Early, deep magnetite-fluorapatite mineralization at the Olympic Dam Cu-U-Au-Ag deposit, South Australia

Abstract

The Olympic Dam iron oxide copper-gold (IOCG)-uranium-silver deposit (South Australia) is hosted in the large Olympic Dam breccia complex within the ~1.59 Ga Roxby Downs Granite. This breccia complex formed through multiple stages of hydrothermal activity and texturally destructive brecciation that affected the granite. The deepest diamond drill hole to date (RD2773, end at ~2,329 m) intersected weakly altered, in situbrecciated granite (~370–2,329 m) and a quartz-phyric felsic unit (~2,010–2,265 m). These two rock units host coarse-grained hydrothermal minerals, from ~2,150 m to the end of the drill hole. The main minerals in this assemblage are magnetite (± hematite), pyrite, fluorapatite, and quartz, with minor disseminated chalcopyrite, sericite, chlorite, rare earth element (REE)-fluorcarbonates, monazite, uraninite, thorite, galena, sphalerite, anhydrite, schorl, rutile, and pyrrhotite. The assemblage is cut by abundant multiphase veinlets and calcite (± fluorite ± barite) veins. A zircon U-Pb age for the felsic unit (1591 ± 11 Ma) implies that this unit is broadly coeval with the granite, whereas U-Pb ages for hydrothermal uraninite (1593.5 ± 5.1 Ma), fluorapatite (1583.3 ± 6.5 Ma), and hematite (1592 ± 15 Ma) indicate that deposition of the U-REE–rich hydrothermal magnetite-fluorapatite-pyrite-quartz assemblage and replacement of magnetite by hematite occurred soon after emplacement of the granitic host rocks. Sm-Nd dating of ubiquitous calcite veins suggests formation at ~1.54 Ga. The deep ~1.59 Ga magnetite-fluorapatite-pyrite-quartz assemblage at Olympic Dam resembles those characteristic of iron oxide-apatite deposits and many other sensu stricto IOCG deposits. This study confirms that the ~1.59 Ga event involved significant and widespread IOCG mineralization in the Olympic Cu-Au province.