Summary:
RD1 drilled through 335m of flat-lying Proterozoic and Cambrian sediments before passing through an unconformity into haematite altered basement rocks. In theory the copper mineralisation was supposed to be in the overlying sediments but assays revealed fine-grained hypogene chalcocite in the basement. RD1 returned 38m @ 1.05%Cu, which was considered uneconomic at these depths at this time, but RD10 changed that with an intersection of 170m @ 2.12%Cu and 0.58kg/t U3O8 and is considered the “Discovery Hole”.
However, following initial underground development in 1985-1987, it became apparent that none of the mineralisation was stratabound and that it was hosted in a highly variable magmatic-hydrothermal breccia complex that displays multiple brecciation, diatreme intrusions, mafic and ultramafic dyke intrusions along with interpreted gravity collapse of high level volcanic edifice material. The deposit is considered to be a member of the Iron Oxide Copper Gold (IOCG) family of deposits and has a close temporal and spatial association with the Hiltaba Suite of granites, dated at 1590Ma and more specifically the more fractionated and oxidised Roxby Downs subsuite (1588Ma).
The main lithologies at Olympic Dam comprise a continuum of breccias starting at the periphery with granite clasts set in little haematite matrix progressing into the centre where the clasts are wholly haematite in a haematite matrix. The principal gangue minerals are haematite, sericite and quartz. Minor gangue minerals include; siderite, chlorite, fluorite and barite.
The dominant sulphide minerals are chalcopyrite, bornite, chalcocite and pyrite. Minor sulphides include; carrolite, cobaltite, galena, sphalerite and molybdenite. Other minerals of interest are; metallic copper, electrum, Ag-, Hg-, Pb-, Bi-selenides and tellurides. The uranium at Olympic Dam occurs mostly as uraninite, coffinite or brannerite with trace amounts in zircon, monazite, florencite and bastinite.