Discriminating between Regional Metamorphic Remobilization and Syntectonic Emplacement in the Genesis of Massive Sulfide Ores
Brian Marshall, Paul G. Spry, 1998. "Discriminating between Regional Metamorphic Remobilization and Syntectonic Emplacement in the Genesis of Massive Sulfide Ores", Metamorphic and Metamorphogenic Ore Deposits, Frank M. Vokes, Brian Marshall, Paul G. Spry
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Metamorphosed deposits predate metamorphism and deformation and are characteristically remobilized. Metamorphogenic deposits form through scavenging in a metamorphosing source volume, liquid-state transfer and concentration at the sink site; the scavenging fluid phase derives from the source volume. Metamorphogenesis is a subcategory of syntectonic-synmetamorphic emplacement, but the latter can be effected by externally derived nonmetamorphic fluids.
Distinguishing between remobilized and syntectonically emplaced deposits is made difficult by: overly simplistic notions of deposit models and geometry; an evolving appreciation of the kinematic interpretation of structural observations; a better understanding of the merits and limitations of many laboratory-based techniques; the potentially overlapping nature of a high degree of liquid-state remobilization and syntectonic emplacement; lack of preparedness to accept polymodal genesis; and inadequate scales of observation coupled with an insufficient range of investigatory techniques. Each of these is discussed and illustrated. The distinction controversy is intensified by the adversarial nature of research within the current research-funding climate.
Successful discrimination between remobilized and syntectonically emplaced deposits requires the integration and refinement of data from multiple lines of investigation. This principle is widely recognized, but its practice is fraught with uncertainty. As a basis for practical discrimination, fifteen types of guidelines are identified and evaluated. Some of the more significant of these involve evidence from solid-state remobilization, preserved primary textures, the gross relationships of ore and alteration to unaltered host rocks, and comparison of deformation event sequences, silicate assemblages, and Pb isotope systematics from ore, alteration, and unaltered host rocks. Because discrete guidelines are rarely definitive and should not be used in isolation, interpretation should adopt a broadly based probabilistic approach.
Application of the probabilistic approach to the Broken Hill (New South Wales) and Cannington (Queensland) deposits, the Mount Isa-Hilton system (Queensland), Dugald River (Queensland), the Zn-Pb deposits of the Mt. Read volcanic rocks (Tasmania), and the deposits of the Kanmantoo Group (South Australia), suggests that they are pretectonic emplacements, substantially modified by regional deformation, metamorphism, and variously related hydrothermal events. Nevertheless, the cases for Cannington, Dugald River and, perhaps, Rosebery and the Kanmantoo Cu deposit, are less well founded. Further resolution of the genetic history of all of these deposits, including the possibility of polymodal genesis, requires refinement of the Pb isotope models for the various regions, better constraint of the pressure, temperature, and chemistry of fluids associated with various parts of the pre-, syn-, and posttectonic evolution of the various regions, and evaluation of the capacity of the various fluids to differentially dissolve, transport, and precipitate the metals of interest.
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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