Fluid inclusion geochemistry of the early Proterozoic Pirilä gold deposit in Rantasalmi,Southeastern Finland |
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| Authors: | M Poutiainen |
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| Institution: | (1) Department of Geology, University of Helsinki, Box 115, SF-00171 Helsinki, Finland |
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| Abstract: | The small Pirilä gold deposit, which is located in the southeastern part of the Svecofennian complex near the Archean/Proterozoic boundary, is hosted by quartz veins and lenses occurring in mica schist. The rocks of the area were metamorphosed under conditions of amphibolite facies. Gold is invariably associated with sulphides. Microthermometry of fluid inclusions in quartz indicates four types of inclusions: (1) weakly saline H2O-CO2 (< 4.0 eq.wt% NaCl) with small amounts of CH4 (< 10 mole% CH4); (2) CO2 (< 10 mole% CH4); (3) CH4; and (4) H2O (< 25 eq.wt% NaCl) with less than 0.85 mole% CO2 in the vapour phase. Texturally these inclusion types are classified as primary (H2O-CO2) and secondary (H2O, CO2 and CH4). Leachate analysis shows that, in addition to Na, the aqueous fluids contain Ca and Fe with minor amounts of K and Mg. The primary H2O-CO2 and the secondary H2O inclusions contain sulphide and unidentified opaque grains, respectively. The secondary CH4 inclusions are often associated with short trails of arsenopyrite grains. Fluid inclusion and geological data suggest ore mineral mobilization, crystallization of host quartz, and deposition of sulphides controlled by the D2 and D3 structures in the presence of a H2O-CO2 fluid mainly during the plastic D3 deformation and during the amphibolite facies metamorphism (i.e. 3.4 kbars/540–670°C). During ductile-brittle deformation (probably D4), precipitation of tectonic remobilized gold from sulphides in fractures occurred in the presence of CH4 and H2O fluids at lowered temperature (< 440°C) and pressure (< 2 kbars). |
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