Gold Deposits in Amphibolite and Granulite Facies Terranes of the Archean Yilgarn Craton, Western Australia: Evidence and Implications of Synmetamorphic Mineralization
J. Ridley, D.I. Groves, J.T. Knight, 1998. "Gold Deposits in Amphibolite and Granulite Facies Terranes of the Archean Yilgarn Craton, Western Australia: Evidence and Implications of Synmetamorphic Mineralization", Metamorphic and Metamorphogenic Ore Deposits, Frank M. Vokes, Brian Marshall, Paul G. Spry
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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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The types of mainly metallic mineralization found in metamorphic terranes are reviewed and an attempt is made to define the genetic relations between the mineralization and the metamorphic events.The terms metamorphosed, metamorphic, and metamorphogenic as applied to ores are also considered.The development of thought and the history of investigations on ores in metamorphic terranes aretraced from the early work in the second half of the nineteenth century onward. Early conceptions ofmetamorphism as an ore-forming process (metamorphogenesis) were seemingly not followed up by theiroriginators, contemporaries, or immediate successors and were neglected until comparatively recentyears. The idea of metamorphism as a modifier of preexisting, mainly sulfidic, but also oxidic, mineralizationwon more immediate and general acceptance in the early decades of the present century. InNorth America, emphasis seems to have been mainly on the deformational aspects of the metamorphism,whereas elsewhere, especially in Europe, the textural and mineralogical results of the metamorphic recrystallizationalso received considerable attention and metamorphism as an ore-forming process hadwon a certain degree of acceptance. This difference in emphasis may perhaps be referred to the differentviews held regarding the initial genesis of the ores in the two regions.The late 1940s and the 1950s witnessed a considerable revision of ideas on ore genesis, especially regardingstrata-bound massive sulfide ores. A parallel revival of interest in the role of metamorphism,probably not unrelated to the foregoing, began in the early 1950s, to begin with concerning metamorphosedores. However, new thoughts concerning metamorphogenesis related to granitization or ultrametamorphismas an ore-forming process began to be published.The following decades witnessed an almost explosive increase in the number of publications dealingwith the effects of metamorphism on ore mineralization of practically all types, but with a definite emphasison sulfide ores of the strata-bound type. One of the most significant breakthroughs in this respectconcerned the world-famous Broken Hill deposit, New South Wales, although the metamorphosed natureof ores in the Scandinavian Caledonides, the North American Appalachians, the Lachlan fold beltof eastern Australia, the Sanbagawa terrane of Japan, the Urals, and Proterozoic fold belts in southernAfrica, have all been thoroughly documented.In recent years, however, the interpretation of many massive sulfidic ores in metamorphic terranes asmetamorphosed has been increasingly questioned, and syntectonic, metamorphogenic, origins havebeen advocated. There is obviously a great need to be able to distinguish more