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  • New chemical analyses of relatively low-grade metabasalt from the Eastern Creek Volcanics, 120-150 km north of Mount Isa, Queensland, show them to be continental tholeiites. A 2-stage model of fractional crystallisation is proposed to explain the major and trace element variation in the suite. The uncommonly high Cu content of the metabasalt (about 200 ppm) is attributed to concentration of an immiscible sulphide phase during fractionation. Examination of all available chemical data has led to the recognition of 5 types of alteration. The Cu content is depleted in metabasalts that are anomalously enriched in K2O, MgO, or CO2, but is not affected in metabasalt enriched in CaO or Na2O. This Cu depletion supports earlier models that attribute Cu mineralisation at Mount Isa to leaching of Cu from the Eastern Creek Volcanics and its redeposition in favourable pyritic, dolomitic sediments of the Mount Isa Group. The ore localisation has been associated with brecciation and appears to depend on the juxtaposition of the Mount Isa Group and the Eastern Creek Volcanics largely by faulting.

  • Mafic igneous rocks were emplaced during deposition of the Robertson River Subgroup, part of the early or Middle Proterozoic Etheridge Group in the Georgetown Inlier. They comprise the Dead Horse Metabasalt and the Cobbold Metadolerite, which forms a multitude of sills both above and below the Dead Horse Metabasalt. They are metamorphosed to lower greenschist to granulite facies, and are probably equivalant to amphibolite and mafic granulite in the Einasleigh Metamorphics. The rocks are geochemically similar to modern mid-ocean ridge basalts (MORB) . However, sedimentological evidence indicates that the Etheridge Group was deposited in shallow water on a shelf or in an epicontinental sea. The mafic rocks may, therefore, be similar to the low-K flood basalts of Greenland and Baffin Island, although the low K could be partly due to depletion during metamorphism. Except for the much lower K, they are similar to the relatively incompatible-element depleted Karoo basalts and dolerites. They are also similar geochemically to some Proterozoic mafic rock suites from the Mount Isa and Pine Creek areas in northern Australia. The mafic rocks from the Etheridge Group may be the expression of convective mantle upwelling, which could have produced extension of the Proterozoic crust and the formation of a shallow epicontinental sea in which the Etheridge Group was deposited. No evidence of any associated rifting occurs in the Georgetown Inlier.

  • The recognition of regional oxygen-isotope depletion patterns in high-level igneous rocks provides a means to discriminate areas potentially prospective for low sulphidation (adularia-sericite type) epithermal gold mineralisation. The coincidence of an extensive regional oxygen-isotope depletion pattern over most of the northern Drummond Basin with a recently discovered epithermal district is consistent with a similar association for younger, world-class epithermal districts in the United States. Reconnaissance whole-rock oxygen-isotope data for Permo-Carboniferous volcanic rocks in the northern Coen Inlier indicate an area of isotopic depletion that correlates with regional stream-sediment geochemical anomalies normally associated with epithermal deposits. The data suggest that the northern Coen Inlier is a region of high epithermal potential worthy of more systematic exploration. In contrast, extensive exploration within and around the Featherbed Cauldron Complex has failed to define any significant epithermal mineralisation. Whole-rock oxygen-isotope values are predominantly near normal, and isotopic depletion is confined largely to the Late Carboniferous volcanic rocks in the southern areas of the complex, particularly along caldera margins where major structures provided pathways for fluid circulation. The data are consistent with earlier observations that the Late Carboniferous sequence is more closely associated with hydrothermal activity than the Early Permian volcanics, and that meteoric fluids were focussed through these major structures, at least during the waning stages of igneous activity.

