Rare earth element (REE) orebodies are typically associated with alkaline igneous rocks or develop as placer or laterite deposits. Here, we describe an economically important heavy (H)REE mineralization type that is entirely hydrothermal in origin with no demonstrable links to magmatism. The mineralization occurs as numerous xenotime-rich vein and breccia orebodies across a large area of northern Australia but particularly close to a regional unconformity between Archean metasedimentary rocks of the Browns Range Metamorphics and overlying Proterozoic sandstones of the Birrindudu Group. The deposits formed at 1.65 to 1.61 Ga along steeply dipping faults; there is no known local igneous activity at this time.
Depletion of HREEs in the Browns Range Metamorphics, together with the similar nonradiogenic Nd isotope composition of the orebodies and the Browns Range Metamorphics, indicates that ore metals were leached directly from the Browns Range metasedimentary rocks. We propose an ore genesis model that involves fluid leaching HREEs from the Browns Range Metamorphics and subsequently mixing with P-bearing acidic fluid from the overlying sandstones in fault zones near the unconformity. The union of P and HREEs via fluid mixing in a low-Ca environment triggered extensive xenotime precipitation.
This mineralization is unlike that of any other class of REE ore deposit but has a similar setting to unconformity-related U deposits of Australia and Canada, so we assign it the label “unconformity-related REE.” Further discoveries of this REE mineralization type are expected near regional unconformities within Proterozoic intracontinental sedimentary basins across the globe.