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.

Considerable exploration interest has been generated by the platinum-group-element (PGE) and Ni-Cu potential of the Archean layered mafic-ultramafic intrusions in the west Pilbara Craton, Western Australia. The Munni Munni Intrusion contains the largest known resource of PGEs associated with a layered intrusion in Australia, and the Radio Hill and Mount Sholl intrusions host significant resources of Ni-Cu-Co sulfides. Titaniferous magnetite layers, remobilized sulfides, and structurally controlled hydrothermal polymetallic deposits have also been a focus for exploration in recent years. The ca. 2.9 Ga Munni Munni, Andover, Radio Hill, Mount Sholl, and Sherlock layered intrusions are a cogenetic suite of high-level (<5 kb) bodies that represent some of the oldest mineralizing systems of their type in the world. Although they display similar field relationships and mineralogical, geochemical, and isotopic features, their contrasting chalcophile metal distribution patterns show that the timing and mechanism of the S-saturation event were critical for the development of PGE-enriched sulfide-bearing layers and basal segregations of base-metal sulfides. The intrusions form thick (>5.5 km) 'dike'-like bodies or relatively thin (0.5-2 km) sheets and sills emplaced at different levels along major lithological discontinuities in the upper crust. Rhythmically layered ultramafic components are generally thinner than, and occur along the northern sides of, more massive overlying mafic components. The ultramafic zones consist of dunite, lherzolite, wehrlite, olivine websterite, clinopyroxenite, and websterite. Inverted pigeonite gabbronorite, magnetite gabbro, olivine gabbro, anorthositic gabbro, and anorthosite comprise the mafic sequences. Olivine and clinopyroxene were generally the first minerals to crystallize, except in the Andover Intrusion, where orthopyroxene preceded clinopyroxene and possibly reflects greater contamination of the parent magma by felsic crustal material. The crystallization of chromite was inhibited in the ultramafic zones by the partitioning of Cr into early crystallizing clinopyroxene, thus downgrading the potential for PGE-chromite associations. The PGE mineralization in the Munni Munni Intrusion occurs in the upper levels of a porphyritic plagioclase websterite orthocumulate layer directly below the ultramafic-gabbroic zone contact. Mineral compositional trends and Nd isotopic data indicate that a Pd-Pt-Au-enriched S-undersaturated magnesian basaltic magma was frequently injected into a small magma chamber during formation of the ultramafic zone. A major influx of more fractionated, S-saturated tholeiitic gabbroic magma related to the resident magnesian magma, rapidly inflated the chamber and induced turbulent magma mixing that resulted in the formation of the PGE-bearing porphyritic websterite layer. In contrast, the parent magmas that formed the Mount Sholl, Radio Hill, Andover, and Maitland intrusions were saturated in S before they were emplaced into the magma chambers. In these intrusions gravitational and structural controls were important for the concentration of PGE-poor (5-400 ppb Pt + Pd + Au) massive sulfides in depressions and structural embayments along the basal contacts beneath the thickest sequences of mafic-ultramafic cumulates. The parent magmas to the west Pilbara intrusions were siliceous high-Mg basalts of Al-depleted komatiitic affinity (Barberton-type) with 9-12% MgO, 15-25 ppm Sc, 12-18 ppm Y, low Al2O3/TiO2 (ca. 11, or half chondrite ratios), and light-rare earth enrichment (chondrite-normalized La/Sm = 2.7, La/Lu = 9.0). They were generated with garnet in the residual asthenospheric mantle with probable involvement of a pre-3.0 Ga subduction-modified lithospheric mantle. Isotopic and geochemical modelling suggests that the magmas were contaminated by ca. 3.0-3.3 Ga Archean tonalitic to granodioritic crust before being emplaced into high-level magma chambers.

