The history and current status of geophysical exploration at the Osborne Cu & Au deposit, Mt. Isa

Abstract

The Osborne deposit lies within the Trough Tank project area, situated some 190 km south-east of Mt Isa, in northwest Queensland. Copper and gold mineralisation occurs within a magnetite-rich Proterozoic ironstone sequence, possibly equivalent to the Mt Norna Quartzite unit of the Soldier's Cap Group. Mineralisation occurs within two shallow north-easterly dipping ironstone units, beneath 20 to 40 m of flat lying mesozoic siltstone. Based on 396 drillholes completed to the end of 1991, the total diluted resource was calculated at 36 Mt at 2.0% Cu and 1.0 g/t Au, using a 1% Cu?Au equivalent cut-off. Results of early geophysical surveys (magnetics, IP and TEM) have been described by Gidley (1988). Although initial interpretation of magnetic data failed to recognise the likely influence of demagnetisation effects, subsequent modelling efforts incorporating a qualitative demagnetisation correction effectively sited drillholes to test the ironstone sequence. Quantitative interpretation of the magnetic response in the area of the deposit, including demagnetisation effects, is the subject of a current M.Sc. study by a co-author (Logan). Fixed and moving loop TEM surveys subsequently defined a strongly conductive zone within the southern portion of the ironstone trend. The next drilling phase tested this position and intersected encouraging thicknesses of sub-economic mineralisation within massive ironstone. Downhole TEM surveys, in conjunction with resistivity logging and physical property measurements, established that conductivity variations within the ironstone relate to the content and degree of remobilisation of magnetite as well as sulphide content. Continuing use of TEM for detection of high-sulphide lenses within the ironstone conductive host was therefore questionable. Step-out reverse circulation drilling continued to confirm the presence of a large mineralised system, although no coherent high-grade intersections were achieved. Vertical derivative enhancement and further modelling of ground magnetic data contributed to the development of structural models for the prospect, which were tested with further drilling programmes, but still failed to reveal any high-grade mineralisation. The first economic drill intersection came after 28 diamond-cored and approximately 70 reverse circulation holes had been completed on the project. Moving-loop coverage of the grid area was completed after drilling had established the presence of high-grade material down-dip from the magnetic anomaly position, in areas poorly coupled with the earlier fixed-loop transmitter loops. This coverage outlined a small conductive zone within the northern high-grade mineralisation. Subsequent geophysical surveys have included gravity, down-hole magnetics, mise-à-la-masse and magnetometric resistivity. Gravity results closely reflect the known distribution of ironstone, and borehole magnetometry has confirmed the inferred orientation of the total field after demagnetisation. Additional electrical techniques have confirmed the conductive zones outlined in TEM data and further indicated that the conductivity of ironstones is not directly related to mineralisation. This is attributed in part to the increase in silica content in high-grade material, at the expense of magnetite, reducing the interconnection of sulphide and magnetite grains. Recent tests with the borehole Radio Imaging Method (RIM) have indicated that this technique may help to confirm inferred correlations of mineralisation between diamond drillholes at separations of up to 70 m. Conductivity of mineralisation at the employed radio frequency (50 kHz) appears to be more coherent than is observed for the TEM technique.