No abstract available

Chemical alteration to certain end-member minerals, such as magnetite, pyrrhotite and pyrite, can produce density and magnetic susceptibility contrasts. These contrasts can be detected using gravity and magnetic surveys. Interpretation of alteration is made possibly by inverting the geophysical data (calculating subsurface properties from the survey results) and combining these inverse results with 3D geological mapping. An application of this method to

No abstract available

Hyperspectral airborne images from the Eastern Fold Belt of the Mount Isa Inlier, were validated as new tool for the detection of Iron oxide Cu-Au (IOCG) related alteration. High resolution mineral maps derived from hyperspectral imaging (4.5m/pixel) enables the recognition of various types of hydrothermal alteration patterns and the localisation of fluid pathways. Four different types of hydrothermal alteration patterns were identified with the hyperspectral mineral maps: (1) Metasomatic 1: White mica mineral maps were applied to map the spatial distribution of regional sodic-calcic alteration in metasedimentary successions of the Soldiers Cap Group in the Snake Creek Anticline. (2) Metasomatic 2: Alteration zoning is evident from albitised granites, assigned to the Williams-Naraku Suite, along the Cloncurry Fault show characteristic absorption features in the shortwave infrared range (SWIR) and can be detected with white mica mineral maps (white mica composition, white mica content, white mica crystallinity index).

The Broken Hill Exploration Initiative (BHEI) started in 1994 and is a joint effort between the New South Wales Department of Primary Industries - Mineral Resources, the South Australian Department of Primary Industry and Resources and the Commonwealth Government through Geoscience Australia. The aim of the BHEI is to provide a new generation of geoscientific data for the Curnamona Province, particularly the Broken Hill-Olary region, as a basis for more effective mineral exploration by industry. This initiative aims to provide the best possible knowledge and information-base for mineral and petroleum exploration investment in western New South Wales and eastern South Australia. The region will benefit from the application of new technologies and exploration methodologies to enhance knowledge of the geological controls of mineral deposit systems. BHEI conferences are held on a regular basis to highlight the geoscientific advances made during the life of the initiative. The contents of this Record are the extended abstracts of oral and poster papers presented at the BHEI conference that was held in Broken Hill on 26-28 September 2006.

Globally, iron-oxide Cu-Au deposits are a prime exploration target given the super-giant status of Olympic Dam, the type example. The most commonly identified primary ingredients for these deposit types are granites, mafic igneous rocks and oxidised host sequences (e.g., Hitzman 2000). The deposits are also characterised by sodic-potassic and potassic alteration which can be significant on a regional scale. Deposits in the Tennant Creek, Cloncurry and Olympic Dam regions are the three main examples of iron-oxide Cu-Au deposits in Australia: all are Proterozoic. A continent scale-spatial analysis of their regional setting reveals some common elements and suggests that these deposit types are unlikely to be found in the Archaean or the Phanerozoic of Australia. In all three cases, intrusive rocks occur in proximity, but whether the metals come direct from these or are leached from the country rock is unimportant to this analysis. The nearby granitic rocks are all fractionated, and show evidence of release of late stage magmatic fluids. Chemically these granites are a very specific type: all strongly oxidised, metaluminous, high temperature, high Ca granites that formed as a result of well above average geothermal gradients from crustal sources. These granites contain anomalously high concentrations of K, Th and U. Such granites are not found in the Archaean and rare equivalent Phanerozoic granitic types are of an order of magnitude smaller in size. This analysis shows that the known Australian deposits occur in regions of present-day anomalously high heat flow. Heat production is enhanced by high concentrations of K, Th and U in the nearby granites, particularly in the Olympic Dam and Cloncurry areas. Such elevated heat production may have resulted in anomalously long-lived hydrothermal cells and enhanced the metal leaching process. The large areal extent of the alteration zones in these two areas may also be a result of this fundamental crustal anomaly. Mafic igneous rocks are common in the vicinity of known Australia deposits, and might be another essential ingredient of an efficient Fe-Ox Cu-Au mineral system. They are regarded as an additional source of Cu for leaching. The regions with known deposits are dominated by oxidised rock packages containing hematite?magnetite. These oxidised packages are regarded as essential to maintaining the high redox state of the metal-bearing fluids. Elsewhere where similar granites intrude reduced packages, particularly those bearing graphite, Cu deposits are rare. Where oxidised host packages are not intruded by granites these deposit types do not occur. Equivalent oxidised host packages intruded by granites are not common in the Australian Archaean or Phanerozoic.

Economic geologists and remote sensing specialists have long been interested in hydrothermal systems for their valuable ore deposits. Alteration mineral assemblages caused by hydrothermal activity typically display certain spectral characteristics due to vibration of the hydroxyl (OH-) anion in the near infrared. This feature can be exploited by satellites using imaging infrared spectrometers. Hyperspectral equipment such as the Australian built HyMap airborne system and the PIMA II field spectrometer collect detailed spectral information, often in contiguous wavelengths, which can be analysed and interpreted to make highly detailed mineralogical and alteration pattern maps . Two Precambrian hydrothermal systems, well suited to testing remote sensing of hydrothermal systems are described here, the first in the northern Flinders Ranges in South Australia, comprising one of Earth?s largest hydrothermal deposits and, the second in the North Pole Dome in the Pilbara region of north western Australia, host to many famous stromatolitic horizons.

