Presentations from 'THE ISHIHARA SYMPOSIUM' held at GEMOC, MACQUARIE UNIVERSITY, JULY 22-24 2003; on a variety of topics ranging from general granite processes to mineralisation associated with magmatism.

Orogenesis in Phanerozoic systems is rapid, diachronous, episodic, and involves the switching of tectonic modes (extension-compression). In contrast, many Archaean orogens have traditionally been viewed as having developed by relatively simple, long-lived, mono-mode deformational processes. New results, however, reveal that the late Archaean eastern Yilgarn Craton (EYC) evolved episodically and rapidly, with a diachronous series of approximately E?W coaxial switches in tectonic mode. Tectonic mode switching changed stress regimes and resulted in the development of `late basins?, the emplacement of granites, and early orogenic gold mineralisation diachronously from east to west (NE?SW). Fluids were driven from the lower crust (and below) via large-scale crustal imbricating thrust faults. These fluids promoted the passage of a compression-extension couplet along a basal detachment by successively `lubricating? faults (preparing the ground), and facilitating a propagating wave of foreland surge (D2a) and hinterland extension (D2E) followed by inversion, uplift and annealing (D2b). In this way, orogenic Au and westward orogenic surge with associated tectonic mode switches are linked. We predict that the compres-sion-extension couplets and early orogenic gold mineralisation propagated from the east to the west diachronously at a rate of ~3-5 m.y. between domains from ~2670 Ma to ~2650 Ma. Multiple mineralising episodes are also a predicted consequence of the orogenic surge model.

Presented at the Evolution and metallogenesis of the North Australian Craton Conference, 20-22 June 2006, Alice Springs. The Tennant Creek goldfield, the third largest goldfield in the Northern Territory, producing over 150 tonnes of gold (Wedekind et al., 1989), was only discovered in the mid-1930s due to the association of gold with ironstone rather than quartz veins. Over the last two decades ironstone-hosted gold deposits have been included in the group of deposits termed iron-oxide copper-gold (IOCG) deposits (Hitzman et al., 1992). Elsewhere in the Northern Territory, prospects with IOCG characteristics have been recognised in the southeastern Arunta (Hussey et al., 2005), and potential for these deposits has been recognised in the Mount Webb area of the Warumpi Province (Wyborn et al., 1998). <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 North Pilbara Terrane has the largest variety of mineral deposits of any Archaean province. It contains the oldest known examples of volcanic-hosted massive sulphide (VHMS), lode Au, porphyry Cu, orthomagmatic Ni-Cu-PGE-V, pegmatitic Ta-Sn and epithermal deposits, with a diversity more characteristic of Phanerozoic mobile belts. Despite this diversity the North Pilbara Terrane appears to lack any major mineral deposits, with the exception of the Wodgina Ta-Sn pegmatite field. Below, we present the metallogenic history of the North Pilbara Terrane in the context of its tectonic development and then compare it to other Archaean provinces to assess controls on metal endowment.

The paper reveiws metallogenic evolution of Australia. A comparison between Archaean, Proterozoic and Phanerozoic metallogeny reveals that in general there exist more similarities between the Archaean and the Phanerozoic that those between the Archaean and the Proterozoic and between the Proterozoic and the Phanerozoic metallogeny. The paper argues that the contribution of plate tectonic processes in the geological evoultion and metallogeny of Australian Proterozoic need revaluation for assessing mineral potetnial of deposit styles which are traditionally considered to be not important but large deposits of each are known to exist in the Proterozoic elsewhere.

