Crystal chemistry of titanite from the Roxby Downs Granite, South Australia: insights into petrogenesis, subsolidus evolution and hydrothermal alteration

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Titanite textures and chemistry have been investigated from the Roxby Downs Granite, host to the Olympic Dam Cu–U–Au–Ag deposit, South Australia. Three textural subtypes of titanite are documented: primary magmatic (cores and rims); deuteric; and hydrothermal (low T recrystallisation). Magmatic cores are defined by enrichment in LREE (~ 3 wt%), Nb (up to 1 wt%) and Zr relative to rims, which typically contain < 1 wt% LREE and Nb, as well as greater concentrations of Al, Ca, Fe and F. Deuteric titanite occurs as overgrowths on pre-existing titanite and other magmatic accessory minerals (magnetite and ilmenite), and is depleted in HFSE compared to magmatic rims, showing geochemical trends consistent with substitution of Ca2+ + Ti4+ ↔ REE3+ + (Al, Fe)3+. Hydrothermal titanite forms as a low-temperature hydrothermal overprint on primary titanite as well as an alteration product of chloritised phlogopite. Applying Zr-in-titanite geothermometry, three temperature ranges are obtained for titanite crystallisation: magmatic cores ~ 765 to 780 °C; rims ~ 705 to 740 °C; and deuteric ~ 680 to 690 °C. Titanite breakdown is a ubiquitous feature of the Roxby Downs Granite, and occurs through interaction with CO2- and F-rich fluids, forming pseudomorphs characterised by the presence of REE-fluorocarbonates, which are subsequently overprinted by REE-phosphates with increased proximity to the Olympic Dam Breccia Complex. This change is related to interaction with fluids containing appreciable PO42− liberated from local dissolution of fluorapatite. Such observations are consistent with and linked to later/retrograde stages in the formation of the Olympic Dam deposit.