Metamorphism of Ni-Cu Sulfides in Mafic-Ultramafic Intrusions: The Svecofennian Sääksjärvi Complex, Southern Finland
Franco Mancini, Heikki Papunen, 1998. "Metamorphism of Ni-Cu Sulfides in Mafic-Ultramafic Intrusions: The Svecofennian Sääksjärvi Complex, Southern Finland", Metamorphic and Metamorphogenic Ore Deposits, Frank M. Vokes, Brian Marshall, Paul G. Spry
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The Sääksjärvi ultramafic-mafic complex is an example of a nickel-copper sulfide deposit in upper amphibolite to lower granulite facies metamorphic rocks. Prograde metamorphic events at 650° to 750°C and 4 to 6 kb have recrystallized the host rocks to olivine-magnesiohornblende-orthopyroxene ± chromite assemblages (metaperidotite) and to magnesiohornblende ± clinopyroxene ± plagioclase ± quartz assemblages (hornblendites and gabbros).
These assemblages were overprinted subsequently by retrogressive serpentinization. Three distinct metamorphic effects have modified the sulfides, namely recrystallization, solid-state flow during deformation, and low-temperature alteration, leading respectively to the following types of mineralization: (1) disseminated sulfides; (2) sulfide veins; and (3) sulfides in serpentinite. The first type has retained primary interstitial structures and has undergone multiple exsolutions during the slow retrogressive cooling. The second type originated by the injection of the ductile sulfides into fracture and shear zones, where mechanical mixing with the more brittle gangue silicates formed sulfide-matrix breccias.
The last type of mineralization originated from the alteration of the primary sulfides during serpentinization. Analyses of the compositions of the sulfides have led to a general model describing the metamorphic effects in such low-grade mineralization in which: (1) disseminated sulfides received some addition of nickel due to reequilibration with olivine; (2) the preferential remobilization of copper sulfides in veins and breccias increased the Cu/(Cu + Ni) ratios; and (3) the low-temperature alteration effectively redistributed the sulfide elements in serpentinites, leading to highly variable Cu/(Cu + Ni) values.
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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