Brothers in arms: the interaction of geology and geophysics in the Mt Isa Inlier

Abstract

The mid-Proterozoic Mt Isa Inlier forms part of the Mt Isa Geophysical Domain, an 800 km long by 200 km wide entity defined by gravity and magnetic patterns. The exposed portion of the Inlier totals about one-third, or 50 000 km2, of the Domain, and contains in known or previously mined deposits about 42 Mt of zinc, 25 Mt of Pb, 10 Mt of Cu, and 2.5 million ounces (77.8 t) of Au, in a variety of geological settings, and valued in 1992 dollars and metal prices at $A100 billion. Not surprisingly, this degree of metal endowment has stimulated geological and geophysical exploration of the 100 000 km2 of the Inlier which is unexposed, and where several discoveries have already been made at depths of 30?50 m. Three major tectonic units are recognised ? a central basement block (Kalkadoon-Leichhardt block) flanked by the Western and Eastern Fold Belts. Basement granites and volcanics are dated at about 1860 Ma and older. The post-basement volcano-sedimentary successions, dated 1800 Ma to 1620 Ma, are related to periods of crustal extension and rifting, especially in the west ? the Leichhardt River Fault Trough ? and also in the Eastern Fold Belt, where the character of rift-related sequences may be masked by high-grade metamorphism. Post-basement granites are dated at about 1740 Ma and 1600 to 1500 Ma. Major deformation episodes D1 to D3 and accompanying metamorphism occurred also in the period 1600?1500 Ma. Filtered gravity data reveal a crustal structure of north-trending alternating belts of high and low density, representing belts of palaeorifting and high-density basic rocks juxtaposed with lower density sediments, felsic volcanics and granites. The filtered gravity linear features are offset by NW to WNW-trending cross-structures, which may be of broad exploration significance. Regional aeromagnetic data are very valuable in definition of subsurface major fault structures, delineation of areas of basic volcanics in the Western Fold Belt as a guide to Isa-style Pb?Zn and Cu deposits, delineation of granite plutons with possible skarn envelopes and U-REE-Cu-Au potential, and identification of quartz-magnetite zones associated with firstly, the Cu?Au deposits of the Starra-type, and secondly with the chemogenic/exhalative sequences which characterise the Pegmont/Broken Hill style of mineralisation, very common in the Eastern Fold Belt. Some regional aeromagnetic patterns may be modified by alteration styles in basalt, by facies variation and changes in metamorphic grade, by zoning in granites, and by superposition of thrusted sequences. Future exploration of prospective but concealed areas demands that geophysics and geology be used and integrated to the greatest possible extent ? that for optimum success, geophysicists, geologists and their respective technologies, truly become brothers-in-arms.