Geophysical and GIS applications to exploration for Proterozoic sediment-hosted Zn-Pb mineralisation, northern Australia

Geophysical and geographic information systems (GIS) methods have been applied to investigate the regional three-dimensional geological setting and geophysical signature of sediment-hosted Zn-Pb mineralisation in the Proterozoic Carpentaria Zinc Belt of northern Australia. The study focused on the setting of two particular Zn-Pb deposits, HYC in the McArthur Basin and Lady Loretta in the Mount lsa Basin. A GIS incorporating geophysical interpretations, geological and geochemical data was designed and implemented for a region in the McArthur Basin encompassing the giant HYC Zn-Pb-Ag deposit. The GIS incorporates geological attributes that encode depth information implicit in the stratigraphic column. This data structure, in conjunction with topological attributes, allows queries based on the stratigraphic relationships of spatial elements. The GIS was used to identify and analyse constraints on base metal metallogeny and to gain insights into the 3-D evolution of the McArthur Basin. Mass balance calculations derived from lithogeochemical data, together with volumes calculated from geophysical and stratigraphic data, indicate that an unrealistic proportion of the carbonate-dominated McArthur Group would have been required to source sufficient base metal to form the HYC mineralised system. At least a major portion of the metals contained in HYC are inferred to have originated deeper in the basin. Automatic identification of the location and magnitude of unconformities on geological maps was used to suggest the location and size of active subbasins at the time of the formation of HYC mineralisation. HYC's situation at the northeastern edge of one of these sub-basins is consistent with topographic and bounding growth fault control on the palaeohydrogeological regime that focused mineralising fluids in the vicinity of the deposit. Petrophysical measurements from the Mount Isa Basin indicate that the only highly magnetic units are the Eastern Creek and Kamarga Volcanics. All other units, including Fiery Creek Volcanics basalts, are at most weakly magnetic. Densities of Proterozoic sedimentary units are largely a function of the relative proportion of carbonate and siliciclastic material. The same, albeit weaker relationship was observed with respect to sonic velocity and resistivity, though acoustic impedance contrasts between basin units remain significant. Pb-Zn ore at Lady Loretta has a high density contrast with the highly pyritic layers enclosing it, which in turn are significantly denser than the host dolomitic siltstone. The Lady Loretta ore is also much more chargeable than any of its host units, but there is little resistivity contrast between the ore and host carbonaceous siltstones. Gravity and IP methods are indicated as most likely to distinguish base metal mineralisation from other conductive bodies in this region. Airborne magnetic and radiometric data flown over the Lady Loretta region were processed and interpreted to identify areas of anomalous response with respect to the 'normal' signatures of individual basin units: Integration of magnetic, radiometric and lithogeochemical data from the Eastern Creek Volcanics has enabled geophysical identification of alteration phases. Magnetite destruction, Fe depletion and possibly K depletion, consistent with chlorite + albite-type alteration, are associated with subdued magnetic signals adjacent to Zn-Pb (Lady Loretta) and Cu (Lady Annie) mineralisation. Anomalously high levels of K and quiet magnetic responses are apparent adjacent to the Mammoth Cu deposits. Variable K, Th and U signals from McNamara Group sequences are interpreted to indicate the location and nature of syn-depositional basin structures and sediment source regions, including evidence of anomalous sub-basin development represented by the Lady Loretta host sequence. The structure and volume of basin units in the Lady Loretta area were inferred from geophysical modelling. Basin structures in this region have been overprinted to varying degrees by the lsan Orogeny. In lower strain areas, there is a WNW trend in pre-Isan Orogeny structures. The area north of the Redie Creek and Leopard Faults appears to have been persistently uplifted from Myally Subgroup through to early McNamara Group time, with a depocentre to the south during this period. The Lady Loretta deposit is situated on the north-western margin of this former depocentre, immediately adjacent to the uplifted area to the north. The proximity to mineralising fluid conduits in the form of bounding or former growth faults was probably a major factor in the formation of the Lady Loretta mineralisation. GIs and geophysics can be used at all scales in exploration for Zn-Pb mineralisation. Analysis of regional geophysics, geology and geochemistry in a GIS can identify provinces and regions containing the requisite source lithologies, basin architecture and trap rocks to generate major Zn-Pb deposits. More detailed analysis of geophysical data can distinguish areas where 'base metal-depleting' alteration may have originated mineralising fluids from potentially rich source rocks such as the Eastern Creek Volcanics. Geophysical analysis can also help to locate structures capable of focusing mineralising fluids in favourable areas and, with gravity and IP methods, directly detect Lady Loretta-style deposits.