This invited keynote address was presented at the Sixth International Hutton Symposium, Origin of granites and related rocks, held at the University of Stellenbosch, South Africa, on 2-6 July, 2007. Felsic magmatism in Australia ranges in age from Paleoarchean to Mesozoic. Extensive episodes occurred in the Paleo-, Meso-, and Neoarchean, Paleo- and Mesoproterozoic, and Silurian-Devonian, and Carboniferous-Permian. Like the Archean elsewhere, the Archean Pilbara and Yilgarn Cratons in Western Australia are dominated by sodic felsic granites of the tonalite-trondhjemite-granodiorite (TTG) suite. This earliest magmatism reflects melting of a basaltic protolith, mostly at high pressures, either within subducted slabs or thickened crust. Sodic granites also form a persistent component of Proterozoic and Palaeozoic magmatism in Australia although in no specific period are they found in the same abundance as for the Archean. The other dominant magmatic series in the Australian Archean are potassic granites. These mostly postdate and largely reflect crustal reworking of the sodic granites. An important sub-class are those with high-silica and often moderately to strongly differentiated compositions. These first appear in the Mesoarchean, but are most extensive in the Neoarchean of the Yilgarn Craton (>100,000 square kilometres within a 25 million year period). Similar high-silica rocks occur in both the Proterozoic and Palaeozoic. Tectonic environments for these rocks are not well understood. One feature of these rocks is elevated thorium (Th) and uranium (U) contents. A wide variety of other (intermediate-) felsic magmatic rocks are present in the Archean to Palaeozoic geological provinces of Australia. These include: high-Mg diorite as well as components of the basalt-andesite-dacite-rhyolite series (locally with boninite-like rocks), which provide strong evidence for modern-style arc-related processes as far back as the Mesoarchean; peralkaline rocks such as syenites (as old as Mesoarchean); and high temperature, Fe- and HFSE-rich rocks, i.e., A-types (as old as Paleoarchean). Perhaps the most significant secular change in Australia is for S-type magmatism, which is rare in the Archean, minor in the Proterozoic, and common in the Palaeozoic. The Australian record shows that, in general, none of the felsic magmatic series are confined to or excluded from any particular time, but rather it is the relative proportions of such rocks which vary with time. Although similar chemistries could reflect similar responses to different processes, the simplest interpretation is that they reflect similar processes. A corollary is that a similar range of tectonic processes has operated from the Paleoarchean to now, but the dominance of particular processes has changed. This conclusion is balanced against observed subtle changes in chemistry which may indicate secular changes in tectonic processes, e.g., slab melting (TTGs) to slab dehydration (calc-alkaline rocks).

Presented at the Evolution and metallogenesis of the North Australian Craton Conference, 20-22 June 2006, Alice Springs. This presentation summarises research work, conducted during 1999 - 2000, aimed at determining the physico-chemical characteristics, timing, and provenance of palaeo-fluid flow across a large portion of the Northern Territory, from the Tanami Region in the west to the Aileron and Warumpi Provinces of the Arunta Region in the east and south, respectively. It also includes the Davenport Province of the Tennant Creek Inlier. The work is based on a study of quartz veining in The Granites, Dead Bullock Soak (DBS) and Tanami goldfields, and approximately 120 outcropping quartz vein clusters scattered throughout the entire region. The research included microthermometric analysis and Raman microprobing of ~3500 fluid inclusions, ~70 ä18O analyses of vein quartz and ~50 40Ar/39Ar age determinations of vein related micas. It resulted in delineation of 11 areas with different fluid characteristics ('fluid domains') and five areas with different age of vein-related micas ('age domains'). Within the Arunta region it also defined a northwest-trending zone where fluids have similar characteristics to the goldbearing fluids of the Tanami region, therefore indicating potential for further discoveries of gold mineralisation. <p>Related product:<a href="https://www.ga.gov.au/products/servlet/controller?event=GEOCAT_DETAILS&amp;catno=64764">Evolution and metallogenesis of the North Australian Craton Conference Abstracts</p>

The absence of basement outcrop and the nearly complete lack of surface expression of mineralisation in the Olympic Cu-Au province is the major impediment to mineral exploration in the province. In such circumstances, analysis of potential field data is one of the usual ways of inferring hidden geology, as high-quality datasets, especially aeromagnetic data, are available for most of the actively explored areas of Australia. Quantitative interpretation of potential field data principally involve 2D forward modelling of profiles, or sections, by skilled interpreters but it can be difficult, and time-consuming, to correctly track structure and geology from one section to the next should one wish to create a 3D model of the geology. To alleviate this problem, we have used a modification of the methods of Li and Oldenburg (1996 & 1998), constrained by known geological information, to give geologically and geophysically consistent solutions to the possible distributions of sources giving the magnetic and gravitational fields observed in a region about Olympic Dam.

