Richter, and 4 co-authors, reported in 2018 that 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 Western Australia hosts deposits of xenotime that are enriched in HREEs 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 concentrations of yttrium (10−3 mol/kg), REEs (1–7 × 10−5 mol/kg), and uranium (4 × 10−5 mol/kg) 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 uranium 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.
The authors concluded that their fluid inclusion data from quartz coevally precipitated with xenotime provide unique insights into the little-known hydrothermal HREE deposits in sedimentary basins. At the Wolverine deposit, xenotime precipitated along with an oxidized assemblage of hematite and minor anhydrite at 100–120°C. This demonstrates that significant REE deposition is feasible at normal basin temperatures, and it therefore raises the potential of unconformity settings for REE exploration.
The authors' data identify SO42– and Cl– as the ligands that enabled REE transport at these low temperatures. Decompression boiling due to faulting-induced volume expansion within the hydrothermal vein system played a key role in xenotime precipitation. Similar sites of tectonic brecciation may therefore be promising exploration targets. The highly saline ore fluid (~25 wt% NaCleq) was not only boiling during xenotime precipitation, but its excess of REEs in equilibrium with xenotime shows that it must have mixed simultaneously with a second fluid to acquire phosphorus. That the availability of phosphorus limited the amount of xenotime precipitation is demonstrated by the high concentrations of Y (1.0 × 10−3 mol/kg) and individual REEs (8.5 × 10−6 to 6.7 × 10−5 mol/kg) in the fluid in equilibrium with xenotime.
The excess metal contents imply that REEs and Y can be effectively transported at typical basin temperatures; the residual fluid at Wolverine is thus still a potent metal carrier. The observation that the REE-bearing fluid also contains excess U strongly supports the idea, already proposed by others based on geological criteria, that the Browns Range hydrothermal fluid is an analogue of the ore-bearing fluid in world-class unconformity-related uranium deposits found in sedimentary basins of the same age elsewhere.
Managed & Built by I2M Consulting, LLC | © 2010-
