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

The Olympic Dam iron oxide copper-gold (IOCG)-uranium-silver deposit is hosted in the large Olympic Dam breccia complex within the ca. 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 intersected weakly altered, in situ brecciated 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 ca. 2 150 m to the end of the drill hole (2 329 m). The main minerals in this assemblage are magnetite (± haematite), 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 (1 591 ± 11 Ma) implies that this unit is broadly coeval with the granite, whereas U-Pb ages for hydrothermal uraninite (1 593.5 ± 5.1 Ma), fluorapatite (1 583.3 ± 6.5 Ma), and haematite (1 592 ± 15 Ma) indicate that deposition of the U-REE-rich hydrothermal magnetite-fluorapatite-pyrite-quartz assemblage and replacement of magnetite by haematite occurred soon after emplacement of the granitic host rocks. Sm-Nd dating of ubiquitous calcite veins suggests formation at ca. 1.54 Ga.
The Olympic Dam iron oxide copper-gold (IOCG)-uranium-silver deposit is hosted in the large Olympic Dam breccia complex within the ca. 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 intersected weakly altered, in situ brecciated 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 ca. 2 150 m to the end of the drill hole (2 329 m). The main minerals in this assemblage are magnetite (± haematite), 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 (1 591 ± 11 Ma) implies that this unit is broadly coeval with the granite, whereas U-Pb ages for hydrothermal uraninite (1 593.5 ± 5.1 Ma), fluorapatite (1 583.3 ± 6.5 Ma), and haematite (1 592 ± 15 Ma) indicate that deposition of the U-REE-rich hydrothermal magnetite-fluorapatite-pyrite-quartz assemblage and replacement of magnetite by haematite occurred soon after emplacement of the granitic host rocks. Sm-Nd dating of ubiquitous calcite veins suggests formation at ca. 1.54 Ga.