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Proterozoic sediment-hosted base metal deposits. CODES/AMIRA Project P.384 Report No. 2, May 1993

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Leaman, DE (1993) Proterozoic sediment-hosted base metal deposits. CODES/AMIRA Project P.384 Report No. 2, May 1993. Technical Report. CODES/AMIRA, Hobart, Tasmania. (Unpublished)

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Abstract

REGIONAL GEOPHYSICS AND BASIN ARCHITECTURE This report describes further advances in the regional basin architecture study. Some revisions and refinements of previously reported material within the Batten Trough are included. The extension of the interpretation southward into the Wallhallow region and eastward onto the Wearyan Shelf is also described. This new work generally confirms the implications of the original study and suggests that the Batten Trough has had a long and complex history. The first two stages of its evolution include large-scale volcanism and irregular extension and uplift. The structural environment of the Wearyan Shelf is shown to be distinct. Most refinements of the Batten Trough study relate to additional peripheral studies, either reported here or under way, and involve small variations in presented diagrams or additional diagrams. The latter include one omitted from the original report and a detailed version of the distribution of basement granitoids. The few seismic sections available have also been reviewed in light of the implications of the gravity-magnetics interpretation and found consistent with it. Some readers of the first report were concerned that the inferences concerning substantial, differentiated and concealed volcanic piles might be suspect. Since this pre-Tawallah Group aspect of the interpretation may ultimately prove crucial to an understanding of the basin, its evolution and its mineralisation, it is necessary to demonstrate that feasible alternatives do not exist. An example of such analysis is included here. This shows that the potential field data do not leave much conceptual uncertainty although many details remain to be worked out. Thick, deep volcanic sections do exist in the McArthur Basin. Others have noted that some interpretation fits were less than ideal and that as an consequence the entire interpretation might be suspect. Lack of fit perfection may result from the nature of the regional data sets and their subsampling for evaluation purposes, the scale of the analysis, and the amount of time actually spent on each analysis. Irrespective of any deficiencies in these factors, an outline of the very rigorous interpretation criteria actually used is included. This outline, and the discussion of volcanic piles, shows that any imperfections are not of consequence at the scale of this study and any refinements are either not justified or not meaningful. The implications of these findings have yet to be appraised and the significance of the Batten Trough, its margins and structures such as the Emu Fault await completion of interpretation across the Bauhinia Shelf. This study will be described in the next report. However, it is evident that a coherent history for the basin and its deep volcanic sequences is evolving. The stratigraphic implications are contrasted with the observed relationships in the nearby Isa Inlier. A future study will test the possible correlations across the Murphy Inlier, South Nicholson Basin and Lawn Hill Platform. GOLD METALLOGENY AND THE Cu-Au ASSOCIATION OF THE AUSTRALIAN PROTEROZOIC - Results of research from a previous Proterozoic Cu-Au project Australian Proterozoic copper-gold deposits, emplaced mainly at 1.6 to 2.0 Ga, are divided into the following categories: (1) breccia-hosted iron oxide-dominated Cu-U-Au-REE deposits spatially associated with fractionated felsic intrusions (273t Au); (2) Au ± cu-bearing iron formations (l28t Au); (3) unconformity-style U ± Cu/PGM/Au deposits (53t Au); (4) iron-oxide-dominated pipes hosted within shears and folds (147t Au); (5) Broken Hill and volcanic-hosted massive sulphides (150t Au); (6) iron-sulphide-dominated veins and replacement zones spatially related to felsic intrusions (148.7t Au), and (7) iron-sulphide-dominated veins and replacement zones spatially related to elements of regional deformation (147t Au). Categories (1) to (4) are mainly confined to Proterozoic rocks, constituting an assocation in which Au and Cu are commonly present together, with variable amounts of U, Bi, Co, W, Se, Te and REE. Most examples in categories 1 to 4 fall into either of two groups: Cu-Au-magnetite±hematite types formed at relatively high temperature (300 to 450 degrees C), and Cu-U ± Au -hematite types formed at 150 to 300 degrees C. We postulate that these ores formed from a common high salinity (15-35 wt. % NaCl equiv.), low total sulphur (= 10-3 to 10-2), high f02 fluid-type, in which metal transport was dominated by chloride-eomplexing. The most effective method of metal deposition was fluid mixing, achieving a synchronous decrease in f02 and temperature. This unusual oxidised fluid association was favoured in high heat-flow extensional settings containing oxidised and/or oxidised-evaporitic sedimentary sequences. The intrusion of oxidised fractionated granites, which are commonly temporally associated with metal emplacement, acted in some places to heat and focus basinal fluids, and in others was the ultimate source of metals.

Item Type: Report (Technical Report)
Publisher: CODES/AMIRA
Date Deposited: 13 Nov 2009 03:43
Last Modified: 18 Nov 2014 04:06
URI: http://eprints.utas.edu.au/id/eprint/9374
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