In the Georgina Basin, the Toko Syncline is considered the most prospective region for petroleum exploration. The syncline contains up to 5000 m of Middle Cambrian to Middle Ordovician sediments and has been the site of six petroleum exploration wells, three of which showed traces of gas, oil, and solid bitumen. Geochemical data for cores from two stratigraphic holes, GSQ Mount Whelan Nos. I and 2, and data from four exploration wells, held on open file at BMR, are used to evaluate the organic maturation and rock potential of the sediments in the Toko Syncline. Source rocks are not recognised in the Ordovician; however, source rocks of generally good quality are present in the Middle and Upper Cambrian. While doubts do exist over the stratigraphic correlations for the wells involved in this study, the Middle Cambrian Marqua beds are considered to be the best source interval. Maturity for oil generation is achieved in the Upper Cambrian to Lower Ordovician Ninmaroo Formation at the Mount Whelan location. However, in the deeper western area, where the better exploration prospects are likely to exist, maturity in Mirrica No. 1 is reached in the Lower to Middle Ordovician Carlo Sandstone. Over-maturity for oil generation, but still in the gas generation phase, is generally observed in the Middle Cambrian. An oil stain in the Lower Ordovician Coolibah Formation in GSQ Mount Whelan No.2 (also described as bitumen plugging porosity) is interpreted as having originated in and having migrated from the Cambrian source rock intervals.

Recent mapping in the Duchess-Urandangi region, covering most of the southern part of the Mount Isa Inlier, has shown that felsic and mafic volcanics occur in most Precambrian stratigraphic units exposed there. Five distinct geochemical suites of felsic volcanics are recognised, each suite probably having been derived from a chemically unique crustal source region. The Leichhardt suite is distinguished by high Sr, and low Zr, Nb and Y abundances, whereas the Argylla suite has low Sr and very high Zr and Nb concentrations. The Bottletree suite is characterised by high Ba, Sr, Zr, and Nb, the Duchess-Corella suite by low Sr and Zr, and high Nb and Y contents, and the Carters Bore suite by high K2O and low Al2O3, Na2O, CaO, Pb, Sr, and Ba. Most of the analysed mafic volcanics from the region are chemically similar, and there is no systematic change in composition from west to east. The mafic volcanics are characterised by low incompatible-element contents and are similar chemically to continental tholeiites of the Karroo province of southern Africa. Silica values for the volcanics and their intrusive equivalents show a well-defined bimodal distribution, probably indicating an extensional crustal regime.

At the Mammoth mine, epigenetic copper mineralisation occurs within the Myally Subgroup of the Haslingden Group, an arenaceous sequence that both includes and overlies basic volcanics. A basic flow in the Myally Subgroup is altered, being enriched in potassium and copper for up to 200 m from the mineralisation. However, underlying metabasalts of the Eastern Creek Volcanics up to 600 m from the mineralisation are depleted in iron and copper and enriched in potassium relative to those further away. Evidence is presented that the Mammoth ore represents copper leached from the Eastern Creek Volcanics and transported along the Mammoth-Mammoth Extended Fault system. This hypothesis is similar to one previously proposed for the copper mineralisation at Mount Isa, although there are differences in the age and lithology of the host rocks in the two areas. Thus, rocks adjacent to major faults transgressing the Eastern Creek Volcanics must be considered prospective for copper mineralisation.

Structural, textural, mineralogical, and geochemical properties of gossans associated with lead-zinc mineralisation in the Carpentarian Mount Isa Inlier of northwest Queensland have been studied. Mineralisation at Mount Isa, Hilton, and Lady Loretta is associated with abundant stratiform pyrite, which, upon oxidation, has formed prominent gossanous ridges at each deposit. The gossans, which have a laminated structure and occasional boxworks and casts after pyrite, generally carry only minor to trace levels of target and pathfinder elements. Where orebodies were close to the surface, cerussite and other secondary lead minerals are abundant, but Zn and Ag have been substantially leached from surface gossans. Immature gossans are found on the Dugald River and Pegmont dposits, both of which have low iron sulphide contents and a partially truncated profile. At Dugald River, the normally mobile elements (e.g. K , S, Zn, Cu, Cd, Tl) have been retained because of reaction between acid solutions and carbonate-rich wall rocks. The Pegmont mineralisation occurs in a quartz-magnetite-garnet-apatite banded iron formation, which, on oxidation, has given rise to a Mn-, Pb-, and Zn-rich gossan with abundant plumbogummite, pyromorphite, and coronadite. Gossans and ironstones from elsewhere within the Paradise Creek Formation, Soldiers Cap Group, Mount Isa Group, and Answer Slate often have similar laminated structures and mineralogy, but their variable geochemical signatures make direct comparison with other deposits difficult. Various statistical techniques have been used to evaluate these geochemical data in relation to gossans and ironstones of known association from throughout the Mount Isa Inlier. Although reflecting some known mineralogical and elemental associations, neither correlation coefficients nor R-mode factor analysis are of use in assigning group membership, because of the diverse geochemical characteristics of the ferruginous outcrops. Stepwise discriminant analysis using Pb, P, Ba, As, Zn, Mn, Co, S, and Sb was found to classify all gossans and ironstones into six geologically distinct groups.

Magnetotelluric techniques have been used to investigate structural trends in the McArthur Basin . Observations were made at 34 sites, extending 450 km across the Wearyan Shelf, the Batten Fault Zone, and the Bauhinia Shelf. For sites on the Wearyan Shelf, the orthogonal components of resistivity are generally similar, suggesting continuous horizontal strata and uniform basement depths. However, lateral changes in resistivity, evident on the Bauhinia Shelf, become extreme in the Batten Trough. For sites near the Emu Fault, the two components diverge at long periods, indicating a major change in structure with a pronounced vertical contact. Resistivities associated with the Tawallah Group appear distinct enough to show that no appreciable thickness of McArthur Group can be present east of the Emu Fault. The data are consistent with geological models based on the assumption that the Batten Trough formed as a syndepositional graben with rapid changes in depositional thickness at the boundary faults.