Proceedings of papers presented at an industry workshop held in Perth, 20 June 2002. Edited by K.F. Cassidy

This paper describes the highlights of AGSO's work in the North Pilbara Project, a joint AGSO-Western Australia Geological Survey (GSWA) effort conducted under the National Geoscience Mapping Accord (NGMA) between 1995 and 2000. One of the principal drivers for AGSO's research in the Pilbara was the possible inapplicability of exploration models and genetic interpretations of Archaean mineral deposits because these models were commonly based upon late Archaean examples. One of our goals was to document the differences between the late Archaean and the early-mid Archaean mineral systems, and to develop regional thematic synthesis datasets so that more robust models could be developed to encompass the entire Archaean era. These datasets, together with our research into new exploration tools, have, and continue to assist exploration in the Pilbara. Our results also have applications to other terranes.

Zn-Pb-Ag mineral deposits, which are the products of specific types of hydrothermal "mineral systems", are restricted in time and space in Australia. These deposits formed during three main periods: ~2.95 Ga, 1.69-1.58 Ga, and 0.50-0.35 Ga. The 1.69-1.58 Ga event, which was triggered by accretionary and rifting events along the southern margin of Rodinia, is by far the most significant, accounting for over 65% of Australia's Zn. With the exception of the 0.50-0.35 Ga event, major Australian Zn-Pb-Ag events do not correspond to major events globally. Over 95% of Australia's Zn-Pb-Ag resources were produced by just four mineral system types: Mt Isa-type (MIT: 56% of Zn), Broken Hill-type (BHT: 19%), volcanic-hosted massive sulfide (VHMS:12%), and Mississippi Valley-type (MVT: 8%). Moreover, just 4% of Australia's land mass produced over 80% of its Zn. The four main types of mineral systems can be divided into two groups, based on fluid composition, temperature and redox state. BHT and VHMS deposits formed from higher temperature (>200?C), reduced fluids, whereas MIT and MVT deposits formed from low temperature (<200?C), oxidized (H2S-poor) fluids. These fluid compositions and, therefore, the mineralization style are determined by the tectonic setting and composition of the basins that host the mineral systems. Basins that produce higher temperature fluids form in active tectonic environments, generally rifts, where active magmatism (both mafic and felsic) produces high heat flow that drives convective fluid circulation. These basins are dominated by immature siliciclastic and volcanic rocks with a high overall abundance of Fe2+. The high temperature of the convective fluids combined with the abundance of Fe2+ in the basin allows sulfate reduction, producing reduced, H2S-rich fluids. In contrast, basins that produce low temperature fluids are tectonically less active, generally intracratonic, extensional basins dominated by carbonated and mature siliciclastics with a relatively low abundance of Fe2+. Volcanic units, if present, occur in the basal parts of the basins. Because these have relatively low heat flows, convective fluid flow is less important, and fluid migration is dominated by expulsion of basinal brines in response to local and/or out-of-area tectonic events. Low temperatures and the lack of Fe2+ prevent inorganic sulfate reduction during regional fluid flow, producing oxidized fluids that are H2S-poor. The contrasting fluid types require different depositional mechanisms and traps to accumulate metals. The higher temperature, reduced VHMS and BHT fluids deposit meatls as a consequence of mixing with cold sewater. Mineralization occurs at or near the seafloor, with trapping efficiencies enhanced by sub-surface replacement or deposition in a brine pool. In contrast, the low temperature, oxidized MIT and MVT fluids precipitate metals through thermochemical sulfate reduction facilitated by hydrocarbons or organic matter. This process can occur at depth in the rock pile, for instance in failed petroeum traps, or just below the seafloor in pyritic, organic-rich muds. Mass balance calculations indicate that the size of a metal accumulation, although controlled at the first order by the mineral system container size, also depends on the efficiencies at which metals are extracted from the source and retained at the trap site. The shear size of minerals systems required to form giant deposits may partly explain why these deposits commonly occur by themselves, without significant satellite deposits. In addition to the size of the mineral system container, metal retention efficiency appears to be the most important determinant of the size of metal accumulations.

The Uranium Systems Project is a key part of the $59m Onshore Energy Security Program (OESP) underway at Geoscience Australia (2006-2011). The project has three objectives: (1) develop new understandings of processes and factors that control where and how uranium mineralisation formed, (2) map the distribution of known uranium enrichments and related rocks in Australia, and (3) assess the potential for undiscovered uranium deposits at regional to national scales. Objective (1) has been addressed initially by reviewing current classification schemes for uranium deposits. Most schemes emphasise differences in host rock type and list 15 or more deposit types. An alternative scheme is proposed that links the apparently separate deposit types in a continuum of possible deposit styles. Three end-member uranium mineral systems are: magmatic-, basin-, and metamorphic/metasomatic-related. Most recognised deposit styles can be considered as variants or hybrids of these three end-members. For example, sandstone hosted, unconformity-related and "Westmoreland" style deposits are viewed as members of basin-related uranium systems and which share a number of ore-forming processes. Identification of the spatial controls on uranium mineralisation is being investigated using numerical modelling, with the Frome Embayment of SA as a first case study. Mapping the distribution of uranium in objective (2) has commenced with the release of a new map of Australia showing the uranium contents of mainly outcropping igneous rocks, based on compilation of whole rock geochemical data. A clearer picture of uranium enrichments is also emerging through cataloguing of an additional >300 uranium occurrences in the MINLOC mineral occurrence database. Finally, the recently completed Australia-wide radiometric tie-line survey is providing a new continent-scale view of uranium, thorium and potassium distributions in surface materials. To assess potential for undiscovered uranium deposits, new OESP data in targeted regions of Australia are awaited, such as airborne EM, seismic and geochronology data.

