The IOCG-IOA Olympic Dam Cu-U-Au-Ag deposit and nearby prospects, South Australia

Ehrig, K., Kamenetsky, V.S., McPhie, J., Apukhtina, O., Ciabanu, C.L., Cook, N., Kontonikas-Charos, A. and Krneta, S

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

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.Olga B. Apukhtina, Vadim S. Kamenetsky, Kathy Ehrig, Maya B. Kamenetsky, Roland Maas, Jay Thompson, Jocelyn McPhie, Cristiana L. Ciobanu, and Nigel J. Coo

Postmagmatic magnetite-apatite assemblage in mafic intrusions: a case study of dolerite at Olympic Dam, South Australia

Published online: 11 December 2015An assemblage of magnetite and apatite is common worldwide in different ore deposit types, including disparate members of the iron-oxide copper–gold (IOCG) clan. The Kiruna-type iron oxide-apatite deposits, a subtype of the IOCG family, are recognized as economic targets as well. A wide range of competing genetic models exists for magnetite–apatite deposits, including magmatic, magmatic-hydrothermal, hydrothermal(-metasomatic), and sedimentary(-exhalative). The sources and mechanisms of transport and deposition of Fe and P remain highly debatable. This study reports petrographic and geochemical features of the magnetite–apatite-rich vein assemblages in the dolerite dykes of the Gairdner Dyke Swarm (~0.82 Ga) that intruded the Roxby Downs Granite (~0.59 Ga), the host of the supergiant Olympic Dam IOCG deposit. These symmetrical, only few mm narrow veins are prevalent in such dykes and comprise besides usually colloform magnetite and prismatic apatite also further minerals (e.g., calcite, quartz). The genetic relationships between the veins and host dolerite are implied based on alteration in the immediate vicinity (~4 mm) of the veins. In particular, Ti-magnetite–ilmenite is partially to completely transformed to titanite and magmatic apatite disappears. We conclude that the mafic dykes were a local source of Fe and P re-concentrated in the magnetite–apatite veins. Uranium-Pb ages for vein apatite and titanite associated with the vein in this case study suggest that alteration of the dolerite and healing of the fractures occurred shortly after dyke emplacement. We propose that in this particular case the origin of the magnetite–apatite assemblage is clearly related to hydrothermal alteration of the host mafic magmatic rocks.Olga B. Apukhtina, Vadim S. Kamenetsky, Kathy Ehrig, Maya B. Kamenetsky, Jocelyn McPhie, Roland Maas, Sebastien Meffre, Karsten Goemann, Thomas Rodemann, Nigel J. Cook, Cristiana L. Cioban