Tanami Region, the largest gold province in the Northern Territory, has produced 122 t of gold, has a known remaining resource of 190 t, and has high exploration potential. Most deposits are concentrated in three goldfields - Dead Bullock Soak (DBS), The Granites and Tanami. Significant mineralisation is also located at Groundrush (0.7 Moz Au), Oberon (0.48 Moz Au) and Crusade (0.1 Moz Au) deposits, as well as the Coyote prospect in WA. Gold mineralisation is late in the tectonic cycle, has a spatial assocaition with late orogenic felsic intrusives, and is coincident with late D5 structures. DBS goldfield (remaining resource 23.6 Mt at 5.6 g/t Au) contains stratabound mineralisation in folded greenschist facies siltstone, BIF and chert of the Dead Bullock Formation. At Callie, which is the largest deposit in the Region, mineralisation is in D5 quartz veins associated with fold closures within metasiltstone. It is dominated by free gold (70%) with some auriferous arsenopyrite. The remaining DBS deposits consist of BIF and chert hosted mineralisation associated with arsenopyrite, pyrrhotite and minor pyrite. The Granites goldfield (remaining resource 1.9 Mt at 3.8 g/t Au) comprises stratabound mineralisation within intensly folded amphibolite facies BIF of Dead Bullock Formation. Gold occurs in sulphides (apy, po, py) disseminated in BIF, quartz and quartz-carbonate veins. The Granites goldfield lies in close proximity to Inningarra and The Granites granitic suites (1815?4 and 1795?5 Ma respectively), and is associated with D5 shearing. Tanami goldfield (remaining resource 6.01 Mt at 3.2 g/t Au) comprise auriferous quartz veins in sub-greenschist facies basalt and interbedded sedimentary units of the Mount Charles Formation. Mineralisation is controlled by three sets of D5 faults striking 350-010o, 020-040o, 060-080o and dipping east to southeast. There is a close spatial relationship with the Coomarie and Frankenia granites (1815?4 and 1805?6 Ma respectively). Gold occurs in sulphides (py, apy, po);and vein textures indicate high level mineralisation. Wallrock alteration involves bleaching of basalt and sediments to produce sericite+quartz?pyrite?carbonate assemblages. Groundrush (resource 3.2 Mt at 4.5 g/t Au) comprises auriferous arsenopyrite in quartz veins within dolerite, but structural relations are still not clear. Titania (resource at Oberon 4.1 Mt at 2.6 g/t Au) contains auriferous pyrite and arsenopyrite in D5 quartz veins within detrital and graphitic sediments of the Killi Killi Formation; Minotaur ( resource 1.0 Mt at 2.4 g/t Au) contains auriferous sulphides (apy, po, py) disseminated in lower amphibolite schist of Dead Bullock Formation. Microthermometric and Raman spectroscopic studies of primary fluid inclusions indicate that Callie fluids changed character with time, characterised by decreasing temperature, increasing salinity and changing gas contents. The fluid conditions were 280-400oC and 6-12 wt% NaCl in pre-ore quartz veins; 220-360oC, 8-22 wt% NaCl eq and abundant CO2 in ore-stage quartz veins; 180-320oC and 12-18 wt% NaCl eq in post-ore quartz veins, and 80-160oC and 14-28 wt% NaCl eq in late post-ore carbonate veins. At The Granites goldfield, the main population of ore stage fluid inclusions had a temperatures of 260-312oC and salinity 4-8 wt% NaCl eq. At the Tanami goldfield, the ore fluids had lower temperatures of 120-220oC and salinity below 12 wt% NaCl eq; fluids here were largely degased. At Groundrush, ore fluids had the highest temperatures of 390-430oC and salinity of 4-10 wt% NaCl eq. Groundrush fluids also contained CO2 and exceptionally high CH4. Based on fluid inclusion data, estimated depths of mineralisation were 8.3 to 5.5 km for Groundrush, 7.5 to 3.8 km for The Granites goldfield, 5.8 to 3.2 km for Callie and 1.5 to 0.4 km for the Tanami goldfield.

