Publication is an abstract

This report presents results of a reconnaissance-scale 40Ar/39Ar geochronological study of gold prospects in the central Gawler Craton. Prospects included in the study were: Tarcoola, Tunkillia, Barns, Weednanna and Nuckulla Hill. The aim of the study was to help constrain the age of mineralisation within individual prospects, and then to consider to what extent gold mineralisation throughout the central Gawler Craton was temporally linked. The age results are seen as a first step in testing the idea of a common origin for the prospects, and thereby testing the concept of a "Central Gawler Gold Province". Nineteen step heating experiments have been completed for samples from gold prospects in the central Gawler Craton. The majority of these samples are sericites from alteration zones spatially associated with gold mineralisation. Sixteen of the nineteen samples yield relatively well-behaved age spectra with the majority of the gas having apparent ages within uncertainty of the age bracket 1570 Ma to 1590 Ma, i.e., contemporaneous with Gawler Range Volcanics (GRV) and Hiltaba magmatism. In the case of the Tarcoola Goldfield, a hornblende age from a mafic dyke and geological relationships between this dyke and cross-cutting sericitic alteration tightly bracket the age of sericitic alteration at ~1580 Ma. Sericite 40Ar/39Ar ages from this deposit therefore appear to closely approximate the time of sericite crystallisation. Interpretation of age data from the other central Gawler Gold prospects is less clear at this stage. While the majority of the new 40Ar/39Ar data are consistent with gold mineralisation at ~1580 ± 10 Ma, the sericite 40Ar/39Ar data can also be interpreted as recording thermal resetting by Hiltaba and GRV magmatism. In the absence of independent evidence for maximum ages, the sericite 40Ar/39Ar ages should therefore be regarded only as minimum constraints on the timing of alteration. A simple first order conclusion is, therefore, that sericitic alteration occurred either contemporaneous with GRV and Hiltaba magmatism or earlier. It should be noted that even if the 40Ar/39Ar ages are interpreted as sericite alteration ages, the relationship between such alteration and gold mineralisation remains to be clearly established at most of these prospects. Addressing this question should be an important component of future studies. It is recommended that any future geochronology should be attempted only after more detailed petrologic studies on individual prospects have been undertaken. These studies are required to establish in more detail the nature and origin of alteration, the relationship between alteration and gold mineralisation, and evaluate the possibility of multiple alteration and/or mineralising events. Detailed studies at prospect-scale may identify critical relative-timing relationships that could be exploited in focussed geochronology studies to augment the reconnaissance-level results presented here. In particular, maximum age constraints for mineralisation are required at most of the prospects to complement the minimum age constraints reported here. As demonstrated at Tarcoola, maximum age constraints in some situations may be obtained from the age of dykes that are cross-cut by alteration and mineralisation. Such "second-phase" geochronology need not necessarily utilise the 40Ar/39Ar method, but may be more appropriately achieved via U-Pb or Re/Os analyses, depending on the particular questions being addressed.

A detailed mapping and geochemical and geochronological study of the Hiltaba Suite in the Tarcoola Region, South Australia, shows significant greater variation in composition than is allowable under the definition of Suite. By extending this work into the rest of the Gawler Craton using existing datasets, the Hiltaba ‘suite’ can be shown to be co-magmatic with the Gawler Range Volcanics. Together the magmatic rocks are grouped as the Gawler Range-Hiltaba Volcano-Plutonic event, and the intrusives themselves grouped as the Hiltaba Association Granitoids. A four-fold Supersuite classification is applied to both the intrusive and extrusive units. A great variation in compositions within the GRHVP reflects source and emplacement conditions. These variations can be correlated to different mineralisation styles observed in the Central Gawler Gold Province and the Olympic Copper-Gold Province.

