RARE-EARTH ELEMENT MINERALOGY OF THE OLYMPIC DAM CU-U-AU-AG DEPOSIT, ROXBY-DOWNS, SOUTH AUSTRALIA - IMPLICATIONS FOR ORE GENESIS

Rare earth elements (REE) and yttrium accompany copper and uranium mineralization within the polymetallic Olympic Dam deposit. The light and heavy REE tend to occur in different host minerals. Most of the LREE are present as the essential structural constituents of LREE fluorocarbonates such as bastnaesite and synchysite, or in phosphates such as florencite and monazite. Yttrium and the HREE occur mostly as minor concentrations in the form of cation substitutions within uranium minerals such as uraninite and coffinite, as well as brannerite to a lesser extent. Xenotime, britholite, yttrium-bearing bastnaesite and unknown Y-Ba-Cu phases are rare and account only for minor yttrium and HREE concentrations. The close mineralogical association of uranium, yttrium and REE is due to the transport of these elements by the same complexes during the ore formation processes. Different stabilities of LREE and HREE complexes caused initial precipitation of LREE into bastnaesite, synchysite, florencite and monazite. Further destabilization of REE from the ore fluids during late-stage hydrothermal activity led to the deposition of yttrium and HREE and to their incorporation into separate mineral phases including xenotime, uraninite, coffinite and brannerite.