At the Sandpiper gold deposit in the Tanami region of northern Australia sericite is intimately intergrown with arsenopyrite in gold-bearing quartz veins and breccias, suggesting sericite crystallisation synchronous with gold-bearing fluid flow. This ore-stage sericite yields a 40Ar/39Ar plateau age of 1785 ± 16 Ma (1?? including all known systematic uncertainties). Recalculation using revised and more precise values for the 40K decay constants and the age of the Fish Canyon Sanidine standard shifts the age to 1792 ± 6 Ma (1???including all known systematic uncertainties). Given the possibility of post-mineralization isotopic resetting this age can be conservatively interpreted as a minimum constraint on the timing of gold deposition although, given local geological relationships and estimates for the argon retentivity of white mica, we consider complete isotopic resetting to be unlikely. The preferred interpretation is, therefore, that the sericite 40Ar/39Ar age indicates the timing of gold mineralization. The sericite age accords with a limited dataset of 207Pb/206Pb xenotime ages of ~1800 Ma from other gold deposits in the Tanami region, interpreted as mineralization ages. The agreement between independently-derived ages from several gold deposits lends support for a widespread gold-mineralizing event at ~1800 Ma in the Tanami region.

The magnitude of systematic uncertainties inherent in comparisons of Proterozoic 40Ar/39Ar- and U-Pb-based ages is illustrated via a consideration of age constraints on two contrasting styles of Palaeoproterozoic mineralization in northern Australia. Published 40Ar/39Ar ages for Au-Cu-Bi-mineralization in the Tennant Creek region range from1825 to 1829 Ma, apparently younger than the timing of deformation bracketed by 207Pb/206Pb zircon ages of igneous rocks at ~1850 Ma. Despite this apparent age offset, propagation of systematic uncertainties in the 40Ar/39Ar ages shows that they are indistinguishable from the ~1850Ma zircon ages. Recalculation using revised estimates of the 40K decay constant and age of the 40Ar/39Ar standard shifts the 40Ar/39Ar ages to 1847-1851 Ma, in excellent agreement with 207Pb/206Pb zircon ages from Tennant Supersuite igneous rocks and with local geological constraints. To the south of Tennant Creek, in the Davenport Ranges, numerous smallWprospects have been interpreted to be coeval with post-tectonic granites of the Devils Suite at ~1710-1720 Ma. New 40Ar/39Ar ages from muscovite selvages on wolframite-bearing veins range from 1697 to 1703 Ma. These ages shift to 1711 to 1717Ma when recalculated using revised estimates for 40K decay constant and age of standard material, bringing them into agreement with existing 207Pb/206Pb zircon ages for Devils Suite granitoids, and consistent with field and geochemical interpretations. These examples highlight the importance of considering systematic sources of uncertainty when comparing radiometric ages determined via different isotopic methods, particularly in the Proterozoic and Archean parts of the timescale. Ongoingwork to calibrate the 40Ar/39Ar and U-Pb-based timescales will further enhance the application of multiple isotopic methods to constrain geological processes.

SHRIMP U-Pb detrital zircon results for five of the principal units within the Lake Frome region

The Pine Creek Orogen located on the exposed northern periphery of the North Australian Craton, comprises a thick succession of variably metamorphosed Palaeoproterozoic siliciclastic and carbonate sedimentary and volcanic rocks, which were extensively intruded by mafic and granitic rocks. Exposed Neoarchaean basement is rare in the Pine Creek Orogen and the North Australian Craton in general. However, recent field mapping, in conjunction with new SHRIMP U-Pb zircon data for five granitic gneiss samples, have identified previously unrecognised Neoarchaean crystalline crust in the Nimbuwah Domain, the eastern-most region of the Pine Creek Orogen. Three samples from the Myra Falls and Caramal Inliers and from the Cobourg Peninsula have magmatic crystallisation ages in the range 2527-2510 Ma. An additional sample, from northeast Myra Falls Inlier, yielded a magmatic crystallisation age of 2671 - 3 Ma, the oldest exposed Archaean basement yet recognised in the North Australian Craton. These results are consistent with previously determined magmatic ages for known outcropping and subcropping crystalline basement some 200 km to the west. A fifth sample yielded a magmatic crystallisation age of 2640 - 4 Ma. The ca. 2670 and ca. 2640 Ma samples have ca. 2500 Ma metamorphic zircon rims, consistent with metamorphism during emplacement of the volumetrically dominant ca. 2530-2510 Ma granites and granitic gneisses. Neoarchaean zircon detritus, particularly in the ca. 2530-2510 Ma and ca. 2670-2640 Ma age span, are an almost ubiquitous feature of detrital zircon spectra of unconformably overlying metamorphosed Palaeoproterozoic strata of the Pine Creek Orogen, and of local post-tectonic Proterozoic sequences, consistent with this local provenance.

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.