The Tanami region is one of Australia?s premier Proterozoic gold provinces, having already produced ~150 t of gold, and still has high exploration potential. This region contains more than 60 gold occurrences including the Dead Bullock Soak, Groundrush and The Granites gold mines as well as several significant gold prospects (Coyote, Crusade and Kookaburra). The Callie deposit (>5 Moz Au total resource) located in the Dead Bullock Soak goldfield is currently the largest mine in this region. Previous studies of the mineral systems associated with the gold deposits in the Tanami region indicate that they formed over a range of depths and were hosted in both greenstone and sedimentary units. Fluid inclusion studies have shown that the ore-bearing fluids were generally of low to moderate salinity with varying amounts of CO2?N2?CH4. Trapping temperatures ranged from 220 to 430 ?C. In order to determine the extent of these gold mineral systems, we have investigated the chemistry of the fluids in regional quartz veins that outcrop in both the Tanami, Birrindudu and northern Arunta. 40Ar/39Ar dating of veins containing mica was also carried out to determine the timing of the veins with respect to the mineralisation in the Tanami region. Epithermal veins outcrop along the southern margin of the Wiso Basin, the northern Arunta, the western Tanami and in the Birrindudu region. Two populations of fluid inclusions were observed in the epithermal veins: a low salinity fluid (<1 wt. % NaCl eq), and a high salinity fluid (>18 wt. % NaCl eq). No gases were detected in either type of fluid inclusion and both homogenised over the range from 120 to 180 ?C. Regional E-W trending mesothermal quartz veins outcrop in the southern Tanami region and a distinctive zone of ENE trending quartz veins outcrop in the northern Arunta whereas both NW trending and ENE trending veins occur in the Birrindudu region. Two populations of fluid inclusions were also observed in these mesothermal quartz veins. The first contained low salinity fluids with CO2>CH4?(N2?graphite). These inclusions homogenised between 320 and 360 ?C. The second population contained high salinity fluids with no detectable gases and they homogenised between 120 and 230 ?C. 40Ar/39Ar dating of quartz veins containing mica showed a distinct difference in the age of the veins in the Tanami and northern Arunta. Mesothermal veins in the Tanami region had ages ranging from 1700 to 1741 Ma while quartz veins in the northern Arunta gave ages ranging from 1432 to 1518 Ma. This suggests that these vein sets formed from two separate fluid flow events.

Legacy product - no abstract available

The Ranger deposit is one of Australia's largest known uranium resources, with current open pit mining of the No. 3 orebody and a total resource of 109,600 tonnes of U3O8 grading 0.08% at this orebody (ERA, January 2011). This unconformity-related deposit is hosted by Paleoproterozoic metasedimentary rocks of the Cahill Formation which is unconformably overlain by sandstones of the Kombolgie Formation. A maximum depositional age of ~1818 Ma is inferred for the sandstones, based on the presence of the Nabarlek Granite of this age in the basement beneath the Kombolgie Formation. Most mineralisation occurs within a largely stratabound shear and breccia zone and is associated with intense proximal chlorite and distal white mica alteration. The Kombolgie Formation is weakly deformed, faulted and weakly chloritised above the mineralisation.

