Hydrothermal formation of HREE-xenotime deposits at 100 °C in a sedimentary basin

Most rare-earth element deposits form from magmatic fluids but there have also been discoveries of heavy rare-earth element (HREE)-enriched hydrothermal xenotime deposits within sedimentary basins. As xenotime is notoriously insoluble, the question arises as to whether these lesser known deposits form at typical basin temperatures or by influx of much hotter magmatic-hydrothermal fluids. The Browns Range District in northern Australia hosts deposits of xenotime that is enriched in HREE and also uranium. The ore bodies consist of fault-controlled hydrothermal quartz–xenotime breccias that crosscut Archean basement rocks and overlying Paleoproterozoic sandstones. Analyses of fluid inclusions show that the xenotime precipitated at remarkably low temperatures between 100 and 120 °C, in response to decompression boiling. The inclusions contain high excess REE concentrations (10−6 to 10−5 molal) and high U concentrations (5 × 10−5 molal) in equilibrium with xenotime at these low temperatures, showing that availability of phosphate limited the amount of xenotime precipitated. The analyses further identify SO42– and Cl- as the ligands that facilitated the elevated REE and U solubilities. These findings establish that significant REE transport and deposition is feasible at basin temperatures and hence they raise the potential of unconformity settings for REE exploration. Moreover, the aqueous metal contents support a genetic link between this type of ore fluid and world-class Proterozoic unconformity-related uranium deposits elsewhere.