  • The Charters Towers district in semi-arid tropical north Queensland has large areas of transported and residual cover. It is host to several major gold and base-metal deposits and has a long history of mineral exploration, but there have been few studies of its regolith. As future exploration will target mineralisation under cover, an understanding of the regolith is essential for interpretation of geochemical and geophysical data. Transported regolith presents further complications. The Puzzler Walls, about 10 km east of Charters Towers, are a group of mesas capped by siliceous and ferruginous duricrusts developed on thick units of deeply weathered Tertiary sediments and Palaeozoic granitoids. The granitoids host mesothennal quartz-vein type gold mineralisation, commonly with associated sulphides, particularly galena. Analysis of ferruginous material from the mesas reveals two main types of iron accumulations: an older ferruginous gravel developed from the weathering of granodiorite, and a younger nodular ferricrete overlying Tertiary/Quaternary sediments. These materials are distinguished on the basis of their position in the landscape, micromorphology (petrology), geochemistry, and mineralogy. The older (Tertiary?) ferricretes have developed in situ on the granodiorite. They consist of ferruginous aggregates of the underlying mottled saprolite, formed by the relative accumulation of iron, as other, more soluble, elements were leached out. The younger (Tertiary-Quaternary?) nodular ferricretes consist of reworked or transported iron nodules coated by several generations of goethite rinds. These nodular ferricretes are interpreted as having formed by the accumulation of iron, precipitated from groundwater on valley sides or oxidation fronts. The geochemistry of both ferricrete types reveals enrichment of transition metals, some rare-earth elements, and elements commonly associated with gold mineralisation in the Charters Towers district (Pb, Sb, As). The last may be useful pathfinders in exploration. It still needs to be established whether this enrichment results from proximity to underlying mineralisation .

  • Volcanic and intrusive rocks of Permian age, ranging from basalt and andesite through granodiorite and dacite to rhyolite, are sparsely but non-randomly distributed in the western Georgetown Inlier, including newly recognised areas in the Croydon region. The basaltic to andesitic rocks are typical intra-plate transitional alkaline rocks, apparently genetically unrelated to the granodiorite/dacite to rhyolite group, which are a suite of fractionated I-type (and possibly A-type) rocks, lower in alkalis, derived from evolved (old) crustal source rocks. The Permian magmatism appears to have been controlled by northwest-trending, and some north-trending, major fractures. Carboniferous igneous rocks in the same region, although only slightly different in chemistry, include a much greater proportion of felsic ignimbrites relative to more mafic extrusive rocks, and are related to north-south aligned and/or elongated sag-type cauldron structures. Total volumes, relative proportions of mafic and felsic rocks, and, to some extent, their compositions, appear to be related to tectonic style.

  • Tightly folded migmatitic rocks, intruded by 1860 Ma granite and younger felsic and mafic dykes, are exposed in a band 95km long and up to 10km wide along the western part of the Kalkadoon-Leichhardt Belt. The migmatites are considered to represent the basement underlying Proterozoic cover rocks, the oldest of which are Ma felsic extrusives (Leichhardt Volcanics) about 1860 Ma old. The migmatites include thinly banded gneiss with mainly concordant leucosomes (metasediments), non-banded gneiss with wispy leucosomes (metavolcanics), and nebulitic granitic gneiss (meta-intrusives). Metamorphism and deformation of the migmatites took place before the intrusion of a cross-cutting granite dyke dated at 1860 ± 32 Ma by U-Pb zircon. Another U-Pb zircon age, 1850 ± 16 Ma, obtained for a migmatitic metadacite, is anomalously young, although within experimental error of a preferred migmatisation age of 1860 - 1870 Ma. Uplift rates of 2-5 mm a year are implied, to account for the inferred brief interval between migmatite formation and ensuing felsic volcanism.

  • Accurate modelling, using seismic and gravity data, has shown that the Donnybrook Gravity High in central Queensland is the result of a complex situation involving three discordant basins. The most significant is an old basin buried deep beneath the Drummond Basin and apparently deposited in a valley carved out of a thick sequence of Silver Hills Volcanics and related acid volcanics. The Drummond Basin wedges out westwards under the Galilee Basin and is bounded to the east by the same acid volcanics that subcrop near the Anakie Metamorphics. The illusion that the Donnybrook Gravity High is associated with the Donnybrook Anticline is the result of an intrabasement granite which introduces a large negative component, cancelling the western flank of a much broader gravity high. After the cancellation, the granite is represented by a low of only 30 ~m.s-2 The already complex situation is further complicated by a topographic feature that introduces ambiguity in the most crucial area of the interpretation . This factor demonstrates the need to combine density profiling with forward modelling.

