Sulfidation and Oxidation Haloes as Guides in the Exploration for Metamorphosed Massive Sulfide Ores
Paul G. Spry, 1998. "Sulfidation and Oxidation Haloes as Guides in the Exploration for Metamorphosed Massive Sulfide Ores", Metamorphic and Metamorphogenic Ore Deposits, Frank M. Vokes, Brian Marshall, Paul G. Spry
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The effects of sulfidation and oxidation on country rocks enclosing metamorphosed massive sulfide deposits constitute an exploration guide to ore. Sulfur and oxygen from the sulfide deposit react with the Fe component of ferromagnesian silicates (e.g., garnet, staurolite, chlorite, biotite, and amphibole) or oxides (e.g., gahnite and högbomite) to produce more Mg- or Zn-rich silicates or Zn-rich oxides with proximity to ore. This halo of Mg-rich or Zn-rich minerals can be superimposed onto zones of premetamorphic alteration or can be developed around ore deposits where signs of alteration are apparently absent. Corresponding zonation patterns are also observed for minerals in the system Ca-Ti-Fe-S-O (e.g., titanite, ilmenite, and rutile). The compositions of ferromagnesian silicates and Zn-rich minerals, as well as the distribution members of the system Ca-Ti-Fe-S-O, are dictated by bulk-rock composition and by a variety of physicochemical conditions including T, P, , , and . Sulfidation-oxidation haloes are most prominent where a large - gradient is apparent around an ore deposit. The presence of pyrrhotite, pyrite, and magnetite (high - conditions) in a metamorphosed orebody that is enveloped by graphite (low conditions)-bearing country rocks ensures such a gradient. The proximity and abundance of graphite in part dictate the width of the sulfidation-oxidation halo to the orebody. Where graphite occurs adjacent to an orebody, the halo will be narrow but if graphite occurs tens to hundreds of meters from sulfide mineralization the likelihood of a wider halo is increased.
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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