The Duchess-Urandangi region covers the southern part of the Precambrian Mount Isa Inlier and small parts of the early Palaeozoic Georgina and Mesozoic Eromanga Basins. The Precambrian is represented by sedimentary rocks and volcanics which have been tightly folded about mainly northerly trending axes, extensively faulted, intruded by numerous granite plutons and mafic bodies, and regionally metamorphosed to the greenschist and amphibolite facies. Basement rocks, older than 1850 m.y., are overlain unconformably by an older cover sequence, between about 1810 and 1700 m.y. old, and a younger cover sequence, between about 1680 and 1600 m.y. old. The cover sequences were laid down mainly in shallow water, and are many thousands of metres thick. They were last deformed on a major scale between about 1600 and 1550 m.y. Two major north-trending fault zones, the Wonomo/Mount Annable/Mount Isa fault system in the west and the Pilgrim Fault Zone in the east, subdivide the region into three parts the western, central, and eastern areas. Correlations across these fault zones are generally uncertain. The igneous rocks are markedly bimodal; rocks of andesitic composition are rare. Five distinct geochemical suites of felsic volcanics are recognised; at least two of them are comagmatic with granite. Of the four granite batholiths present, one, the oldest, has I-type characteristics, and the others are mainly uranium-rich A-types. Most mafic lavas and intrusives resemble continental tholeiites in their geochemistry. A regional gravity high corresponding to the Mount Isa Inlier is characterised in the region by elongate Bouguer anomalies trending north-south, parallel to geological trends; relative highs are mainly over metasediments and mafic rocks, and lows are mainly over granites. Broad aeromagnetic zones correlate well with the known geology. Most mafic volcanics and intrusives and some metasediments, felsic volcanics, and granites are significantly magnetic. Regional radiometrics are strongly influenced by the presence or absence of uranium-rich granites. The Precambrian rocks are hosts to numerous, mainly small deposits of copper, gold, silver, lead, zinc, cobalt, tungsten, uranium, silica, and calcite, many of which have been exploited. Most copper deposits are localised along shear zones, especially in carbonaceous slates, calc-silicate rocks, and metabasalt. Most of the gold has been obtained as a by-product of copper-mining, as at the Duchess mine, the most productive in the region, from which 27 717 t Cu (average grade 12.3%) and 69 474 g Au were produced between 1906 and its closure in 1920. Silver ore has been obtained from two mines and cobalt from one. Several stratiform lead-zinc-copper deposits, at present uneconomic, occur within the Kuridala Formation and Soldiers Cap Group; the best known deposit is that at the Pegmont prospect. Major deposits of phosphate rock occur within the Palaeozoic Georgina Basin.The geological evidence suggests that the Mount Isa Inlier evolved during the Proterozoic in an intracratonic tectonic setting, rather than at a continental margin. Since about 1500 m.y. ago the inlier has been an essentially stable tectonic unit.
