The Geophysical Signature of Mt Isa Cu, Zn-Pb-Ag Ore Bodies

At a regional scale the Mt Isa Cu and Zn-Pb-Ag deposits do not have a noticeable gravity or magnetic response. In addition there is little recent literature on the geophysical signature of the deposits and the data which is available is dated.  Fallon and Busuttil 1992 and Valenta 2020 provide summaries of the available geophysical data. Given that pyrite extends up to 10 km north of economic grade mineralization within the Urquhart Shale and across a width of >1km, pristine mineralization would have  strong IP, EM and MT responses however this data is not readily available.

Leaman 1991, reported that a ground gravity survey was commenced in 1981 and extended 50km N-S and 20km E-W.  Nominal station spacing was 1km excepting an area of 5x5km centred on the mine which was covered by 250m station spacings. Leaman 1991 only report on the immediate mine area survey.  Valenta 2020 digitized and interpolated the data.

There is an clear correlation in the east between the Bouger gravity anomalies and the Eastern Creek Volcanics (Cromwell Metabasalt).  To the west much of the gravity field is low relief.  The mean value at the Mt Isa Mine is about 13 mGal but with a  10 mGal range.  The data shows coincidence of the gravity high with mineralization.  A gravity low to the east of mineralization may be related to low density leached rocks (Valenta, 2020).

Eastern Creek Volcanics ( green ) with copper orebodies (blue cross-hatch) (Valenta, 2020) projected to surface and the near surface copper geochemistry (red) from drillhole data (Conaghan et al 2003) plotted over Bouger Gravity (Leaman 1991 and Valenta 2020).

References

Conaghan, E.L., Hannan, K.W. & Tolman, J. 2003. Mount Isa Cu and Pb-Ag-Zn deposits of NW Queensland, Australia. Regolith Expres­sion of Australian Ore Systems, CRC LEME Geochemistry Special Monograph Series. 3pp

Fallon, G. and Busuttli, C., 1992. An appraisal of the geophysical effects of the Mount Isa ore bodies, Exploration Geophysics

Leaman, D. (1991b). Surface gravity and magnetic responses of mineralization, Mt. Isa, Northwest Queensland, Australia. Geophysics, 56(4), 542-549.

Valenta, R., 2020, NW Mineral Province Deposit Atlas, Mt Isa Cu and Pb-Zn-Ag Orebodies, Sustainable Minerals Institute

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