Fluid inclusion geochemistry of the early Proterozoic Piril� gold deposit in Rantasalmi, Southeastern Finland

Abstract

Abstract. The small Pirilfi 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 H20-CO2 ( < 4.0 eq.wt % NaCI) with small amounts of CH4 ( < 10 mole % CH4); (2) CO2 ( < 10 mole % CH4); (3) CH4; and (4) H20 ( < 25 eq.wt % NaC1) with less than 0.85 mole % CO 2 in the vapour phase. Texturally these inclusion types are classified as primary (HEO-CO2) and secondary (H20, 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 H20-CO z and the secondary HzO inclusions contain sulphide and unidentified opaque grains, respectively. The secondary CH 4 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 D 2 and D 3 structures in the presence of a HzO-CO z fluid mainly during the plastic D 3 deformation and during the amphibolite facies metamorphism (i.e. 3.4 kbars/540-670~ During ductile-brittle deformation (probably D4), precipitation of tectonic remobilized gold from sulphides in fractures occurred in the presence of CH 4 and HzO fluids at lowered temperature (< 440~ and pressure (< 2 kbars).The Pirii/i gold deposit is a base metal-bearing mineralization hosted by quartz lenses and veins in an early Proterozoic Svecokarelian complex in Rantasalmi, southeastern Finland. The deposit lies within the RaaheLadoga zone (Kahma 1973(Kahma , 1978, which is a boundary between the Archean and Proterozoic crust in central and eastern Finland (Fig. 1). The estimated ore reserves of the deposit are 0.15 Mt with an average grade of 8 g/t Au and 30 g/t Ag (Makkonen and Ekdahl 1988). The rocks of the Pirilfi area have suffered a polyphase deformation and an amphibolite facies metamorphism (Kilpel/iinen 1988;Korsman 1977). A fluid inclusion study was undertaken to obtain information on the temperature and composition of the fluids and to impose further constraints on the origin of the deposit. On the regional scale, fluids associated with the progressive stage of metamorphism and deformation were studied by Poutiainen (1990).