Metamorphism of Komatiite-Hosted Nickel Sulfide Deposits
Komatiite-hosted magmatic nickel sulfide deposits fall into two major categories: sulfide-rich deposits at basal contacts of komatiite flow units (type 1, exemplified by the Kambalda deposits), and diffuse sulfidepoor disseminations within large extrusive bodies of olivine cumulate that are formed in major lava-flow pathways (type 2, exemplified by the giant Mt. Keith deposit in the Agnew-Wiluna belt of Western Australia).
Metamorphism of komatiites produces a very wide range of nonsulfide assemblages, despite the relatively restricted compositional range of the rocks; a crucial variable is the of the metamorphic fluid. The sulfide mineralogy of the komatiite-hosted deposits is influenced by the temperature and composition of the metamorphic fluid. Typical greenschist-facies hydration gives rise to serpentinites, hosting assemblages very rich in pentlandite and in some cases heazlewoodite. Reduction reactions associated with the serpentinization front give rise to Ni-Fe alloy-bearing assemblages. Oxidized fluids associated with low-temperature talc-carbonate alteration give rise to vaesite, polydymite, and millerite coexisting with pyrite and hematite.
Despite the foregoing commonality, the role of metamorphism in modifying the two types of deposit is very different. In type 1 deposits, the relative modal proportion of sulfide to silicate minerals is generally high, and the bulk of the nickel in the ore resides within sulfide minerals. In this situation, metamorphic reconstitution of the rock has relatively little impact on the overall nickel tenor of the sulfide fraction. However, metamorphism may still have important consequences on mineral grain size and the nature of silicate-sulfide intergrowths, a common consequence being the development of bladed or triangular-textured intergrowths of sulfide with metamorphic olivine. Associated tectonism may result in mobilization of massive sulfide bodies into low strain zones, but there is no evidence for any significant upgrading of sulfide ores by this mechanism.
In type 2 deposits, both the sulfide and silicate components of the ore may contain substantial proportions of the total nickel budget. Partitioning of nickel between silicate and sulfide fractions is a sensitive function of the metamorphic mineral assemblage. Under favorable circumstances, metamorphism can give rise to almost complete residence of nickel in sulfide minerals. The economic viability of type 2 deposits rests on the high nickel tenor of the disseminated sulfide component of the ore.
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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