The role of zircon in hydrothermal heavy REE mineralisation: The case for unconformity-related ore deposits of north-west Australia

Zircon is hailed for its chemical and physical durability, but can undergo extensive chemical and structural modification due to radiation damage via the emission of alpha (α) particles, and subsequent low-temperature hydrothermal alteration. Here, we investigate Archean zircons from arkosic metasediments of the Browns Range Metamorphics (BRM) to evaluate their role in the formation of local unconformity-related heavy rare earth element (REE) ore deposits, within the Browns Range Dome, Western Australia. We determine that the heavy REE inventory of the BRM are primarily hosted in zircon, and that these zircons have a wide range of major element totals (77 to ~100 wt%, including low SiO2 and ZrO2 contents), and high and variable 'non-formula' components (U, Th, Y, REE, Nb, P, Al, Ca, Fe, Ti, F and OH− or H2O). Concentrations of Y + REE in some cases exceed 8 wt%. Extensive radiation damage (metamictisation) is confirmed by structural features including porous and amorphous domains, cavities, and voids. The lack of regional thermal events over an extended period likely prevented thermal annealing of these radiation-damaged zircons. Uptake of non-formula elements in metamict zircon, most likely during sedimentation in the late Archean, promoted further radiation damage, such that these grains remained highly susceptible to alteration by subsequent hydrothermal fluid circulation. We propose that the unconformity-related REE mineralisation was formed by saline fluids leaching Y + REE (and possibly P) from metamict zircon in the BRM, followed by ore mineral precipitation in fault zones near, and along the regional unconformity. More broadly, this model of ore formation may be relevant to other basin-hosted mineral systems, and could be used to guide exploration for unconformity-related REE deposits in Australia, and globally.