The Olympic Dam copper-uranium-gold-silver-rare earth element deposit, South Australia: A geophysical case history

Abstract

The Olympic Dam polymetallic deposit is located 520 km northnorthwest of Adelaide, South Australia. The deposit has an inferred resource of 2660 Mt at 1.2% Cu, 1.4 kg/t U3O8, 0.5 g/t Au. The deposit occurs in a hydrothermal breccia complex within Mesoproterozoic crystalline basement of the Stuart Shelf. It is overlain by 260 m of flat-lying sedimentary rocks and has no surface expression. The mineralisation is irregularly distributed within the breccia complex, occurring predominately in haematite-rich matrix material. The deposit was located in 1975 by Western Mining Corporation by the drilling of coincident gravity, magnetic and tectonic targets, identified with respect to a geological conceptual model. The Olympic Dam deposit coincides with a 17 mGal gravity anomaly, caused by haematite-rich polymict breccias. The source of a broad 1600 nT magnetic anomaly, which is closely associated with the deposit, is interpreted as being due to several geological entities, including magnetite in the deposit and the host granite. Electrical geophysical surveys have been tried over the Olympic Dam deposit since discovery. Due to the disseminated nature of the sulphide mineralogy, emphasis has been placed on IP techniques. Initial experimentation was not encouraging, due to the low signal-to-noise ratio and the dominance of electromagnetic coupling. However, in 1980 a high-power transmitter and digital IP receiver were employed to combat the above two factors. The deposit was covered with 400 m dipole-dipole surveys with an 800 m line spacing. These data detected an apparent resistivity low, restricted to the southern and central sections of the deposit. The cause of this low is interpreted as being porous, matrix-rich, haematitic breccias, in which the dominant form of haematite is a black crystalline variety. Pore fluids are highly saline, with an apparent resistivity of between 0.1 and 0.05 Ohm.m. The source of a positive phase-angle response is not well understood, but it has a close spatial relationship with the extent of the haematite-rich breccias, whether these breccias are mineralised or not. This interpretation is not consistent with the results of downhole logging using relatively small geometric survey parameters. This inconsistency reflects the difficulty in trying to reconcile small-scale physical property measurements with the larger scale surface measurements, for such a complex inhomogeneous mineralised environment.