ELIAS, 1988. Petrology and structure of the early Proterozoic Pirilä gold deposit in southeastern Finland. Bull. Geol. Soc. Finland 60, Part 1,[55][56][57][58][59][60][61][62][63][64][65][66] 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 F : and F 3 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 quartzcummingtonite 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.
The Osikonmäki gold deposit is hosted by an early Proterozoic tonalite close to the Archean-Proterozoic boundary along the Raahe-Ladoga suture zone. The structure, petrology and chemistry of the Osikonmäki intrusion are typical of synkinematic I-type granitoids having calc-alkaline affinity. The deposit was discovered by boulder tracing and is situated in a deformed shear zone. Gold is concentrated in open F3 synforms plunging 20 degrees to the ESE. The dominant ore minerals are pyrrhotite, arsenopyrite, löllingite and chalcopyrite, occurring as disseminations or in places as compact veins and stringers. Gold occurs as native gold and electrum in both sulphide and silicate phases and is invariably accompanied by Bi-bearing minerals with variable Se and Te contents. The measured mineral resources in the eastern part of the shear zone are 2 Mt at 3 g/t Au and 0.77 % As. Mineralogical and geochemical alteration is limited to the sheared and mineralized zone itself, manifested by biotitization, weak silicification and potassic alteration.
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