Regional Metamorphism and Ore Formation: Evidence from Stable Isotopes and other Fluid Tracers
Christoph A. Heinrich, Anita S. Andrew, Matthias D. Knill, 1998. "Regional Metamorphism and Ore Formation: Evidence from Stable Isotopes and other Fluid Tracers", Metamorphic and Metamorphogenic Ore Deposits, Frank M. Vokes, Brian Marshall, Paul G. Spry
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How can stable isotopes and other fluid tracers be used to interpret the formation of ore deposits hosted by regional metamorphic terranes? The central question of metamorphogenic ore formation versus a regional metamorphic overprint must consider two basic constraints, as imposed by mass-balance and ore-metal solubility. First, regional metamorphism of an existing orebody generally involves rock-dominated fluid-rock interactions that rarely permit significant modification of preexisting elemental or isotopic compositions at a scale greater than that of an orebody. Second, the generation of a significant ore deposit during regional metamorphism requires, just as in any ore-forming hydrothermal system, a large amount of fluid from a km3-sized and generally rock-dominated source to be focused through a small and generally fluid-dominated depositional site.
Well-documented case studies are reviewed to illustrate these general principles. The small metamorphosed Pb-Zn-As deposit of Lengenbach (Alps) and the large metamorphogenic Cu deposit of Mount Isa (Queensland) illustrate why deposits hosted by metacarbonate rocks often yield unambiguous stable isotope evidence for a relationship between metamorphism and metal accumulation. In metamorphosed base metal deposits and metamorphogenic gold deposits hosted by common silicate metamorphic rocks (taking the base metal deposits at Ducktown, Tennessee and Cobar, NSW, and the lode-gold deposits at Bendigo-Ballarat, Victoria, as examples), isotopic and other fluid tracers alone do not yield conclusive evidence for any necessary relationship between hydrothermal metal enrichment and regional metamorphism. Structural and geological observations at a scale greater than that of an orebody are essential to interpret the isotopic data and the origin of such deposits. Metamorphogenic base metal mineralization is inherently rare because it requires rather atypical conditions of regional metamorphism, including a large supply of salt for metal complexing. The process is, however, difficult to identify isotopically, and may therefore be more common than reported. Mesothermal gold deposits are the only well-established group of major deposits formed by upward-focused flow of fluids with at least a component originating from prograde metamorphic devolatilization. Their common occurrence can be related to liberation of H2S, which forms extremely stable gold complexes, by prograde metamorphic reactions. Isotopic tracer data do not alone preclude an important precious metal addition from magmas or other deep sources, even in these clearly metamorphogenic deposits.
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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