The Tarcoola Goldfield in central South Australia is hosted by the Palaeoproterozoic Paxton Granite and Tarcoola Formation sediments. Both of these units are intruded by narrow dykes of the Lady Jane Diorite. Crosscutting relationships and Ar-Ar dating of sericite and hornblende demonstrate that veining, alteration and mineralization occurred synchronously with intrusion of the diorite dykes at ~1580 Ma. The Lady Jane Diorite is considered to be part of the Gawler Range?Hiltaba Volcano-Plutonic event. Mineralization at the Perseverance deposit in the Tarcoola Goldfield is hosted by both granite and the lower parts of the Tarcoola Formation. Alteration intensity and mineralization are related to fracture density, suggesting the structural regime was an important control on ore location and deposition. However, at the Tarcoola Blocks mine, mineralization is hosted entirely in the Tarcoola Formation, and very high gold grades are recorded in carbonaceous horizons, suggesting chemical processes in ore deposition. Primary fluid inclusions from mineralized quartz veins are dominated by two types, a two-phase aqueous type and a three-phase H2O-CO2(l)-CO2(v)?CH4 type. Both inclusion types have salinities of <10 wt% eq. NaCl, with evidence of other salts being present. Both types have Th between ~380?C and 150?C. At the Perseverance deposit, both inclusion types occur together in single quartz grains as primary inclusions, and are interpreted to have formed from a single evolving fluid, indicating immiscibility. Pb isotopes of galena from gold-bearing mineralization are in equilibrium with K-feldspar and whole-rock samples of the Paxton Granite. This is consistent with a local source for Pb and possibly other ore components, but not all potential reservoirs were able to be tested. Variably altered and gold mineralized samples that had Paxton Granite precursors show a loss of Nd and a trend from ?Nd(1580 Ma) values of about -6.2 in slightly altered samples towards more positive ?Nd values of about -3.3 in high grade gold samples. Of the local lithologies, the Lady Jane Diorite has ?Nd(1580 Ma) values of about +0.15 and is the most probable source of the more primitive Nd isotopic components in mineralized samples. Whether the Lady Jane Diorite is a cause of mineralization at Tarcoola, or both it and the mineralization are symptoms of the same tectonothermal event at ~1580 Ma can not be resolved with the available data. The demonstrated age relationship between the diorites and mineralisation at Tarcoola, the Nd data at Tarcoola, and the presence of similar dioritic dykes in all of the known gold prospects in the Central Gawler Gold Province are strongly suggestive that such dyking was an essential ingredient in gold mineralization throughout this province.

Deposits of the iron oxide copper-gold (IOCG) family are diverse in composition, ranging from relatively reduced magnetite-rich to hematite-dominated oxidized styles that may contain major uranium resources. These variations reflect an interplay between multiple hydrothermal fluids with different properties and the varied host lithological settings, each of which ultimately relate to differences in the tectonic evolution of IOCG provinces. - Early Mesoproterozoic and late Paleoproterozoic terranes of southern Australia host the 7.7 Bt Olympic Dam Cu-Au-U deposit as well as numerous lesser known IOCG±U deposits and prospects. They represent the spectrum of magnetite- to hematite-dominated IOCG deposit styles. Magnetite-rich styles of IOCG alteration and mineralization occur in the eastern Gawler Craton and western Curnamona Province. These styles developed at mesozonal to epizonal crustal levels between ~1610 Ma and ~1575 Ma when the terranes were subject to low pressure - high temperature metamorphism and compressional deformation. Magnetite-rich alteration was spatially and temporally associated with high-temperature A-type granitoids of the Hiltaba Suite and with mafic magmas, although the magnetite-forming fluids show major chemical and isotopic contributions from non-magmatic sources. Magnetite-rich alteration varies between IOCG districts and includes biotite-albite or albite-clinopyroxene-actinolite or K-feldspar. Generally minor quantities of chalcopyrite, gold, pyrite and rare pyrrhotite occur in magnetite-rich alteration. - Higher grade IOCG ±U mineralization is associated with hematite-rich alteration (± sericite, chlorite, carbonate) which generally overprinted magnetite-rich assemblages and formed at epizonal crustal levels prior to ~1575 Ma. In the Olympic Dam district uplift and exhumation inferred between ~1595 Ma and ~1575 Ma resulted in superposition of hematitic ±U mineralization on magnetite-rich alteration. Brittle deformation structures and the temporal and broad spatial association of hematitic alteration with bimodal volcanism and alkaline mafic magmatism are consistent with an extensional setting, although supporting data are not yet comprehensive. - The switch from syn-orogenic mesozonal magnetite-rich IOCG formation to post-orogenic epizonal hematite-rich IOCG mineralization is suggested to reflect a change from compressional to extensional tectonism at ~1595-1590 Ma. Contrary to propositions of an 'anorogenic' setting for the Olympic Dam deposit, back-arc settings have been advocated recently. However, several features including the composition of the Hiltaba Suite and co-magmatic volcanics are inconsistent with typical backarc settings. Alternatively, intracontinental orogenesis was closely followed by extension and uplift. A possible driver was convective removal or delamination of thickened lithospheric mantle, triggered by convergence at distant plate margins. This model reconciles magma compositions and timing of volcanism with a switch from compression to extension.

Comprehensive studies of the well preserved, Paleoarchean Panorama volcanic-hosted massive sulfide (VHMS) district provide for the first time definitive evidence that Zn, Pb, Cu, Mo and Ba were leached from the base of the volcanic pile and redeposited at the top in VHMS deposits. This leaching provided more than enough metal to form known deposits, implying that direct input of metal is not required. Sulfur is depleted from the base of the volcanic pile, in line with an increase in Fe2O3/FeO and hematite alteration. These data, combine with sulfur isotope data, indicate that seawater sulfate reduction was facilitated by the oxidation of rock FeO to hematite at high temperature in the H2S stability field. This is the first time such processes have been demonstrated regionally in ancient VHMS mineral systems. The data presented here require Paleoarchean seawater to be sulfate-bearing.