Browsing by Author "Höytiä, Henri"

Komatiites are ultramafic rocks produced in high-degree (> 20 %) partial melting of the mantle. They can be emplaced as extrusives and subvolcanic dikes and sills. Komatiitic melts are of high-temperature, low-viscosity, and naturally enriched in base and precious metals, e.g., Ni, Cu, and the platinum group elements (PGE). Consequently, they are able to form komatiites-hosted Ni-Cu-PGE deposits that globally comprise a significant source for base and precious metals. For such ores, the sulfur-undersaturated komatiitic melts need to reach sulfur saturation. This can be achieved by gaining external sulfur from, e.g., sulfide-bearing country rocks. Komatiites are found in Archean to Proterozoic greenstone belts. Due to their age, primary mineral composition and original spatial position of komatiites have been modified by metamorphism, deformation, as well as, erosion. Geochemical studies of assimilation signals, depletion/enrichment in base and precious metals together with volcanological and stratigraphic studies are critical in exploration of komatiite-hosted Ni-Cu-PGE deposits. The Eastern Lapland Archean domain (ELAD) is a granite-gneiss-greenstone terrain with abundant komatiitic rocks, located in the municipalities of Salla and Savukoski in Northern Finland. It is characterized by amphibolite-facies metamorphism and predominant thrust tectonics, which have destroyed the majority of primary magmatic characteristics in the area. In this thesis, ore potential of the ELAD komatiites is evaluated on the basis of field observations and samples collected during geological mapping of komatiitic bodies of the area. Studies of thin section petrography, whole-rock geochemistry and mineral chemistry of olivine and spinel have been performed. In addition, previously analyzed whole-rock data provided by the Geological Survey of Finland (GTK) are utilized. The komatiitic lavas and cumulates of ELAD were derived from parental melts of komatiitic basaltic composition. Cumulates formed from these melts are critical for exploration. They have been interpreted as basal cumulates of thick komatiitic sequences or cumulates of major magma pathways. Furthermore, komatiites crystallized from Cr-undersaturated melts and komatiites showing depletion or enrichment with respect to Ni are found in the area. Both of these features have been attributed to potentially mineralized environments. Also, sulfide-bearing country rocks, which provide a potential source for sulfur, are present. Nevertheless, strong alteration of the ELAD komatiites hampers geochemical interpretations.