Sources of ore fluid components in IOCG deposits

Many diverse hydrothermal copper deposits containing iron oxides are classified as IOCG deposits. Genetic models need to consider a number of different ore fluid components that may not all have the same source in an individual ore system, and also that economic IOCG deposits may include examples in which key components (e.g. Cu) had different sources. The principal components of interest are (1) water ± other volatiles, (2) chlorine, (3) sulphur, (4) Fe, Cu, Au (the diagnostic ore components), and (5) other potentially economic components (e.g. U) that may be differently-sourced to the Cu and Au. Geochemical studies are providing growing insights into source contributions to IOCG deposits but their interpretation is commonly limited by inadequate baseline data for lithospheric to regional scale reservoirs and/or by incomplete understanding of natural fractionations that may affect the chemical systems. Where contributions from contemporary igneous rocks are evident it is generally not clear whether these came directly from a volatile phase evolved from the magmas or by leaching from previously consolidated rocks. Key aspects of current knowledge include: (1) IOCG deposits formed in a variety of different hydrological systems including (a) high level systems in which cool surficial fluids were able to interact with deeply-sourced fluids, and (b) deeper systems, some but not all of those for which suitable data are available, display evidence for a direct input of magmatic fluid; (2) Despite uncertainty about compositions of source reservoirs and fractionations with silicate minerals, there is good evidence that many IOCG ore fluids had salinities derived from more than one source and that these were not the same in each case (e.g. ±magmatic, ±dissolved evaporite/meta-evaporite, ±evaporated surface water); (3) Only circumstantial evidence links the principal economic metals in IOCG deposits to any particular source such as (a) the correspondence of higher Cu concentrations in fluid inclusions with typical (if not characteristic) magmatic Br/Cl ratios in the Cloncurry district where there are contemporary intrusions displaying evidence for evolution of Cu-rich brines, and (b) the correlation of Cu endowment and contemporary mantle-derived REE in deposits in the Gawler craton. Many gaps still remain to be filled in the knowledge base for IOCGs and the potential sources of their components. Nevertheless there is already good evidence that there are fundamentally different genetic types of IOCG deposit