New reconnaissance studies of Au mineralisation, alteration, geochronology, and palaeo-fluids in the central Gawler Craton support the existence of a major Au metallogenic province. The results show that Au mineralising systems were active along a >300 km belt at similar times (~1570 Ma to ~1590 Ma; Fraser et al., 2004, this volume) during the early Mesoproterozoic. The scale of this metallogenic belt is a positive indicator for Au prospectivity. Recent company results suggest that the potential for economic resources is high. The main similarities in the Tarcoola, Tunkillia, Nuckulla Hill, and Barns Au systems are: <ul><li>Gold occurs in both disseminated and veinlet-hosted styles. </li> <li>The main host rocks are ~1680-1720 Ma granitoids and, at Tarcoola, metasedimentary rocks (partly carbonaceous).</li> <li>Hydrothermal alteration is characteristically zoned around the Au mineralisation, with intense sericite-pyrite alteration and quartz veining proximal to Au mineralisation, and chlorite, epidote, hematite alteration distal (metres to 100s of metres) to mineralisation.</li> <li>Deformation was synchronous with the proximal sericitic hydrothermal alteration and mineralisation and was generally of brittle to brittle-ductile style including shearing.</li> <li>Au is associated with disseminated and vein-hosted pyrite and minor to trace galena, sphalerite, and chalcopyrite.</li> <li>Iron oxides have very low abundance in mineralised zones, where they are present mainly as haematite, manifest as de-magnetised zones in magnetic images.</li> <li>Hydrothermal white micas at Tarcoola, Tunkillia, Barns, and Weednanna all yield similar Ar-Ar ages of ~1570-1600 Ma. Sericitic alteration and Au mineralisation are interpreted to have formed within this period.</li> <li>Multiple fluids were present in some systems, but one fluid-type occurs in each of the Au prospects: a low-moderate salinity (up to 10 eq. wt.% NaCl; mostly 3-6 %) fluid with homogenisation temperatures mostly in the range of ~200 C to 300?C and trapping temperatures of ~300-400 C. This fluid is commonly associated with CO2-rich fluid inclusions.</li> <li>Galena yields broadly similar Pb isotope compositions at Tarcoola and Tunkillia. </li></ul> Differences between the Au mineralised systems at Tarcoola, Tunkillia, Nuckulla Hill, and Barns, such as the intensity of deformation, relative abundances of base metal sulfides, and sulfur isotope values, reflect differing local structural and lithostratigraphic settings. The Weednanna Au and nearby Mawson Au-Cu prospects bear some similarities to the other Au prospects of the central Gawler gold province, but we consider them to be possible hybrids of the early high-temperature alteration style of iron-oxide Cu-Au systems overprinted by Au hydrothermal systems. Late-stage epithermal-style quartz-carbonate-adularia veins in the Nuckulla Hill and Weednanna areas indicate potential for epithermal Au mineralisation in the region. Preliminary depth constraints suggest high-level crustal environments of mineralisation, possibly around 3-6 km deep, at temperatures up to 300-400 C. Spatial and genetic relationships with Hiltaba Suite magmatism remain cryptic, despite the broadly coeval timing of magmatism and Au mineralisation. The Au mineralisation style does not neatly fit deposit classes such as 'lode-gold', 'orogenic/mesothermal gold', 'intrusion-related gold', 'epithermal gold', or 'porphyry Au'.

This GSQ-GA geochronology record presents a compilation of 16 new zircon U-Pb SHRIMP geochronological results from the Mount Isa and south Nicholson region, Queensland. This data was collected through the collaborative GSQ-GA geochronology project in August 2008 as part of the National Geoscience Accord (NGA) and in support of ongoing geoscientific investigations and regional geological mapping by the GSQ in the Mount Isa and south Nicholson regions. In addition, an appendix contains Sm-Nd whole rock analyses of selected samples. The acquisition of the Sm-Nd data in Appendix A was funded wholly by Geoscience Australia. A separate report for each sample is presented, which contain a brief geochronological interpretation as well as information on sample location and geological context. Three samples were analysed from the south Nicholson Basin from the WESTMORELAND and LAWN HILL 250K mapsheets, with an additional sample (Westmoreland Conglomerate) taken from the base of the Macarthur Basin. The twelve remaining samples were taken from various units in the greater Mount Isa region from the CLONCURRY, DOBYNN, DUCHESS, MOUNT ISA and URANDANGI 250K mapsheets. One sample was submitted for SHRIMP analysis but which failed to yield sufficient zircons for further investigation (rhyodacitic porphyry from within the Corella Formation).

This Record contains zircon U-Pb geochronological data obtained between September 2001 and June 2002 on rocks from the Yilgarn Craton, Western Australia. In addition, data are presented for rutile from sample 2001969019A. Zircon from this sample was analysed in the first year of the project (Fletcher et al., 2001). Additional zircon data for this sample are also reported here.

Legacy product - no abstract available

This GA Record documents SHRIMP data co-funded (50/50) by GSNSW under the National Geoscience Agreement, and obtained during the 2008-09 financial year.