Age, geological setting, and paragenesis of heavy rare earth element mineralization of the Tanami region, Western Australia

Metasedimentary rock-hosted heavy rare earth element (HREE) mineralization occurs as numerous orebodies distributed across a large district of the Tanami region of central Australia, close to a regional unconformity between Archean metasedimentary rocks of the Browns Range Metamorphics (BRM) and overlying Proterozoic Birrindudu Group sandstones. The orebodies consist predominantly of quartz, xenotime, and minor florencite and occur along steeply dipping structures within a stockwork of hydrothermal veins and breccias. Paragenetic stages of the mineralization include (1) a pre-ore stage of a greenschist-facies overprint of detrital minerals including quartz, alkali feldspar, plagioclase, and muscovite aligned in the premineralization foliation; (2) syn-ore quartz and white mica alteration associated with a multistage mineralization of the ore minerals, primarily in breccias and veins; and (3) a post-ore stage of veining and brecciation forming several generations of quartz, plus hematite, barite, anhydrite, and pyrite. In situ U–Pb dating of xenotime from several deposits/prospects yielded an age range for mineralization of 1.65 to 1.60 Ga; this timeframe lacks local magmatism or orogeny and is significantly younger than the ca. 1.72 Ga 40 Ar/ 39 Ar age of the pre-ore muscovite. Far-field stresses associated with the distal Isan and Liebig Orogenies are invoked as drivers of large-scale fluid flow and fault (re)activation in the region. We propose that ore formation was achieved via fluid leaching of REE from the BRM, followed by fluid mixing in fault zones, especially in the vicinity of the unconformity between the BRM and overlying Birrindudu Group sandstones. This mineralization style shares many features with unconformity-related U deposits, and there is significant potential for discovery of further REE orebodies of this style, especially in the vicinity of regional unconformities, in intercontinental sedimentary basins.