The Central Gawler Gold Province is poorly known and poorly explored, principally because of the extensive development of regolith. CRC LEME is collaborating with Geoscience Australia and the Minerals Resources Group in the South Australian Department of Primary Industry and Resources to reduce the risk attached to exploration through the regolith in this terrain. We have integrated the interpretation of geological, geophysical, and calcrete geochemical data at known prospects in order to develop a strategy for generating ranked drilling targets. This ranking strategy carries the assumption that Au anomalies in calcrete formed in in-situ regolith materials are associated with Au mineralisation at depth. However, this assumption ignores the possibility that Au is moving laterally through the regolith in groundwater. It has been established that groundwater is mobilising significant quantities of Au in areas of the central Gawler Craton. Au may be precipitated within the regolith when the chemical or physical conditions governing mobilisation change. This process could generate "false" Au anomalies. Our research suggests that regolith geochemistry (surface and sub-surface) should be interpreted against the groundwater chemistry and an understanding of groundwater flow paths. Whereas groundwater chemistry can only be established by assaying samples from drillholes, groundwater flow paths can be mapped using a combination of geology and geophysics. At prospect scale, these results can be used as vectors to primary mineralisation, thereby refining targeting strategies.

This work presents a "snapshot" of research into the crustal architecture around the Olympic Dam Iron Oxide Cu-Au deposit, and makes some preliminary inferences on the fault architecture and alteration systems on the basis of filtering, interpretation, modelling ad inversion of public domain potential field datasets. At the time of presentation (Dec 2002), this work was still ongoing. The main results from this work are that the Neoarchean-Mesoproterozoic basement geology in the region around Olympic Dam has a history consistent with areas exposed on Eyre Peninsula to the south and west; that the system was actively deforming at the time of alteration and mineralisation which occurred at ca. 1590 Ma; that faults show geophysically detectable alteration patterns which suggest they transported and buffered at least two generations of fluids related to ore deposition; and that the regional wall rocks are reasonable sources for metals that were concentrated in the Olympic Dam deposit. However some of the other initial inferences (e.g. for an overall regional extensional tectonic environment during ore deposition) were not borne out by further testing and modelling.

This talk was presented at the Gawler Craton 2002: State of Play conference held in Adelaide, December 2002.

We report new and reinterpreted geological and geophysical results for the basement to the Stuart Shelf, in the north-eastern Gawler Craton. Regridding of gravity and magnetic datasets at optimal cell sizes allows resolution of basement structures with subtle geophysical expression. New processing techniques applied to these data, such as multi-scale edge detection (worming), and reassessment of available drill cores permit a reinterpretation of the stratigraphy and structure of units underlying the Pandurra Formation and Neoproterozoic cover sequences. In particular, we describe a three-fold Palaeoproterozoic basement sequence, analogous to that exposed in the southern Gawler Craton on the Eyre and Yorke Peninsulas. From west to east, we identify deformed BIF, schists, and gneisses equivalent to the Hutchison Group; orthogneisses equivalent to the Donington Granitoid Suite; and deformed and preserved metasedimentary rock equivalent to the Wallaroo Group. Intruded into the basement are structurally-controlled, high-level plutons of the ~1590 Ma Hiltaba Suite. These magmas fed extensive, flat-lying felsic sheets of the Gawler Range Volcanics (GRV), as well as more localised mafic centres equivalent to the Roopena Volcanics. Forward modelling of potential-field data and worming reveal that basement appears to have formed in a thick-skinned, transpressive regime. Structures suggestive of duplexes, megaboudinage, positive flower structures, and thrust stacks with non-ramp, flat geometry are consistent with modelled solutions. A similar structure to (or extension of) the Kalinjala Shear Zone is inferred to lie beneath the Stuart Shelf and GRV. In contrast, the ~1590 Ma volcanic-plutonic province appears to have formed in an overall extensional regime, with plutons elongated NE/SW in inferred dilational jogs within a conjugate dextral transtensional fault system. Thicker depocentres of GRV also appear to have formed in graben and half-graben nested above reactivated basement faults. To the south-west, four major sheets of GRV are inferred to rest on a basement of Archaean paragneisses. There is no geophysical requirement for a massive, sub-horizontal, mafic underplate. All geophysical anomalies can be explained with reference to realistic petrophysical properties of basement rocks found elsewhere in the Gawler Craton. Mass-balance calculations for a deposit such as Olympic Dam show that the source-rock volume for Cu is in the order of 102-103 km3. Under the hypothesis that the mineral system is controlled by faulting related to ~1590 Ma extension, faults of about 50-100 km by 10 km are required to create a large enough strain-envelope to ensure that fluids have access to the required volume of source-rock; and that those fluids may be mobilised and transported (Cox et al., 2001). Further, faults of this size are capable of tapping fluids from a variety of rock-types. Assuming that the regional NNW-to NW-trending transtensional structures penetrate to 10 km, their interpreted lengths are sufficient for them to have imposed a first-order control on the mineral system. The loci of mineralisation may be controlled by the second-order, NE- to ENE-trending, normal faults that connect the first-order regional structures and define the margins of the dilational jogs. The limiting factor to the size and spacing of deposits may be the quantity of metal available to the system, particularly Cu.