The world class Callie lode gold mine is one of several gold deposits located at the Dead Bullock Soak goldfield in the Northern Territory Tanami Desert, about 550km northwest of Alice Springs. Its high grade and high tonnage make it an important system to understand in terms of regional mineralising processes. The aim of this research was to elucidate the important factors contributing to the formation of such a large deposit, and what features might be important for refining exploration strategies in the region.

The Tanami region has become one of Australia?s premier Proterozoic gold provinces, having already produced 122 t of gold, and still has high exploration potential. This region contains more than 60 gold occurrences including three established gold fields (Dead Bullock Soak, The Granites and Tanami) as well as several significant gold prospects (Groundrush, Titania, Crusade, coyote and Kookaburra). The Callie deposit (>5 Moz total resource) located in the Dead Bullock Soak goldfield is currently the largest mine in this region. Fluid inclusion studies indicate that the ore fluids in the Dead Bullock Soak and The Granites goldfields were low to moderate salinity (4 ? 10 wt.% NaCl eq), moderate to high temperature (260 ? 460 ?C) and gas rich. In contrast, ore fluids in the Tanami goldfield were low temperature (120 ? 220 ?C) with only minor CO2. O and H isotopic data are consistent with either a metamorphic or magmatic origin for the ore fluids with some mixing with meteoric fluids evident in the Tanami district. The fluid inclusion data also indicate that the deposits have formed over a range of physico-chemical conditions and depths. Groundrush appears to have formed at the greates depths and has the most reduced (CH4-rich) fluids. The Granites goldfield and the Callie deposit formed at shallower depths. The Granites fluids were CO2-rich but also had variable N2 and CH4. The Callie fluids show only small variations in temperature and salinity and are more oxidised with only CO2 and N2 being detected. The Tanami deposits appear to have formed at the shallowest levels and are dominated by low-salinity aqueous fluids, although some CO2-bearing fluids have also been detected. Considerable uncertainty surrounds the age of gold mineralisation. The spatial relationship between many of the gold deposits and granitoids has led to proposed genetic links between granite intrusion and mineralisation. These granites have since been dated at between 1825 and 1795 Ma. Preliminary 40Ar/39Ar results from sericite from the Carbine pit in the Tanami goldfield yields a total gas age of 1810 Ma, i.e. similar to the age of the granites. However, 40Ar/39Ar analysis of biotite in mineralised veins from Dead Bullock Soak indicate an age of 1720 ? 1700 Ma. This suggests that mineralisation at Callie may be related to fluid migration driven by the Late Strangways Orogeny that was responsible for widespread deformation and metamorphism in the Arunta Province to the south-east. Results of this study indicate that gold deposits of the Tanami region are surprisingly diverse, with some being basalt and dolerite hosted, some in banded iron formations and some being sediment hosted. Moreover, fluid inclusion data shows that the deposits formed over a range of depths and 40Ar/39Ar dating may indicate the occurrence of more than one mineralising event. Consideration of this diversity from a mineral systems perspective, highlights the likelihood that undiscovered gold deposits in the Tanami region will have a range of characteristics and may occur in a number of different lithologies.

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Preface to special issue on North Pilbara mineral deposits; no abstract

