Authors / CoAuthors
Mernagh, T.P. | Bastrakov, E.N. | Wyborn, L.A.I.
Abstract
Controversy continues on the origin of gold deposits in metamorphic belts and the role of magmatism in these regions. We adopt a Minerals Systems approach to analyse and compare some chemical processes related to the formation of major Australian Au-dominated deposits that have been classified as either orogenic or intrusion-related. Fluid inclusion data was compiled from deposits in the Archaean Yilgarn Craton, the Proterozoic Tanami, Pine Creek and Paterson areas, and the Palaeozoic western Lachlan Fold Belt. On a regional scale, and a deposit scale, the dominant lithologies in each area are mafic and felsic igneous rocks, graphite-bearing clastic sediments and banded iron-formations. Significantly, evaporites are absent from all areas. A clear spatial association exists in the Tanami, Telfer and Pine Creek regions with reduced granites. The complied data show that the deposits form over a wide range of temperature-pressure conditions (<200 to >600ºC, <1.4 kbar) and that they involve fluids with broadly similar major chemical components (i.e. H2O+NaCl+CO2± CH4 ± N2). The main difference is that Telfer, Tanami and Pine Creek deposits have higher salinity fluids. Elsewhere, deposits classified as orogenic gold deposits have low salinity fluids (typically <10 wt.% NaCl eq.) with CO2 contents ranging from 10 to 25 mol.% (Ridley & Diamond, 2000), whilst intrusion-related gold deposits may show evidence of higher CO2 and/or high salinity fluid inclusions (Thompson & Newberry, 2000). Processes thought to cause gold precipitation in both types of deposits are fluid-rock interaction (e.g. desulphidation), phase separation, or fluid mixing. We have re-examined the impact of the H2O-NaCl-CO2 system on the nature of the dominant gold precipitation mechanisms at different crustal levels (Fig. 1). The latter infers different roles of chemical (fluid-rock interaction) vs rheological (phase separation and/or fluid mixing) host-rock controls on gold deposition. This also implies that at the site of deposition, similar precipitation mechanisms operate at similar crustal levels for both orogenic and intrusion-related gold deposits
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nonGeographicDataset
eCat Id
63529
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Cnr Jerrabomberra Ave and Hindmarsh Dr GPO Box 378
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- External PublicationAbstract
- ( Theme )
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- economic geology
- ( Theme )
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- geochemistry
- ( Theme )
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- mineral deposits
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
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2005-01-01T00:00:00
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