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.

The Kangaroo Caves zinc-copper deposit in the Archaean Panorama District in the northern Pilbara Craton, Western Australia contains an Indicated and Inferred Mineral Resource of 6.3 million tonnes at 3.3% zinc and 0.5% copper. The Kangaroo Caves area is characterised by predominantly tholeiitic volcanic rocks of the Kangaroo Caves Formation, which is overlain by turbiditic sedimentary and volcanic rocks of the Soanesville Group. Zinc-copper mineralisation is hosted mainly by the regionally extensive Marker Chert, the uppermost unit of the Kangaroo Caves Formation, and structurally controlled by D1 synvolcanic faults. The upper area of the deposit is characterised by quartz-sphalerite ± pyrite ± baryte ± chalcopyrite, whereas the lower area contains mainly chlorite-pyrite-quartz-carbonate-sericite ± chalcopyrite ± sphalerite. Laser ablation inductively coupled plasma mass spectrometry analyses show that cobalt-nickel ratios in pyrite are significantly greater in the upper, zinc-rich area (median copper/nickel = 0.4) of the deposit than the lower, zinc-poor area (median copper/nickel = 5). Structural analysis of the Kangaroo Caves area combined with Leapfrog modelling of ore and trace element distribution shows that the deposit is predominantly an elongate sheet of zinc mineralisation (-1%), which plunges ~30° to the northeast and is approximately 1000 metres in length. The morphology of the Kangaroo Caves deposit was retained from its original formation, despite rotation during the D2 event. Variations in hydrothermal alteration assemblages, including the copper and nickel contents of pyrite within the deposit and underlying dacite, are interpreted to be the result of variations in the influx and mixing of seawater with upwelling volcanogenic fluids during zinc-copper mineralization. At the Kangaroo Caves area the cobalt-nickel ratio of pyrite can be used as an exploration vector towards high-grade zinc-copper mineralization.

The global distribution of mineral deposits in cratons, belts and districts shows that they are not equally and uniformly endowed with metal. Some cratons are highly fertile (e.g. Yilgarn Craton for Archaean greenstone gold and nickel) and there are those which are almost barren (e.g. Archaen greenstone belts in the Pilbara Craton). Within belts the distribution is equally non-uniform. For instance more than 80% of gold resources in the Yilgarn are concentrated in the Kalgoorlie Terrane of the Eastern Goldfields. At a first level the total endowment can be used to compare mineralised belts and districts, however the distribution of deposit sizes in them can provide a second level constraint on their fertility, in particular the nature and intensity of metal accumulation versus metal dispersion. More enigmatic from this point of view are belts and districts in which the total metal endowment is contained in one or two giant and/or super-giant deposits, such as the Broken Hill in New South Wales, Norilsk-Talnakh in Western Siberia, and Olympic Dam in South Australia. These mjaor deposits represent single "bull elephants" in an "elephant country". Cumulative frequency distribution curves of metals of major mineralized cratons, belts and districts are used to compare the nature of their metal endowment. The analysis shows that the curves of "elephant-bearing" belts and districts are remarkably different from those of "average" belts and districts, and that regional and/or district scale geological factors could have played a significant role in controlling metal endowment. A comparison of curves for belts and districts with similar endowment can be used to assess potential for yet to be discovered deposits and to assess their relative size.

This publication contains the abstracts from the Evolution and Metallogenesis of the North Australian Craton conference held in Alice Springs, Australia in June 2006.

The secular distribution of zinc deposits is pulsed and related to changes in Earth processes and conditions, including the supercontinent cycle and oxygenation of the atmosphere and hydrosphere. Deposits hosted by volcanic successions formed during the assembly of supercontinents along convergent margins, probably as the consequence of high heat flow and a greater likelihood that such tectonic systems are preserved. Siliciclastic-hosted and carbonate-hosted deposits post-date the first oxygenation event as fluids that formed these deposits were oxidized. Siliciclastic-hosted deposits formed both during assembly and breakup of supercontinents, whereas carbonate-hosted deposits formed during supercontinent or microplate assembly.

