The magmatic source of IOCG deposits: Paragenetic and thermodynamic evidences

The precipitation sequence hematite-magnetite-pyrite-chalcopyrite is widely documented in Andean IOCG deposits (e.g. Sillitoe, 2003; de Haller et al., 2006; de Haller, 2006; Marschik and Fontboté, 2001). This oxidized sequence is typically characterized by the presence of mushketovite (pseudomorphosis of magnetite after specular hematite). Strange enough, a reduced ore mineral precipitation sequence pyrrhotite-pyrite-chalcopyrite coexists with the oxidized sequence in some Andean IOCG deposits (e.g. Raúl-Condestable and Candelaria-Punta del Cobre).
The redox state of hydrothermal fluids is controlled internally by the dissolved redox buffering species (O2, H2, SO2, SO4--, HSO4-, H2S, HS-, etc.; see Giggenbach, 1987 and 1997), and externally by the redox state of the host rocks. Aside from sedimentary rocks containing organic carbon (reductant), the main redox buffer is the Fe contained in the rocks, as Fe++ and Fe+++. Sulfur in rocks (as sulfides or sulfates) can control the redox state of a fluid if solid and aqueous sulfur buffer couples are involved through wall-rock reaction (e.g. H2S-SO4--). The reactivity of wall-rock (and its capacity to buffer the redox state of the fluid) is enhanced at temperatures below about 350°C, where SO2 starts to dissociate into water, generating sulfuric acid:
4SO2 + 4H2O = 3H+ + H2S + 3HSO4-
Detailed studies on several Andean deposits allow to state that the oxidized sequence in Andean IOCG deposits is indicative of oxidized mineralizing magmatic fluids circulating at high fluid/rock ratio, while the reduced ore mineral association indicates that the magmatic fluids interacted at a low fluid/rock ratio with reduced host rock and/or mixed with heated (by the thermal anomaly of the magmatic-hydrothermal system) and reduced (at equilibrium with the host rock) externally-sourced brines (seawater or evaporite derived). In both oxidized and reduced sequences, the redox state of the involved fluids progressively converge at or near the Fe2+/Fe3+ "rock buffer" line (Giggenbach, 1987 and 1992, Einaudi et al., 2003) during cooling as a result of wall rock reaction and fluid mixing. This explains why the later lower temperature pyrite-chalcopyrite succession is shared by both oxidized and reduced mineral associations.
Finally, the sequence hematite-magnetite-sulfides is widely documented in IOCG deposits worldwide as well as in skarn and porphyry copper deposits, and might be considered as a field evidence for mineralizing magmatic fluids (transporting metals and sulfur) having been released by I-type magmas (i.e. oxidized).