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