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Gold Deposits in Amphibolite and Granulite Facies Terranes of the Archean Yilgarn Craton, Western Australia: Evidence and Implications of Synmetamorphic Mineralization

By
J. Ridley
J. Ridley
Institut für Isotopengeologie und Mineralische Rohstoffe, ETH-Zentrum, CH-8092, Zürich, Switzerland,
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D.I. Groves
D.I. Groves
2
Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, University of Western Australia, Nedlands 6907, Australia
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J.T. Knight
J.T. Knight
3
BHP, P.O. Box 6062, East Perth, Western Australia 6892, Australia
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Published:
January 01, 1998

Abstract

A significant proportion of gold production from the Archean Yilgarn craton in Western Australia has come from lode- and vein-style deposits hosted in amphibolite and granulite facies metamorphic rocks rather than in greenschist facies terranes, which are the normal hosts of such deposits in greenstone belts. The deposits in higher metamorphic-grade terranes have many similarities to those in lower-grade terranes, particularly the structural controls on deposit siting and form, and metal inventories.

Mineral assemblages in ore zones and surrounding alteration halos of deposits in high-temperature terranes are interpreted as belonging to either a high-temperature or a retrograde paragenesis. In metamorphosed igneous host rocks, quartz, biotite, Ca amphibole, plagioclase, and clinopyroxene, with generally only minor carbonate, are characteristic of the high-temperature paragenesis. Chlorite, sericite, and carbonate minerals, most commonly as patchy and partial replacement of higher-temperature minerals, belong to the retrograde paragenesis. Equilibration of the high-temperature assemblages was at temperatures and pressures similar to those of the local peak of metamorphism, or at slightly lower temperatures. Different high-temperature assemblages are related systematically to host-rock metamorphic grade. The retrograde minerals grew at greenschist or prehnite-pumpellyite facies conditions. Ore mineral assemblages are pyrrhotite dominated in almost all deposits with, in some cases, minor pyrite and chalcopyrite. In other assemblages, there is an important arsenopyrite component, with loellingite forming commonly in the cores of arsenopyrite grains. Compositions of ore minerals indicate variable equilibration at high temperatures or under greenschist facies conditions. Stable isotope compositions of vein minerals generally indicate equilibration at near-peak metamorphic conditions.

Combined textural, structural, and geochemical data show that mineralization most likely occurred at near-peak metamorphic conditions in the majority of these gold deposits. Gold introduction at greenschist conditions during the retrograde evolution of the terranes is ruled out from some of the common mineralogical settings of gold. It is generally in equilibrium with high-temperature gangue minerals and can be as invisible gold in loellingite that was overprinted by arsenopyrite prior to cooling below amphibolite facies conditions. Additionally, there is generally a close correlation of gold grade and specific high-temperature gangue assemblages. The retrograde paragenesis in the deposits is considered to be unrelated to gold introduction, although retrograde processes may have caused some ore remobilization. Differentiation between synpeak metamorphic mineralization and mineralization at lower-grade conditions during the prograde history is not clear in every deposit, in part because of textural equilibration of minerals in the alteration halos, such that replacement relations and paragenetic sequences can rarely be determined. Synpeak metamorphic mineralization is, however, indicated by the structurally late timing of many ore-controlling structures, the relatively low deformation state of many veins compared to that of wall rock, and the lack of a C and O isotope signature indicative of decarbonation, despite the low carbonate content of the ores in higher-grade terranes. Subtle differences in element enrichment and depletion patterns in the alteration halos of deposits in different grade host rocks may reflect stability of different minerals at the time of mineralization.

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Contents

Reviews in Economic Geology

Metamorphic and Metamorphogenic Ore Deposits

Frank M. Vokes
Frank M. Vokes
Volume Editor
Department of Geology and Mineral Resource Engineering Norwegian University of Science and Technology Trondheim, Norway
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Brian Marshall
Brian Marshall
Volume Editor
Department of Applied Geology University of Technology Sydney NSW 2007 Australia
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Paul G. Spry
Paul G. Spry
Volume Editor
Department of Geological and Atmospheric Sciences Iowa State University Ames, Iowa 50011 USA
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Society of Economic Geologists
Volume
11
ISBN electronic:
9781629490182
Publication date:
January 01, 1998

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