Legacy product - no abstract available

The late Archean Lake Harris Komatiite near the centre of the Gawler Craton is the only documented komatiite in South Australia, and the most eastern occurrence of such primitive ultramafic rocks in Australia. An integrated program of airborne magnetics, gravity, and core drilling was successful in 'seeing through' an extensive thin cover of Cainozoic alluvial sediments to define the distribution and volcanic architecture of the ca. 2520 Ma komatiitic flows. The greenstones form a series of sub-parallel east-northeast-trending sinuous magnetic high features flanked by large ovoid to elongate magnetic highs and lows that correlate with Archean-Proterozoic granitic bodies. The Lake Harris Komatiite is a steeply dipping greenstone sequence metamorphosed to middle amphibolite facies during the ca. 2440 Ma Sleafordian Orogeny and sheared during the ca. 1700 Ma Kimban Orogeny. The greenstones consist of cumulate komatiite (anhydrous 43-32% MgO), high to low Mg komatiite (32-18% MgO), komatiitic and tholeiitic basalt (<18% MgO), felsic volcanics, minor metasedimentary rocks, pyroclastics, and banded iron formation. They extend over 300 km in three subparallel east-northeast-trending belts that appear to be isoclinally folded around east-northeast axes and tectonically dismembered to the south by the Yerda Shear Zone. Komatiitic rocks have been confirmed by drilling in the three belts, but the absence of outcrop and structural complexities prevent detailed stratigraphic correlations within and between the belts. The komatiitic rocks display a range of textures that largely reflect the different habits of olivine and its alteration products. These include feather-quench and spinifex-textured chilled flow tops, fractured and jointed flow tops, with joints filled by metamorphic olivine or amphibole, massive aphyric variants, and various types of olivine-rich cumulates. Mesocumulates and orthocumulates are prominent with low porosity adcumulates comprising less than 10% of the more primitive cumulus zones. Lower to middle amphibolite facies assemblages are dominated by metamorphic olivine, with minor to trace amounts of igneous olivine, orthopyroxene, chromite, Cr-spinel, magnetite, ilmenite, and sulfide. Trace sulfides (pyrrhotite, chalcopyrite, pentlandite, pyrite, marcasite, polydymite violarite, heazelwoodite, millerite) form very small single-phase disseminated grains and coarser disaggregated grains. Their distribution largely reflects metamorphic and serpentinization alteration controls, with high Ni/S ratios and probable S loss from the more magnesian parts of the flows. The Lake Harris whole-rock data do not show any obvious Ni depletion during fractionation, but indicate a strong olivine control in dominantly S-undersaturated environments. Low S (100-600 ppm S) and high Pd+Pt (5-30 ppb) contents, and Ti/Pd ratios of 2 to 4 x 105 for the komatiitic rocks are similar to fertile S-undersaturated Archean komatiites hosting Ni-Cu-PGE deposits. The Lake Harris Komatiite has chemical (parent magma composition of 28% MgO, Al2O3/TiO2 = 16, depleted light REE) and initial Nd isotope (Nd = +2.8 to +3.0 at 2520 Ma) characteristics similar to 'typical' Al-depleted Archean komatiites, and there is no clear evidence of chemical modification by processes associated with contemporary subduction processes. Coherent patterns of trace elements (Th, Nb, REE, Ti, Y, Zr, and P) and typical initial Nd isotope signatures indicate a late Archean komatiitic system involving a depleted mantle source and no obvious crustal contamination. Its generation is consistent with mantle plume activity within a convective mantle system that probably exploited a lithosphere that was stretched and thinned by extension and/or thermal erosion in an intraplate environment.

The Ranger 1 unconformity-related uranium deposit in the Northern Territory of Australia is one of the world's largest uranium deposits and is currently Australia's second largest producer of uranium. Mineralisation at the Ranger, Jabiluka and other major unconformity-related deposits in the Alligator Rivers Uranium Field (ARUF) occurs in Paleoproterozoic metamorphic basement rocks immediately beneath the unconformity with the overlying and partly preserved Paleo- to Mesoproterozoic McArthur Basin. New geochronological data and observations of the relative timing of tectonothermal and hydrothermal events at the Ranger 1 number 3 orebody provide fresh insights on the spatial-temporal controls on ore formation in this deposit and more broadly in the ARUF. Uranium mineralisation and associated alteration at Ranger were fundamentally controlled by reactivated D1 shear zones that were initially localised by rheological contrasts and graphite-bearing units within the metasedimentary Cahill Formation. A new model of the Ranger 1 ore-forming mineral system attempts to reconcile the key features of the Ranger deposit and those of other major deposits and regional alteration in the ARUF. The results of the study demonstrate the importance of pre-ore structural and chemical architecture of the basement in controlling the location of uranium mineralisation. As in most recent models, McArthur Basin-derived, oxidised, diagenetic brines are envisaged as crucial in transporting uranium downwards into reduced basement rocks where uranium was deposited either by fluid-rock reaction and/or by fluid mixing. However, we propose a different architecture of fluid flow in which these basinal fluids penetrated extensive volumes of basement rocks beneath the unconformity to leach uranium, particularly from Archean felsic orthogneisses across basement paleo-highs. Possibly driven by convection and/or extensional tectonism during the period ~1725 Ma to ~1705 Ma the uranium was transported laterally through microfractures in this regional sub-unconformity alteration zone, along the unconformity, and via fault networks until chemical gradients were encountered in D1 shear zones in the Cahill Formation. Uraninite deposition at Ranger most likely resulted from reduction of ore fluids via reaction with pre-existing Fe2+-bearing minerals and/or via fluid mixing, although the evidence for mixing remains cryptic. Simultaneously, intense Mg metasomatism generated very acidic and silica-rich fluids that migrated laterally and downwards within the D1/D2 shear zones to dissolve and silicify adjacent carbonate rock units, explaining the observed spatial association of chloritic ore breccia adjacent to, and in places above, thinned and silicified carbonate rock units, both at the Ranger and Jabiluka deposits. The possible role of fluid mixing above and below the unconformity requires further study to better understand why no significant uranium deposits have yet been discovered within the McArthur Basin, a characteristic that appears to differentiate the ARUF from the Athabasca Basin in Canada.

This Record provides information to accompany the usage of two web-based map sheets showing the time-space distribution of Proterozoic Large Igneous Provinces (LIPs) in Australia. The maps should be studied in context with other maps in this national series: the Map of Australian Proterozoic Mafic-Ultramafic Magmatic Events (which presents all magmatic events, large and small), and the Map of Australian Archean Mafic-Ultramafic Magmatic Events, together with their accompanying Geoscience Australia Records. Together, these are a comprehensive source of primary information on the place of mafic-ultramafic magmatism in the evolution of the Australian continent, and will be of interest to explorers in the search of magmatic ore deposits of nickel, platinum-group elements, chromium, titanium, and vanadium.

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.