Petrology and structure of the Early Proterozoïc Pirilä gold deposit in southeastern Finland

The Pirilä gold deposit is located in the southeastern part of the Ladoga-Botnian Bay Zone, in a volcano-sedimentary environment metamorphosed under PT conditions of the amphibolite facies. Metavolcanic rocks include felsic and intermediate pyroclastics and tholeiitic to komatiitic lavas. The stratigraphically underlying metasedimentary rocks are mainly graywackes with some limestone and calc-silicate rocks. Sulfide and gold bearing quartz veins and lenses occur in a transitional zone between volcanic and sedimentary units, the latter including a narrow iron formation of silicate, oxide, and sulfide facies. The quartz veins and lenses were emplaced during the F2 and F3 deformational phases. The most common ore minerals are arsenopyrite, löllingite, pyrrhotite, and pyrite. In some places there is also abundant galena, sphalerite, and chalcopyrite. Gold and sulfides are associated with either quartz lenses and veins or with quartz-cummingtonite rock. The gold occurs predominantly as inclusions in arsenopyrite or in löllingite, and is only rarely visible macroscopically. The grains are mostly electrum, the silver content of the analyzed grains ranging between 10.6 and 60.0 percent. Silver also occurs as small dyscrasite inclusions in galena. The in situ ore reserves of the Pirilä deposit are 150,000 t at 8 g/t gold and 30 g/t silver. Smaller gold occurrences in a similar stratigraphic position are found near the Pirilä deposit. Further indications of gold are met along the same stratigraphic horizon over a distance of at least 30 kilometers.