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

Abstract

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.