THE ROLE OF FLUORINE-BEARING MAGMATIC-HYDROTHERMAL FLUIDS IN THE FORMATION OF THE SUPERGIANT OLYMPIC DAM U-AU-CU-AG DEPOSIT, SOUTH AUSTRALIA

The Olympic Dam Iron Oxide Copper Gold deposit is a supergiant, comprising 8.3 Gt of ore enriched in U, Cu, Au and Ag. Olympic Dam is also a significant fluorine (F) and REE anomaly in the crust. Fluorite is present as veins and disseminated in the ore body. Fluorine occurs in all the major alteration minerals, such as chlorite, sericite, carbonate and apatite, and is also present in REE-minerals such as bastnaesite, britholite and florencite. A simple calculation, based only on the fluorite proportion, gives a minimum estimate of 390 Mt of F within the defined ore body. The immediate host to the ore is a breccia complex derived from and within the Roxby Downs Granite, a Mesoproterozoic intrusion, which is part of the Gawler Range Volcanics (GRV)-Hiltaba Suite (HS) silicic Large Igneous Province (SLIP). The SLIP represents a total volume of 100,000 km3 of magma, and is contemporaneous with the formation of Olympic Dam deposit. We have found abundant evidence of the F-rich nature of the SLIP magmas. Magmatic fluorite is present as inclusions in phenocrysts in the GRV lavas and as an interstitial phase in the HS intrusions. Fluorite daughter crystals are also present in melt inclusions hosted in quartz phenocrysts of the GRV lavas. The SLIP magmas were therefore saturated in F. The fluorine content of the melt inclusions is up to 1.4 wt% and decrease with increasing SiO2. Melt inclusions are systematically enriched in F, REE, U and HFSE compared to the whole rock, suggesting that this suite of elements was mobile. The presence of vesicles filled with fluorite and F-REE-Zr-bearing phases also indicates the concentration of these elements into a late-magmatic fluid phase. The difference of the F content between the melt inclusions and the whole rock implies that ~250 Gt fluorine was exsolved by the end of the crystallization. We conclude that the mineralizing hydrothermal fluid at Olympic Dam inherited its F-rich character from the host SLIP, and that the deposit is directly related to the exsolution of a late magmatic F-bearing this fluid had an exceptional capacity to transport HFSE in solution, especially U.