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

The authors were engaged in geological reconnaissance work in the Fitzroy Crossing - Halls Creek area, during the period 16th August to 23rd September, 1948. The principal object of the investigation was to examine the area covered by the Mt. Ramsay Sheet of the Army Series. Work was extended beyond the limits of this sheet however in order to obtain information with regard to the age relationship of some formations, and a visit was made to the Wolf Creek Meteorite Crater 63 miles south of Halls Creek. During the survey, work was concentrated chiefly in mapping the distribution of rocks and different geological periods, determining their relationships and economic possibilities, and recognising areas warranting more detailed investigations. Accompanying geological plans and aerial photographs are included.

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.

This study reports results of mass transfer calculations using chemical modelling software (HCh) to determine chemical parameters that may have a significant effect on turbidite-hosted gold deposition in Phanerozoic metamorphic terranes. The geochemical models herein consider an 18 component system (Al-As-Au-C-Ca-Cl-Cu-Fe-H-K-Mg-N-Na-O-Pb-S-Sb-Si) and thermodynamic data to simulate a number of geochemical processes including fluid-rock interaction, gas partitioning and mineral precipitation in veins. Each modelling run consists of four parts, namely (1) the minerals predicted to precipitate in the vein, (2) the composition of the fluids in the vein, (3) the predicted alteration assemblage of the host-rocks due to fluid-rock interaction and (4) the composition of the fluids during fluid-rock interaction. Results of the modelling are in good agreement with observed mineral assemblages in variably-endowed orogenic gold provinces (central Victoria and NE Tasmania, Australia; Buller Terrane, New Zealand; Meguma Terrane, Canada; Sierra de Rinconada, Argentina) and illustrate that gold can be precipitated efficiently and over a wide temperature range (350 - 200 C in this study) from low-salinity, mixed aqueous-carbonic fluids containing up to 0.1m CO2. The modelling shows that the absence of certain physicochemical processes (e.g., boiling) or fluid constituents, such as low total sulphur or lack of CO2 may inhibit gold transport and precipitation in some environments. All the modelling runs, except the one involving a low total sulphur fluid, predict the precipitation of sulphides in the host-rocks due to desulphidation processes. However, the highest gold grades are predicted to occur in the vein mainly from partitioning of H2S into the vapour phase during phase separation. This indicates that the efficiency of gold mineralisation is dependant on the concentration of CO2 and other gases in the fluid because of their effect on immiscibility (in both closed and open systems) and the efficient transport and precipitation of gold. We also investigated what effect the composition of a range of source rocks (i.e., granite, turbidites, greenstones, auriferous exhalative interflow sediments) have on gold solubility, as this relates to masses of gold that are accessible by leaching fluids and hence to ore transport and formation. Our thermodynamic calculations suggest that the chemical composition of the hypothetical source rock has only a small influence on the solubility mainly influenced by the initial concentration of gold in the rock.

No abstract available

Abstract The Palaeoproterozoic, from 2100 to 1800 Ma, is recognised as the third largest period of orogenic gold mineralization. In contrast to earlier Archean orogenic gold episodes which occur predominantly in greenstone terranes, supracrustal sedimentary rocks became increasingly important as hosts in the Palaeoproterozoic. Unusually iron-rich 1840 Ma marine mudstones in the Tanami region host one world class gold deposit and many other gold deposits. Fluid-rock modelling at 350°C suggest a strong correlation between gold grade and these iron-rich, fine-grained sedimentary rocks and suggest that gold may precipitate in the iron-rich sediments in the first stage of mineralization, before remobilization of the gold further enhances the grade of the deposit. New regional stratigraphic correlations for similar iron-rich rocks to those in the Tanami region are suggested with ~1860 Ma gold-bearing stratigraphy in the Pine Creek region and potentially with ~1860 Ma host rocks in the Tennant region. These Northern Australian Palaeoproterozoic iron-rich sedimentary rocks could be linked globally to similar aged iron-rich and gold-bearing sedimentary rocks in Homestake, U.S., Ghana, West Africa and elsewhere. From about 2400 to 1800 Ma the Palaeoproterozoic is also marked by the occurrence of mainly Superior-style BIF's, which are attributed to the progressive oxygenation of the deep oceans resulting in the global scrubbing of iron from the oceans. The high iron concentrations noted in pre-1800 Ma marine sediments in Northern Australia could also be related to this same process and help explain the anomalous concentration of orogenic Au deposits from 2100 to 1800 Ma.

Sandstone uranium mineral systems

Legacy product - no abstract available

The eastern Yilgarn Craton (EYC) of Western Australia is Australia's premier gold and nickel province, and has been the focus of geological investigations for over a century. Geoscience Australia, in conjunction with partners in the Predictive Mineral Discovery Cooperative Research Centre conducted a series of projects between 2001 and 2008 (Y4 project team, 2008). This article summarises the highlights and new findings from the research, many of which challenge previous paradigms on the tectonics and architecture, as well as the relationship of gold to structure, magmatism and metamorphism. Although a Yilgarn-based study, the results have general implications for other Archaean terranes.