As part of the North Pilbara NGMA Project, AGSO (now Geoscience Australia), together with Newcastle University and the Geological Survey of Western Australia (GSWA), have been conducting a research program to document the geological setting, characteristics and genesis of Au deposits of the North Pilbara Terrane. This record summarises some results of this research program. This research has concentrated on turbidite-hosted lode Au deposits in the Indee and Nullagine areas as well as basalt and ultramafic-hosted deposits in the Mt York-Lynas Find area. In addition to these areas, AGSO's research also concentrated on epithermal deposits in the Indee area, and less detailed studies were undertaken on lode Au deposits at Gold Show Hill and Klondyke. This research program was designed to complement recent (e.g., Neumayr et al. [1993; 1998] on the York deposits and Zegers [1996] on the Bamboo Creek deposits) and ongoing (e.g., D. Baker, University of Newcastle] at Mt York-Lynas Find) programs conducted at the other institutions. This Pilbara Gold Record is supported by an extensive GIS dataset, providing many new digital data sets, including a number of variations of the magnetics, gravity, and gamma-ray spectrometry. A solid geology map, and derivative maps, mineral deposits, geological events, and Landsat 5-TM provide additional views. This data set complements the 1:1.5 Million scale colour atlas (Blewett et al., 2000).

No abstract available

Existing age constraints for geological events in the Tanami Block come predominantly from U-Pb geochronology of i) detrital zircons in sediments, and ii) magmatic zircons in granitoids. These constraints have been used together with observed and inferred geological relationships to help constrain timing of stratigraphy, magmatism, deformation, metamorphism and Aumineralisation (e.g. Vandenburg et al., 2001). Ongoing GA/NTGS zircon geochronology is continuing to refine our understanding of the stratigraphy and magmatic history of the Tanami, with attendant implications for tectonic evolution. In this regard it is noteworthy that detrital zircon ages of ~1815 Ma from the Killi Killi formation require either (or both) a revision of existing stratigraphy, or that the so-called Tanami Orogenic Event significantly post-dates ~1815 Ma, in contrast to previous estimates of ~1845 - 1830 Ma. However, detrital and magmatic zircons can provide no direct constraints on timing of deformation, metamorphism and Au-mineralisation, and consequently our current understanding of these processes in the Tanami region is relatively poor, despite being critical to predictive exploration models.

In July 2000, Geoscience Australia (then the Australian Geological Survey Organisation) joined with the Northern Territory Geological Survey (NTGS) in the North Australia NGA (National Geoscience Agreement) Project (NAP), a three year program to assist NTGS in their regional mapping and metallogenic programs in the southern Northern Territory.

The Tanami Region of northern Australia has emerged over the last two decades as the largest gold producing region in the Northern Territory and one of the top three Palaeoproterozoic gold provinces in the world. Gold occurs in epigenetic quartz veins hosted by metasediments and mafic rocks, and in sulphide-rich replacement zones within iron formation. Many of the deposits are hosted in the hinges of pre-existing anticlines; others are hosted within zones of extensive structural reworking or in highly competent rocks. Limited geochronological data suggests that mineralisation occurred at about 1810 Ma, during a period of extensive granitoid intrusion. Most deposits are associated with D5 faults and shear zones that formed in a sinistral transpressive structural regime with ?1 oriented between ESE-WNW and ENE-WSW. Structures active during D5 include ESE-trending sinistral faults that curve into north-trending reverse faults localised in supracrustal belts between and around granitoid domes. Granitoid intrusions also locally perturbed the stress field, possibly localising structures and deposits. The reverse faults are interpreted as relay faults, and forward modelling indicates that all faults extend into the mid-crust. In areas characterised by the north-trending faults, orebodies also tend to be north-trending, localised in dilational jogs or in fractured, competent rock units. In areas characterised by ESE-trending faults, the orebodies and veins tend to strike broadly east at an angle consistent with tensional fractures opened during ESE-directed transpression. Many of these deposits are hosted by reactive rock units such as carbonaceous siltstone and iron formations. Ore deposition occurred at depths ranging from 1 to 11 km from generally low to moderate salinity carbonic fluids with temperatures from 200 to 430?C, similar to lode gold fluids elsewhere in the world. We interpret these fluids as the product of metamorphic dewatering caused by enhanced heat flow, although it is also possible that the fluids were derived from coeval granitoids. Lead isotope data suggest that the ore fluids had a similar source to the granitoids, possibly a mid-crustal reservoir. Gold deposition is interpreted to be caused by: (1) CO2 effervescence and reduction of ore fluids caused by interaction of the moderately oxidised fluids with carbonaceous siltstone; (2) sulphidation of host iron formations; (3) depressurisation and effervescence of fluids caused by pressure cycling in shear zones; and (4) boiling and fluid mixing at shallow levels. Deposits in the Tanami Region may illustrate the continuum model of lode gold deposition suggested by Groves (1993) for Arch?an districts.