The Palaeoproterozoic Tanami Region is one of the most rapidly developing gold provinces in Australia. Its steadily growing gold resource currently stands at 12.5 Moz, including past production of 4.1 Moz. The entire Region contains some 60 gold occurrences. Most of these are concentrated in three goldfields ? Tanami, The Granites and Dead Bullock Soak (DBS). Mined deposits usually contain reserves of 0.01-0.1 Moz Au. Unique in size and a notable exception is the Callie deposit (DBS goldfield). Prior to 30 June 2001, this open cut and underground mine produced 1.7 Moz Au and there is a remaining underground resource of 4.3 Moz Au. Other significant deposits include Groundrush (0.5 Moz Au), Titania (0.3 Moz Au) and Minotaur (0.1 Moz Au). The Coyote deposit in Western Australia also appears to contain significant mineralisation, but its resource has not been announced as yet. In 1999, the Northern Territory Geological Survey commenced a major multidisciplinary project in the Tanami Province. This was designed to facilitate mineral exploration in the region by provision of a new generation of geological maps and the development of mineralisation models. Results of this work to date have been published in Hendrickx et al 2000, Dean 2001, Vandenberg et al 2001 and Wygralak and Mernagh 2001. Earlier reported metallogenic work (Wygralak and Mernagh 2001) concentrated on the delineation of physico-chemical characteristics, and the origin and evolution of hydrothermal fluids in the Tanami, The Granites and DBS goldfields. Pilot work has also been performed on the newly discovered Groundrush deposit. Mineralising fluids in each of the goldfields have a unique physico-chemical signature. Fluids in the Tanami goldfield have a temperature range of 120-220oC and contain almost no gases. Mineralisation occurred at shallow depths of 0.4-1.8 km. Gold was precipitated as a result of decreasing pressure and temperature. In The Granites goldfield, fluid temperature was in the range 260-312oC and the fluid contained significant amounts of CO2 mixed with minor CH4 and N2. Gold precipitated due to reaction of the fluid with host rocks containing magnetite, graphite and, in the case of the Bullakitchie deposit, carbonates. The depth of mineralisation is estimated at 3.8?7.5 km. In the Callie deposit (DBS goldfield), mineralising fluid had a temperature of 310-330oC and contained CO2 and N2, but no CH4. Gold precipitation occurred at a depth of 3.2?5.8 km as a result of the reaction of fluid with carbonaceous sediments. In the Groundrush deposit, fluid temperature was in the range 390-430oC. Fluid was dominated by CH4 and there was a minor amount of CO2. Mineralisation occurred at depths of 5.7-8.3 km and phase separation was the precipitation mechanism.

Precambrian layered mafic-ultramafic intrusions in Australia have recently generated considerable exploration interest for their platinum-group element (PGE: Pt, Pd, Rh, Ru, Os, Ir) and Ni-Cu-Co potential. Exploration has been stimulated by the discovery of potential world-class deposits (Voisey?s Bay, Canada; west Musgraves), high metal prices (notably Pd, Pt, and Rh), and a perception that many favourable intrusions are under-explored for different styles of orthomagmatic and hydrothermal mineralisation. Despite the renewed interest, Ni production associated with layered intrusions accounts for only 3% of Australia?s Ni production, and PGE production is currently restricted to the Archaean komatiitic-volcanic associations of the Yilgarn Craton. Exploration programs (see Hoatson & Blake 2000) for Precambrian layered intrusions vary considerably for different styles of precious- and base-metal mineralisation. The four styles of mineralisation considered here are believed to have the greatest potential in the following major orogenic domains: (1) Stratabound PGE-bearing sulphide layers: Yilgarn Craton, Pilbara Craton, Musgrave Block, Gawler Craton; (2) Stratabound PGE-bearing chromitite layers: Halls Creek Orogen, Albany?Fraser Orogen, Yilgarn Craton; (3) Basal segregations of Ni-Cu-Co?PGE sulphides: Musgrave Block, Pilbara Craton, Yilgarn Craton, Halls Creek Orogen, Arunta Block, Gawler Craton; and (4) Hydrothermal PGE remobilisation: Pilbara Craton, Arunta Block, Halls Creek Orogen, Yilgarn Craton, Musgrave Block, Gawler Craton. During the exploration of layered intrusions it is important not to be `blinkered? to a particular model, but to maintain a flexible innovative approach and consider different styles of orthomagmatic and hydrothermal mineralisation at different stratigraphic levels in the intrusion. It should also be borne in mind that it took more than 20 years of intensive exploration to define the J-M Reef of the Stillwater Complex, and it was not until the 1990s that a significant Au-PGE layer (Platinova Reef) was found in the Skaergaard Intrusion, East Greenland?an intrusion which has been investigated in great detail for more than 60 years.