Browsing by Author "Kuikka, Antti"

Kalliojärvi, Ania and Erkkilä gold deposits are located in the Pirkanmaa migmatite belt in southern Finland. Nearest city is Tampere, approximately 15 km northeast of the deposits. The native gold occurs with sulphide dissemination in quartz veins and shear zones in hydrothermally altered mica schist, gneiss and granite-pegmatite intrusions and black schist. Black schist is common in Erkkilä deposit. Tourmaline pegmatite is common in Ania deposit. Gold grades are in the range of 0,1-26,7 g/t. The peak of metamorphism has reached upper amphibolite fasies in Erkkilä deposit (K-feldspar + sillimanite isograd). In Kalliojärvi the metamorphic index mineral is staurolite, which indicates middle amphibolite facies conditions. In Ania the metamorphism has progressed to the lower amphibolite facies stability field of garnet. These metamorphic associations indicate pressure of 8 kb to 6 kb and temperature of 790 to 525 °C. The hydrothermal alteration is strong in the mineralized zones, demonstrated by silicification and potassium metasomatism, which altered feldspars to sericite and biotite to chlorite. Plagioclase has also altered to albite and potassium feldspar, epidote and calcite and ilmenite has altered to rutile. In the mineralisation process fluids have enriched in the same ore forming elements in all three deposits. These elements include Fe, Cu, Ni, Co, Au, Ag, Hg, Pb, Bi, Sb, Te, S, Se, As. The major ore minerals are arsenopyrite, löllingite, pyrrhotite, pyrite and chalcopyrite. Other ore minerals are native bismuth, gold, bismuth tellurides, maldonite and oxide minerals. The major ore forming process has been open space filling. The ore minerals have crystallized as disseminations in intergranular spaces between silicates and carbonates, in quartz- and epidote-clinozoisite veins and walls of the fluid channels. The arsenopyrite geothermometry shows crystallization temperature from 438 to 609 °C with pyrrhotite and löllingite. The sulfur activity (logS2) has been from -3,70 to -8,30. Fluids transported gold predominantly as a reduced sulphur complex and a thioarsenide complex. The first generation native gold is intergrown and as inclusions in arsenopyrite and in grain borders of composite löllingite and arsenopyrite grains. The second generation native gold occurs as open space fillings in the gangue fractures, intergranular spaces between silicates and as inclusion in potassium feldspar and sulphides, Bi-, Sb- Te-minerals and metallic bismuth. The main crystallization of native gold and tellurides took place in the epizonal temperature range from 150 to 300 °C. According to the mineral liberation analyses in Ania the grain size of gold, 86,10 wt. %, is from 32 to 106 µm, in Erkkilä 98,22 wt. % is from 32 to 355 µm and in Kalliojärvi 76,02 wt. % is from 32 to 106 µm. In heavy mineral concentrates gold occurs as free grains and also intergrown with arsenopyrite and bismuth. A small portion of gold is as inclusions in other minerals in arsenopyrite and silicates. The mineralisation and crystallization of the ore minerals is epigenetic i.e. it was active after the last metamorphic event in shear zones of the host rock. All three gold deposits have crystallized after a complex, accretionary to collisional orogenic evolution of Svekofennian. The ore mineral crystallization occured in the hypozonal-epizonal temperature range from 600 to 150 °C.