Bastnäsite U–Th–Pb geochronology records Cryogenian overprinting in a Mesoproterozoic IOCG system

Bastnäsite is a major rare earth element (REE) host in a range of geological environments and typically contains low, yet measurable concentrations of U, Th, and Pb. Despite this, bastnäsite is seldom used as a robust geochronometer because of typically low U content, variable Th/U ratios, compositional heterogeneity, and ill-constrained closure temperatures. Here, coupled U–Pb and Th–Pb geochronology is applied to texturally and compositionally distinct bastnäsites from Oak Dam West, South Australia, a Mesoproterozoic iron oxide-copper–gold system. The variations of composition and texture hint at formation during distinct geological events. Complementary hematite U–Pb geochronology provides an independent framework to assess reliability of bastnäsite ages. Millimetre- to centimetre-sized pods of fine-grained low-Th bastnäsite from deep low-sulphide hematite breccias yield U–Pb ages around 1590 Ma, consistent with coexisting hematite. High-Th bastnäsite from shallow levels of the orebody occur as 50–400 μm rosette-like aggregates with Th–Pb ages between 718 and 681 Ma. These younger ages are interpreted to record a fluid-driven event that remobilised and reprecipitated REE and U. Rosette bastnäsite is also distinguished by positive Ce anomalies and associations with oxidised alteration mineral assemblages. Petrographic, geochemical, and geochronological evidence support a model in which oxidised fluids, plausibly linked to Cryogenian subglacial meltwater infiltration in the upper orebody along reactivated structures promoting dissolution–reprecipitation of earlier REE mineralisation. Coupled U–Pb and Th–Pb bastnäsite geochronology is a powerful tool to resolve multistage histories in REE-bearing ore systems and highlights the potential for Snowball Earth-related fluid circulation to drive oxidative metal remobilisation in buried ores.