  • The middle Miocene macropodoids of the Riversleigh, Bullock Creek, and Kangaroo Well local faunas are the oldest recorded assemblages of kangaroos. Five new species are described here. One, Bulungamaya delicata n. gen. and sp., is placed with Wabularoo naughtoni Archer, 1979 in the new potoroid subfamily Bulungamayinae. Gumardee pascua Ii n. gen. and sp. is placed within the Potoroinae. Balbaroo camfieldensis n. gen. and sp. and Balbaroo gregoriensis n. sp. are placed in a new macropodid subfamily, the Balbarinae. Galanarla tessel/ala n. gen. and sp. is placed in the Macropodidae , but cannot as yet be assigned to a particular subfamily. All the new species are from the Riversleigh local fauna, except B. camfieldensis from the Bullock Creek local fauna. The previously known Kangaroo Well macropodid appears to be referable to the genus Balbaroo. The Bulungamayinae is represented by forms which, although related to potoroines, have bulbous premolars and lophodont molars. The Balbarinae includes generally plesiomorphic forms that may have been ancestral to other macropodids. None of the macropodoid genera described here are known from more recent deposits. This suggests that a radical transition in the dominant kinds of macropodoids took place between middle and late Miocene times.

  • The Proterozoic I-type Kalkadoon and Ewen Batholiths and their comagmatic extrusive equivalents, the Leichhardt suite, form an association covering at least 5000 km2 in the central part of the Mount Isa Inlier. U-Pb zircon data and some Rb-Sr total rock data show that these rocks crystallised from melts emplaced between 1840 and 1870 m.y. ago and are the oldest dated igneous rocks in the Inlier. Chemically and isotopically, these granites are relatively uniform and, compared with most other Mount Isa granites , they have higher Sr and Al2O3 contents, and lower TiO2 , Zr, Nb, and Th contents . These chemical characteristics appear to be restricted to felsic igneous rocks known to be older than 1800 m .y. and may be useful in identifying the older felsic melts of the Mount Isa Inlier. The source for the rocks of the Kalkadoon- Ewen- Leichhardt association is estimated to have had an SiO2 content of 55-60 per cent. Relative to other large Palaeozoic and Mesozoic I-type batholiths elsewhere, this Mount Isa association is enriched in K20, Rb, Th, U, La, Ce, Zr, and Nb, and depleted in CaO, MgO, Ni, and Cr. The least isotopically disturbed granites of the association have relatively low initial 87Sr/86Sr ratios (about 0.704), which implies that the age of the source for these melts was not much older than the age of their emplacement. As chemically and isotopically similar granites occur in most Proterozoic areas of Northern Australia, it is inferred that during the period 1900-2100 m.y. a significant mantle differentiation event took place, during which large volumes of material were accreted to the base of the crust in these areas. Post-emplacement metamorphism and deformation , which have a maximum age of 1640 m.y. , caused significant textural and mineralogical changes in the Kalkadoon Batholith, but had a lesser effect on the Ewen Batholith . Igneous textures are commonly preserved in the Ewen Batholith, but the Kalkadoon Batholith, which has been metamorphosed from lower greenschist to upper amphibolite grade, shows significant isotopic disturbances.

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    The radiometric, or gamma-ray spectrometric method, measures the natural variations in the gamma-rays detected near the Earth's surface as the result of the natural radioactive decay of potassium (K), uranium (U) and thorium (Th). The data collected are processed via standard methods to ensure the response recorded is that due only to the rocks in the ground. The results produce datasets that can be interpreted to reveal the geological structure of the sub-surface. The processed data is checked for quality by GA geophysicists to ensure that the final data released by GA are fit-for-purpose. This radiometric potassium grid has a cell size of 0.00083 degrees (approximately 87m) and shows potassium element concentration of the Thomson Orogen, Qld, 2011 in units of percent (or %). The data used to produce this grid was acquired in 2011 by the QLD Government, and consisted of 298198 line-kilometres of data at 400m line spacing and 80m terrain clearance.