Ian Withnall

ABSTRACT Fe-rich tholeiites intruded the lower Etheridge Group of the Georgetown Inlier, north Queensland during the late-Paleoproterozoic (ca. 1685–1640 Ma). Emplacement of the tholeiites occurred during a prolonged period of rift-related sedimentation associated with extension. The tholeiites comprise two lithologically distinct units: the extrusive Dead Horse Metabasalt (DHM), a conformable 1000 m-thick series of fine- to coarse-grained basalts and pillow lavas, and the intrusive Cobbold Metadolerite (CMD), a multitude of sills and lesser dykes that range up to 500 m thick.The DHM and CMD are comagmatic, and have compositions typical of relatively evolved, low-K, Fe-rich continental (rift) tholeiites. Immobile major and trace element patterns suggest that the tholeiites were derived from a depleted upper mantle source (initial ɛNd values range between +2.6 and +5.3). The parental melts evolved along a trend of increasing Fe-enrichment (Fe2O3* 6.81–21.11 wt%) via closed-system fractionation, with little or no replenishment. Several samples show distinct REE patterns, consistent with limited crustal contamination. Trace element and isotopic data show that the crustal contaminants were derived from the host metasediments of the lower Etheridge Group.The age and geochemical characteristics of the Etheridge Group Fe-rich tholeiites show striking similarities to similar mafic sills in the southeastern Curnamona Province around Broken Hill in western New South Wales, and to basaltic lavas and dolerites of the Soldiers Gap Group in the eastern edge of the Mt Isa Block (some 400 km west of the Georgetown Inlier), suggesting a spatial link between these terranes during the Paleoproterozoic. They may have formed along a more than 2000 km segment of a ca. 1685–1640 Ma volcanic passive margin, tectonically and magmatically akin to the modern East Greenland margin.

The Balcooma Metavolcanics, in the eastern part of the mainly Proterozoic Georgetown Province, consist mostly of felsic metavolcanic and metasedimentary rocks. They were intruded by small microgranite and granite plutons, and porphyry dykes and sills before being deformed. Amphibolite facies metamorphism accompanied D1 and D2. The Balcooma Mylonite Zone separates the Balcooma Metavolcanics from the Proterozoic Einasleigh Metamorphics to the

Small mesothermal vein quartz‐gold‐base‐metal sulfide deposits from which some 20t of Au‐Ag bullion have been extracted, are the most common gold deposits in the Georgetown region of north Queensland—several hundred were mined or prospected between 1870 and 1950. These deposits are mostly hosted by Proterozoic granitic and metamorphic rocks and are similar to the much larger Charters Towers deposits such as Day Dawn and Brilliant, and in some respects to the Motherlode deposits of California. The largest deposit in the region—Kidston (>138t of Au and Ag since 1985)— is substantially different. It is hosted by sheeted quartz veins and cavities in brecciated Silurian granite and Proterozoic metamorphics above nested high‐level Carboniferous intrusives associated with a nearby cauldron subsidence structure. This paper provides new information (K‐Ar and Rb‐Sr isotopic ages, preliminary oxygen isotope and fluid‐inclusion data) from some of the mesothermal deposits and compares it with the Kidston deposit. All six dated mesothermal deposits have Siluro‐Devonian (about 425 to 400 Ma) ages. All nine of such deposits analysed have δO quartz values in the range 8.4 to 15.7‰. Fluid‐inclusion data indicate homogenisation temperatures in the range 230–350°C. This information, and a re‐interpretation of the spatial relationships of the deposits with various elements of the updated regional geology, is used to develop a preliminary metallogenic model of the mesothermal Etheridge Goldfield. The model indicates how the majority of deposits may have formed from hydrothermal systems initiated during the emplacement of granitic batholiths that were possibly, but not clearly, associated with Early Palaeozoic subduction, and that these fluid systems were dominated by substantially modified meteoric and/or magmatic fluids. The large Kidston deposit and a few small relatives are of Carboniferous age and formed more directly from magmatic systems much closer to the surface.

The Anakie Metamorphic Group of central Queensland is a multiply deformed sequence of psammitic to pelitic metasedimentary and mafic igneous rocks. Although the depositional age is still uncertain, this paper presents K‐Ar dates from four samples which indicate that the sequence was deformed at about 500 Ma in the Middle Cambrian, and thus forms part of the Delamerian‐Ross Orogen of

An unknown sedimentary sequence was first recorded during a Geoscience Australia/ Geological Survey of Queensland/ pmd*CRC deep seismic reflection survey in the Mount Isa Inlier and adjacent undercover terrains, during 2006/07. The sequence occurs unconformably underneath the Carpentaria Basin succession in the Julia Creek area, east of Cloncurry in north Queensland, and is named the Millungera Basin. A section through the basin is recorded along seismic line 07GA–IG1, recorded between north of Cloncurry to east of Croydon. In this section three internal sequences are noted—with two strongly reflective units separated by a poorly reflective unit. As well as deep crustal seismic reflection profiles, magnetotelluric profiles were collected along the same traverse. These data show a moderately conductive Millungera Basin underlying the strongly conductive Carpentaria Basin. Zones of limited reflectors beneath the basin in the seismic sections have been interpreted as granites, raising ...

... These rocks, separated from the Eina-sleigh Metamorphics by a mylonite zone and younger granite bodies, have been studied in the Greenvale area (Arnold and Rubenach, 1976 ... W ' e > S o 0 cd ki ki & hi t; > 2 S e0) .S . ! > 0 0 k' lumu4-> S'? ^<. 3 WS 'O e -^ -i3 as &, ki Ucu 0 cfl ...

The Balcooma Metavolcanics, in the eastern part of the mainly Proterozoic Georgetown Province, consist mostly of felsic metavolcanic and metasedimentary rocks. They were intruded by small microgranite and granite plutons, and porphyry dykes and sills before being deformed. Amphibolite facies metamorphism accompanied D1 and D2. The Balcooma Mylonite Zone separates the Balcooma Metavolcanics from the Proterozoic Einasleigh Metamorphics to the

Introduction The cornerstone of plate tectonic models for the New England Orogen over the past quarter of a century was that through Late Devonian and Early Carboniferous time, the New England Orogen was a classic convergent continental margin with parallel west to east belts representing volcanic arc, forearc basin and accretionary wedge assemblages. Bryan et al. (2001) have questioned this interpretation by proposing that the rocks attributed to a forearc basin setting actually formed in a backarc basin, and that no arc volcanics of Devonian or Carboniferous age are ...