Abstract attached

The Herberton/Mount Garnet area is situated in north Queensland, southwest of Cairns (Fig. 1). It is bounded by latitudes 17°15'S and 17°45'S, and by longitudes 145°00'E and 145°30'E, and comprises 2885 sq km. The area is covered by the Herberton and Mount Garnet 1-mile Military map sheets, and lies within the Atherton 1:250,000 Sheet area. Almost the whole of the productive part of the Herberton Tinfield* is covered by the two 1-mile map sheets.

An orogenic cycle typically follows a sequence of events or stages. These are basin formation and magmatism during extension, inversion and crustal thickening during contractional orogenesis, and finally extensional collapse of the orogen. The Archaean granite-greenstone terranes of the Eastern Yilgarn Craton (EYC) record a major deviation in this sequence of events. Within the overall contractional stage, the EYC underwent a lithospheric-scale extensional event between 2665 Ma and 2655 Ma, resulting in changes to the entire orogenic system. These changes associated with regional extension include: the crustal architecture; greenstone stratigraphy; granite magmatism; thermo-barometry (PTt paths); and structure. Synchronous with these changes was the deposition of the first significant gold, and it is likely that the intra-orogenic extensional event was one of the critical factors in the region's world-class gold endowment.

A diverse range of mineralisation, including porphyry and epithermal deposits, intrusion-related gold and other metal deposits, iron oxide-copper-gold (IOCG) deposits and orogenic gold deposits all have linkages to crustal growth and magmatic arcs. Furthermore, all of these deposit types are associated with fluids containing H2O, CO2 and NaCl in varying and differing proportions. In all cases, it can be argued that magmas are a key source of hydrothermal fluids for these types of mineral system, and that subduction processes are critical to controlling fluid chemistries, the metal-bearing capabilities of the fluids and depositional processes. The differences on typical/bulk fluid chemistries between deposit types can be explained in part by differences in the P-T conditions of fluid segregation from its magmatic source. The most significant control here is the pressure at which fluid forms from the magma as this has a strong effect on fluid CO2/H2O values. This is clearly exemplified by the rare occurrence of readily detectable CO2 in deep porphyry systems (Rusk et al., 2004). On the other hand, fluid Cl contents (which strongly influence its base metal carrying capacity) are very sensitive to the magma's bulk composition. However, only some subduction-related magmas are fertile, and the differences do not seem to be due solely to variations in effectiveness of depositional processes. So what controls the volatile content of the magmas? Isotopic and other evidence, in particular for S and Cl, shows (unsurprisingly) that the greater contents of these elements in arc magmas compared to other melts is due to contributions from subducted materials, although there may be additional, lower crustal sources of Cl. Variations in the budget of volatiles subducted may thus play a role in controlling the chemistry of magmas and associated hydrothermal fluids, but variations within individual arcs suggests that again this is not the entire story.

DRAFT Australia's Resources Supporting Economic Growth in the Nation and the Region Paul J Kay Geoscience Australia A new book on Australia's geology viewed through the lens of human activity has been prepared by Geoscience Australia for the 34th International Geological Congress (IGC). Geological factors influencing the nation's recent economic development make up one chapter of the IGC book. Australia's long geological history, fringing passive margins, limited recent deformation and overall landscape stability has formed and preserved a vast quantity of high quality bulk commodity resources. The nation's educated workforce, system of government and legal framework has provided a sound, stable foundation allowing the geological legacy to be utilised through a large export industry for societal and national benefit. The bulk resources of coal, iron, aluminium and liquefied natural gas (LNG) account for more than 50 percent of Australia's export earnings, sustaining the nation's economic success and the lifestyle of the Australian people. Mining has been a cornerstone of the Australian economy since the 19th century gold rushes and importance the resources sector has increased markedly since the mid 20th century, largely a consequence of accelerating export income from the bulk commodities. The industrialisation of Asia has provided the demand, driving infrastructure investment in remote regions of Australia. Advances in technology combined with massive economies of scale and sound public policy have enabled access to the resource and helped to satiate the growing regional market. Responding to changes in the existing status quo, be they trade or societal, will require ongoing interactions between the geosciences and other disciplines to maintain and improve Australia's standard of living.

Globally supracrustal sedimentary rocks are known to preferentially precipitate gold between 2400 Ma and 1800 Ma (Goldfarb et al. 2001). The Palaeoproterozoic Tanami and Pine Creek regions of Northern Australia host one world-class gold deposit and many other gold deposits in anomalously iron-rich marine mudstones (Figure 1). New fluid-rock modelling at temperatures between 275 - 350C suggest a strong correlation between gold grade and these Palaeoproterozoic iron-rich, fine-grained sedimentary rocks.

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