An Appraisal of the Geophysical Effects of the Mount Isa Ore Bodies

G. N. Fallon; S. Busuttil

doi:10.1071/EG992133

ISSN: 0812-3985
E-ISSN: 1834-7533

An Appraisal of the Geophysical Effects of the Mount Isa Ore Bodies
Authors G. N. Fallon^a and S. Busuttil^a
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^a MIM Exploration Pty Ltd, Locked Mail Bag 100, Mount Isa, QUEENSLAND, Australia
Publisher: Australian Society of Exploration Geophysicists
Source: Exploration Geophysics, Volume 23, Issue 1-2, Mar 1992, p. 133 - 139
DOI: https://doi.org/10.1071/EG992133
- Published online: 01 Mar 1992

Abstract

In past years, regional to detailed gravity and aeromagnetic surveys have been employed to develop a greater understanding of the geology in and around the Mount Isa copper and silver-lead-zinc ore bodies. In recent years, several downhole electromagnetic (DHEM) surveys have been conducted, primarily to target further massive sulphide mineralisation. This paper will present some of the results from these geophysical surveys.

Gravity data show a complex but clearly anomalous effect over the mineralisation. A large negative anomaly is correlated with a zone of deep oxidation and leaching in the footwall, with a complex but strong positive anomaly associated with the silica dolomite alteration and silver-lead-zinc mineralisation.

Aeromagnetic data over the ore zone are partly obscured by the cultural effects of the city and mine. The removal of the cultural effects has been attempted by measuring the field at a number of terrain clearances and then calculating an actual vertical gradient. These vertical gradients were then used to compute a ‘culture’ filter. The data, after filtering the cultural effects, suggested that the silver-lead-zinc ore bodies produce an anomaly with a magnitude of approximately 35 nT.

Two holes have been probed using DHEM. The results show a complex superposition of responses from many sources and demonstrate the applicablilty of DHEM in exploring for base-metal mineralisation, within the conductive environment at Mount Isa. They also show the heterogeneous nature of the system given the differing responses from both holes.

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References

Bell, T, H., Perkins, W. G., and Swager, C. P. (1988). ‘Structural controls on localisation and development of syntectonic mineralisation at Mt Isa, Queensland’. Econ. Geol.83, 69–85.
Blake, D. H. (1987). ‘Geology of the Mount Isa Inlier and environs, Queensland and Northern Territory’. Bur. Miner. Resour. Aust. Bull.225.
Breiner, S. (1973). ‘Applications manual for portable magnetometers.’ Geometrics, p 30.
Clarke, D. A. (1980). ‘Magnetic properties of rocks from the Mount Isa Area’. CSIRO Report 1149R.
Clarke, D. A. (1991). ‘Magnetic properties of samples from the Mount Isa Copper Orebody’. Report for MIM Exploration Pty Ltd (unpublished).
Croxford, N. J. W. (1968). ‘A mineralogical examination of the McArthur lead-zinc-silver deposit’. Proc. Aust. Inst. Min. Met.226 (2).
Forrestal, P. J. (1990). ‘Mount Isa and Hilton silver-lead-zinc deposits’. 927–934. In F. E. Hughes (ed), ‘Geology of the mineral deposits of Australia and Papua New Guinea’. Australian Institute of Mining and Metallurgy, Melbourne.
Leaman, D. E. (1991a). ‘Surface gravity and magnetic responses of mineralisation, Mount Isa, NW Queensland’. Geophysics56, 542–549.
Leaman, D. E. (1991b). ‘Geophysical constraints on structure and alteration of the Eastern Creek Volcanics, Mt Isa, Queensland’. Aust. J. Earth Sci.38, 457–472.
Mathias, B. V., and Clark, G. J. (1975). ‘Mount Isa copper and lead-zinc orebodies Isa and Hilton mines’. In C. L. Knight (ed.), Economic deposits of Australia and Papua New Guinea, vol 1, Metals. Australian Institute of Mining and Metallurgy, 351–372.
Perkins, W. G. (1984). ‘Mount Isa silica dolomite and copper orebodies.’ Econ. Geol.79, 601–637.
Shalley, M. J., and Lord, J. R. (1967). ‘A test of exploration methods at the northern leases’. M.I.M. Technical Report GEO-98.
Smith, R. J. (1966). ‘Mount Isa gravity test survey’. Bur. Miner Resour. Aust. Rec. 1966/50.

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Article Type: Research Article

Keyword(s): conductive environment; downhole electromagnetics; gravity; magnetics; Mount Isa; structure

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An Appraisal of the Geophysical Effects of the Mount Isa Ore Bodies

Abstract

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