The Moonta Domain forms the southern part of the Olympic Cu-Au province on the eastern margin of the Gawler Craton. Historical production comprises over 330,000 tonnes of Cu from vein and shear-hosted mineralisation in the Moonta-Wallaroo district. The domain basement comprises metasediments and metavolcanics of the Palaeoproterozoic Wallaroo Group (~1760?1740 Ma) which were deformed and metamorphosed to upper greenschist-amphibolite facies during the Kimban Orogeny (~1720 Ma). These rocks were further deformed and intruded by granitoids and minor mafic intrusions of the Hiltaba Suite between about 1600 Ma and 1575 Ma. There is a close spatial association of high temperature Fe-Na-Ca-K metasomatism of the Wallaroo Group and Hiltaba Suite intrusions. Conor (1995) termed the most strongly altered rocks the Oorlano Metasomatites, although metasomatic mineral assemblages within this rock association vary widely. Intense albite-actinolite-magnetite ? carbonate ? epidote ? pyrite alteration of metasediments is strongly associated with the contact zones of Hiltaba Suite granites, particularly the Tickera Granite. More distal albitisation of the Wallaroo Group is common but is not generally associated with significant sulphides. Biotite ? albite ? magnetite ? quartz ? apatite ? monazite ? tourmaline alteration is commonly associated with pyrite ? minor chalcopyrite, and is particularly widespread south of Moonta where numerous magnetic and non-magnetic Hiltaba Suite granitoids (previously grouped as Arthurton Granite) intrude the Wallaroo Group. Late chlorite and K-feldspar alteration is typically of restricted extent, but may also be associated with sulphides. Biotite-rich alteration typically forms irregular magnetic anomalies, including a major 5 x 15 km alteration zone near Weetulta, and possibly a large area (~30 km x 40 km) of strongly magnetic rock beneath Spencer Gulf. Fluid inclusion data indicate that highly saline, multi-cation fluids are associated with the alteration. Preliminary U?Pb SHRIMP dating of hydrothermal monazite from biotite-rich alteration in the Weetulta and Wallaroo areas yields ages of approximately 1585 Ma and 1620 Ma respectively. The Weetulta district data indicate a close temporal relationship of the biotite alteration and Hiltaba Suite magmatism. However, the older Wallaroo district age suggests hydrothermal activity may have commenced prior to intrusion of Hiltaba Suite granites. Regional metamorphic and alteration characteristics of the Moonta Domain are similar to those of the Fe-Cu-Au mineral province of the Mt Isa Inlier Eastern Succession, where there are strong links between magmatism, regional albitisation, and Fe-Cu-Au mineralisation (eg., Oliver et al., 2001). Biotite-magnetite metasomatism commonly occurs proximal to major Fe-Cu-Au ore deposits in the Mt Isa Eastern Succession. The shear-hosted Cu lodes and associated alteration at Wallaroo may be an analogue in the Moonta Domain. However, apart from some very minor drill intersections in prospects in the Weetulta district, no other significant Cu-Au mineralisation associated with biotite-magnetite alteration has yet been discovered in the Moonta Domain. Given that most of the Proterozoic basement of the Moonta Domain is concealed by up to 100 metres of Neoproterozoic to Cainozoic sediments and remains largely untested by drilling, the potential for discovery of Ernest Henry-style Fe-Cu-Au deposits in the Moonta Domain remains high.

The mid to late Archaean Yule Granitoid Complex lies in the westernmost part of the East Pilbara Granite-Greenstone Terrane, and is dominated by generally potassic, felsic (66-76% SiO2), calc-alkaline granitoids, with Sr-depleted, Y-undepleted compositions and mostly moderate to large negative Eu anomalies. These characteristics all indicate the significant involvement of a felsic crustal source. Typical Archaean Tonalite-Trondhjemite-Granodiorite (TTG) series are not found, even amongst the very oldest (>3.4 Ga) granites. The most TTG-like granitoids are post-3270 to 2930 Ma in age, are confined to the westernmost part of the complex, and share some similarities with granites found in the West Pilbara Granite-Greenstone Terrane. An abundance of post 2950 Ma granite also contrasts the Yule Granitoid Complex with complexes in the eastern part of the East Pilbara Granite-Greenstone Terrane, where such rocks are uncommon. This feature, and the presence of c. 2945 Ma Mungaroona Granodiorite (high-Mg diorite suite) in the western part of the Yule Granitoid Complex, is probably related to the tectonic development of the adjacent Central Pilbara Tectonic Belt.

No abstract available

Detrital zircon from sandstone bodies intersected in three recent exploration wells on the North West Shelf were analysed and dated using the SHRIMP (Sensitive High Resolution Ion Microprobe) at Geoscience Australia to test the technique as a tool for understanding the provenance and sediment transport pathways of reservoir facies in the region. Chevron, Hess and Santos, the operating companies for exploration permits WA- 365-P, WA-390-P and WA-281-P respectively, collected 3-5 kg of cuttings from the wells Guardian-1 and Hijinx-1 (Triassic Mungaroo Formation on the Exmouth Plateau of the Carnarvon Basin) and Burnside-1 (Jurassic Brewster Sands from the Browse Basin). All three wells were drilled in 2009-2010. Samples were prepared at Geoscience Australia with 70-80 zircon grains randomly selected for analysis following standard data acquisition and processing procedures to provide a statistically meaningful representation of detrital ages in each sample.