 200°C. In the added presence of aqueous ferrous chloride (FeCl₂), a redox buffer is set up that is strongly oxidizing (e.g., roughly 15 log units higher in O₂ fugacity compared to hematite-magnetite at 300°C, pH = 4, and aCl^- = 1). The concentration of FeCl₄^- in equilibrium with hematite increases exponentially with increase in temperature, salinity, or decrease in pH. Once formed, FeCl₄^- will oxidatively dissolve sulfide minerals and release metals, just as Fe³⁺ breaks down sulfide minerals in acid mine drainage systems.

The FeCl₄^- compound may play a key role in the formation of iron-oxide-copper-gold (IOCG) deposits, as well as other deposits (such as oxidized porphyry/skarns), that contain abundant hematite. Models for these deposit types that rely on transport of dissolved iron as ferrous species must identify an oxidant to drive precipitation of hematite. Whereas molecular O₂ serves this role in low temperature waters, it is unlikely that O₂ could be present deep in the crust where the deposits in question are formed. In contrast, hematite is very common, and is especially ubiquitous in Mesoproterozoic sedimentary basins that contain large IOCG deposits (e.g., Olympic Dam). If iron is transported as FeCl₄^-, then dissolution and precipitation of hematite are simple, non-redox reactions triggered by changes in temperature, salinity, or pH. In turn, reactions involving iron greatly influence the mobility and precipitation of other metals such as gold, copper, or silver. At the aqueous ferric/ferrous boundary, 10s of mg/L of Au and Ag, and 100s of mg/L of Cu are soluble as chloride complexes at T > 200°C: uranium is also highly soluble as UO₂²⁺ or a simple ion pair. Transport of iron as FeCl₄^- may also help to explain the common presence of hematite daughter minerals in fluid inclusions from porphyry copper deposits, without the need of diffusion of H₂ gas out of the host mineral.