Mobilization of insoluble Fe(III) and phosphate in sulfate- and carbonate-rich fluids

Iron(III) oxides and phosphates are considered to be insoluble in geofluids but are common products of hydrothermal mineralization. We investigated the capability of sulfate- and/or carbonate-rich fluids to dissolve hematite and apatite using a hydrothermal diamond anvil cell (HDAC). We found that substantial Fe(III) can be dissolved in these fluids, challenging the view that hydrothermal Fe transport requires the predominance of Fe(II). This provides insights into the genetic link between sulfate-rich lithologies (e.g., evaporites and carbonatites) and Fe mineralization in iron oxide-apatite (IOA), iron oxide copper-gold (IOCG), and carbonatite-related deposits. In addition, the co-mobility of phosphate and Fe(III) in these fluids not only explains their coexistence in a series of deposit types, but also implies that some conventionally immobile components (e.g., high field strength elements) may be mobile in sulfate- or carbonate-rich “unconventional” fluids.