This address was presented at the 2008 Australian Nickel Conference held in Perth, 22-23 October 2008. Geoscience Australia has released a detailed, web-based colour map (at 1:5 000 000 and 1:10 000 000 scales) 'Australian Proterozoic Mafic-Ultramafic Magmatic Events (Sheets 1 and 2)'. This new map is the third and final component of the Proterozoic magmatic event series that show, for the first time, the geographic extent and age relationships of Proterozoic mafic and ultramafic rocks, and associated mineral deposits throughout the continent. The maps (`Proterozoic mafic-ultramafic magmatic events of Western Australia' and 'A Synthesis of Australian Proterozoic Mafic-Ultramafic Magmatic Events. Part 2: Northern Territory and South Australia') were produced in close collaboration with the State and Northern Territory geological surveys.

This presentation, presented at PDAC 2004, provides an insight into the nickel-sulphide industry of Australia in 2004 - a very dynamic and exciting time for the industry that is rapidly changing, through regular announcements of new discoveries, intersections, upgraded resources, and new mineralised provinces. Nickel-sulphide deposits in Australia are mainly associated with Archaean komatiites and Archaean Proterozoic mafic intrusions, but some unusual Phanerozoic deposits occur in eastern Australia. The majority of Australia's nickel production (~80%) is derived from komatiite deposits in the Yilgarn Craton of Western Australia. The Eastern Goldfields Province of this craton hosts one of the greatest concentrations of Archaean komatiite-hosted nickel deposits in the world, several of which are world class (>1 Mt Ni). Exploration activities in Australia are currently focussed on mafic-ultramafic rocks in Late Archaean and Proterozoic provinces. Exploration has been stimulated by the discovery of new deposits (Flying Fox, Daybreak, Armstrong, Daltons, McEwen, Nebo-Babel), recognition of different styles of mineralisation (Avebury), and the protracted period of elevated nickel metal prices. There is considerable potential for finding new deposits associated with komatiites and mafic intrusions, particularly under shallow cover.

This address was presented at the Central Australian Basins Symposium, Alice Springs, Northern Territory, 16-18 August, 2005. The Neoproterozoic sedimentary successions of the Centralian Superbasin (Walter et al 1995) in the northwest Paterson Orogen, Western Australia, are host to several major mineral systems including: gold-copper (Telfer, Magnum), base metal (Nifty, Maroochydore) and uranium (Kintyre). Geological mapping of the region by Bagas and others (1991-2003) at 1:100 000 scale is being followed by a new Geoscience Australia and Geological Survey of Western Australia National Geoscience Agreement project, to acquire new airborne geophysical data, and develop an understanding of the depositional history of the Neoproterozoic Paterson stratigraphy, its subsequent deformation(s) and its mineral systems. This project will investigate and integrate sedimentary facies, geophysical properties, structural elements and other post-depositional processes, such as granite emplacement, to understand their relationships to the mineral systems in the region. The suggested links of Bagas et al (1995, 1999) between the Paterson Neoproterozoic succession and the Centralian Superbasin will also be investigated.