Mineralizing events in the North Pilbara Terrain of Western Australia occurred between 3490 Ma and 2700 Ma and include the oldest examples in the world of many ore deposit types. The mineralizing events were pulsed and associated with major volcano-plutonic (volcanic-hosted massive sulfide [VHMS], porphyry Cu, Sn-Ta pegmatite, mafic-ultramafic-hosted Ni-Cu-PGE, Cr and V, and epithermal deposits) and deformation events (lode Au?Sb deposits). In many cases, the mineralizing events are associated with extension, either in rifts, pull-apart basins or back-arc basins. Although mineralizing events occurred throughout the evolution of the North Pilbara Terrain, the most significant deposits are related to the development of the Central Pilbara Tectonic Zone (CPTZ). The CPTZ is sandwiched between the older East and West Pilbara Granite-Greenstone Terranes. Four significant volcano-plutonic and three significant deformation events occurred in and around the CPTZ between 2950 and 2840 Ma, a relatively short period in the evolution of the North Pilbara Terrain. Mineralization in the East and West Pilbara Granite-Greenstone Terranes was less intense and occurred over a much longer period. Compared to other Archean granite-greenstone terranes, the North Pilbara Terrain is poorly endowed: the only known world-class deposit in this region is the Wodgina Ta-Sn deposit. This lack of major mineral deposits may relate to the low rate of crustal growth of the North Pilbara Terrain. If such is the case, then the long history of crustal development and extensive recycling in the Pilbara is responsible for both the diversity of mineral deposits therein and, partly, the apparent poor endowment of the North Pilbara Terrain.

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Poorly exposed Paleoproterozoic sandstones and siltstones of the Killi Killi Formation record developement of a large turbidite complex. Killi Killi Formation sediments were eroded from the uplifted ~1860 Ma Nimbuwah and Hooper Orogens as indicated by detrital zircons with sediment deposition at ~1840 Ma. Facies analysis, isopach maps and detrital zircon populations, combined with Sm-Nd data from the Tanami region and Halls Creek Orogen, confirm the previously suggested correlation of the Paleoproterozoic successions in the Eastern zone of the Halls Creek Orogen and the Tanami region. Detrital zircons from the Aileron Province suggest the turbidite complex extends into the Arunta region, however, high metamorphic grade precludes direct facies comparisons in the Arunta region. Portions of the turbidite complex in the Tanami region are dominated by mudstones, consisting of low-density turbidites and associated hemipelagites, that potentially acted as a redox boundary to gold-bearing fluid. Gold prospectivity in turbiditic systems is increased within these mudstone sequences with the potential for further gold discoveries.

Report summarises results from the Offshore and Onshore Energy Security Progams undertaken between 2006 and 2011.

A number of Paleoproterozoic layered mafic-ultramafic intrusions in the central part of the Halls Creek Orogen of East Kimberley, Western Australia, have been explored for platinum-group elements (PGE), chromium, nickel, copper, cobalt and gold. Here we report on the halogen geochemistry of apatite and biotite in a number of these intrusions. Interstitial apatite is ubiquitous in these intrusions and, in most samples, tends to be relatively enriched in F- and OH-endmembers and relatively poor in Cl (< 20 mole %). Fluorapatite occurs in the more evolved igneous rocks and in marginal samples that apparently have been contaminated by metamorphic country rock. Cl/F ratios generally increase with bulk rock molar Mg/(Mg + Fe) ratios, as observed in other intrusions. Only a few samples show Cl-enrichment as high as that seen in the Stillwater and Bushveld complexes beneath the major stratabound PGE deposits. The most Cl-rich compositions observed occur in the upper part of the Springvale intrusion, where it is associated with troctolite, and in a single sample from the McIntosh intrusion. For the former intrusion, it is suggested that volatiles migrating out of the lower part of the mafic stratigraphy stabilized olivine at the expense of pyroxene. Associated biotite tends to be low in both Cl and F, containing no more than 10 mole % of these components. It is concluded that the East Kimberley intrusions contained a low to moderate volatile component that, during the combined processes of crystallization, degassing and fractionation of interstitial halogen-bearing minerals, was able to produce a late, mobile interstitial silicate liquid or volatile-rich fluid phase of variable Cl/F content that gave rise to most of the observed variations within any given intrusion. The exceptions include some marginal samples that appear to have been affected by country rocks, either during emplacement (assimilation) or during later metamorphism. The generally low Cl/F ratio of apatite, the lack of primary amphibole and the high background sulfur concentrations of the East Kimberley intrusions suggest that these magmas were relatively dry. The possible development of high-grade, PGE-enriched horizons by late-stage hydrothermal processes that could have mobilized significant amounts of the PGE and sulfur is considered to be of low potential.