Browsing by Subject "Ostrobothnian Schist Belt"

Graphite is formed mainly by graphitization processes from organic precursor in high-grade metamorphic conditions, or precipitation of carbon from carbon-bearing fluids. Quality of the graphite, i.e. crystallinity, flake size and purity, determines its use and commercial value. Demand for graphite has been on the rise and is expected to increase more in the near future. The Korsnäs region in Ostrobothnia, Western Finland, is a prominent target for a graphite exploration based on the region’s geological history, i.e. the Svecofennian accretionary orogeny and high-grade metamorphic conditions. Graphite is ubiquitous in the area, and has been enriched mainly in the metapelitic schists intercalated within the paragneisses. The aim of the master’s thesis is to study the mineralogical characteristics and quality of the graphite, as well as estimate the economic importance of the graphite occurrences in the area of Korsnäs. Additionally, the origin of the graphite formations and their relationship to the regional metamorphic conditions are considered in this research. Mineralogical characteristics and quality of the graphite flakes were studied from the polished thin sections with optical microscopes and scanning electron microscope. Crystallinity of the graphite flakes was analyzed by a Raman spectroscopy, which has been used widely to measure orderly structure of a carbonaceous material. Peak temperatures of the metamorphic conditions were estimated from the Raman results using geothermometers. Based on the petrographic analysis, graphite is abundant, and the morphology refers to a flake graphite. Large proportion of the flakes are over 0.3 mm on their c-axis, which can be considered as large and the most valuable type of a flake graphite. Crystallinity of the graphite was determined from the Raman spectra using the peak intensity ratio (R1) of the graphite band and the main disordered band. The analyzed graphite flakes show good values for the R1 parameter (mean value 0.05 ± 0.05), indicating high crystallinity, and consequently, high-grade metamorphic conditions or precipitation from carbon-bearing fluids for the formation mechanism. The estimated peak temperatures are ranging from 650 to 695°C, which are consistent with the mineral assemblages of the migmatite and paragneiss samples, but inconsistent with the graphite-bearing metapelitic schists. The biotite-garnet and biotite-garnet-cordierite (sillimanite) paragneisses does not show any meaningful evidence of a retrograde overprint, but it is apparent that the graphite bearing samples have experienced retrograde metamorphism and hydrothermal alteration. This is evident from the lack of high-grade metamorphic minerals, presence of lower metamorphic grade minerals, and the estimated high peak temperatures for the formation of the graphite. Consequently, the R1 parameter suggests that the graphite in these rocks has not been affected by the retrograde metamorphism. The origin of the high-crystalline graphite appears to be a graphitization of biogenic material, but fluid-deposited graphite cannot be completely ruled out. Some carbon enrichment may have occurred by fluid-rock interactions. Further studies and detailed analyses, such as locating the possible high-grade deposits and determining the purity of the graphite, are needed. In conclusion, the high-crystalline graphite in the Korsnäs region is of good quality and shows prominent results for the commercial value.