Metamorphic Fluids and their Relationship to the Formation of Metamorphosed and Metamorphogenic Ore Deposits
I. Cartwright, N. H. S. Oliver, 1998. "Metamorphic Fluids and their Relationship to the Formation of Metamorphosed and Metamorphogenic Ore Deposits", Metamorphic and Metamorphogenic Ore Deposits, Frank M. Vokes, Brian Marshall, Paul G. Spry
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Metamorphic rocks produce fluids as devolatilization occurs during prograde metamorphism or as melts (which act as temporary repositories for fluids) crystallize during the early stages (>650°C) of cooling in high-grade metamorphic terranes. Metamorphosed shales and graywackes, which make up much of the sedimentary component of the upper crust, initially contain approximately 4 wt percent H2O, which may be liberated during the metamorphic cycle. These fluids may combine with others derived from external sources (e.g., synmetamorphic igneous intrusions or surface-derived fluids), and have the potential to transport heat, cause metasomatism, alter the rheology of the rocks, or form ore deposits. Metamorphic fluid flow in the crust is probably initially widespread, as fluids are derived from much of the rock mass, and then becomes increasingly channeled as fluids are focused along higher-permeability layers or along structures such as faults or shear zones. This type of flow path promotes ore genesis as metals can be scavenged from a large volume of rocks, with deposition occurring where fluids are focused and flowing down temperature. Most metamorphic fluids are dominated by H2O, with variable CO2 and minor amounts of other species (e.g., F, Cl, B, and S). At high to moderate metamorphic grades, H2O and CO2 are miscible at all XCO2 values unless significant salt is present. Such fluids transport some metals (e.g., Cu, Au, Ag) relatively efficiently but not base metals. Thus, the variety of metamorphogenic ore deposits will be limited unless input of saline fluid from other sources (e.g., igneous bodies) occurs, or the terrane is composed of significant volumes of meta-evaporites. However, remobilization of preexisting orebodies may occur during metamorphism and deformation.
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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