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Fluid evolution of the Paleoproterozoic Jokisivu orogenic gold deposit, SW Finland

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Title: Fluid evolution of the Paleoproterozoic Jokisivu orogenic gold deposit, SW Finland
Author(s): Kuusela, Tytti
Contributor: University of Helsinki, Faculty of Science, Department of Geosciences and Geography
Discipline: Geology
Language: English
Acceptance year: 2016
The Jokisivu gold deposit is a structurally controlled orogenic gold deposit that was formed during the Svecofennian orogeny when the Pirkanmaa belt, a turbidite dominated subduction zone complex, was pushed below the Tampere schist belt in the north. The Jokisivu gold deposit formed in a brittle-ductile shear zone and gold mineralization is related to the youngest deformation stage. The main focus of this study was in fluid inclusions in quartz veins. Quartz grains host fluid inclusions that carry information of the prevailing P–T–X conditions during fluid events in the quartz veins. According to the properties of fluid inclusion assemblages (FIA), six fluid inclusion types were distinguished. Fluid inclusion types A1, A2 and A3 are aqueous and fluid inclusion types B1, B2 and B3 are aqueous–carbonic. Relative chronology of the fluid inclusion types was determined by cross cutting relationships of fluid inclusion assemblages. Relative chronology of fluid inclusion types from oldest to youngest is A1, A2 and A3, B3, B1 and B2 indicating that aqueous fluids came in first and aqueous–carbonic fluids followed them. Microthermometry was conducted for all the FIAs. The melting point of CO2 was generally below the CO2 triple point ranging from -59.15°C to -56.73°C indicating the presence of other gaseous phases that were later confirmed with Raman spectroscopy to be CH4, N2 and even H2S. Raman spectroscopy was conducted for qualitative analysis of the phases presentn the inclusions. Quartz crystallization temperatures were calculated using titanium in quartz geothermometer (TitaniQ). TitaniQ was well suited for Jokisivu samples because titanium concentrations in the hydrothermal quartz were quite constant. Combining TitaniQ P–T slopes and fluid inclusion assemblage isochores from microthermometry and Raman spectroscopy yielded a good estimate for P–T conditions of fluid entrapment of 390–470°C and 170–345 MPa. The estimated temperature range is higher than the previous studies have suggested. However, the P–T range estimated for Jokisivu is in accordance with the P–T conditions of other Svecofennian orogenic gold deposits. Judged by the mineralogy of the quartz vein - host rock contact, the most important factor triggering gold precipitation in Jokisivu has probably been fluid-rock interaction. Strong sulfidation of host rock reduced the sulfur fugacity and enabled gold precipitation.

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