Geology, geochemistry, fluid inclusion characteristics, and U–Pb age studies on iron oxide–Cu–Au deposits in the Kolari region, northern Finland

Abstract

Several iron oxide–copper–gold deposits are known in the Kolari region, in the western part of the Central Lapland greenstone belt, northern Finland. They are hosted by clinopyroxene-dominated skarns that were formed near to contact zone between ca. 1860 Ma Haparanda Suite intrusions and > 2050 Ma Savukoski Group supracrustal rocks. All deposits are located within or next to shear and fault zones forming parts of the major, NNE-trending, Kolari shear zone. Three of the Fe–Cu–Au deposits, Kuervitikko, Cu-Rautuvaara, and Laurinoja were studied; all contain significant amounts of Cu (0.1% to 4.5%) and Au (0.1 to 6.6 ppm). At Laurinoja and Kuervitikko, Cu and Au are hosted by ironstone and skarn. At Cu-Rautuvaara, the host rock is a magnetite-disseminated albitite. The deposits have a distinct metal association of Fe–Cu–Au ± Ag, Bi, Ba, Co, Mo, Sb, Se, Te, Th, U, LREE. The wall and host rocks are intensely altered and display a deposit-scale zonal pattern. The distal alteration zone is characterised by albite ± biotite, K-feldspar, and scapolite and the proximal zone by clinopyroxene–magnetite ± amphibole, scapolite, calcite, and sulphides. Mass balance calculations indicate that Al 2O 3, TiO 2, and Zr were immobile during alteration. The calculations also indicate that significant quantities of Fe 2O 3, CaO, CO 2, S, Cu, Au, Bi, and Te were added to the proximal altered rocks. The main gains in the distal altered rocks are in Na 2O, K 2O, and Ba. Fluid inclusion data suggest that the fluids which circulated in the rocks during the main mineralisation event and subsequent brittle fracturing were highly saline (≤ 56 wt.% NaCl) H 2O ± CO 2 fluids. The temperature during the main mineralisation event was between 450 and 550 °C and the pressure was 1.5 to 3.5 kbar. Based on U–Pb age data of magmatic zircon from altered hanging wall diorite and granite that brecciates the ore, the age of the deposits is between 1864 ± 5 and 1766 ± 5 Ma. The 1797 ± 5 Ma age of zircon in skarn, combined with the 1810 to 1780 Ma ages of the metamorphic titanite in altered wall rocks and skarn, suggest that the deposits were most likely formed ca. 1800 Ma. This age has been interpreted to be broadly contemporaneous with the D 3 thrusting event in the Kolari region during which the Kolari shear zone was activated. The data presented in this work are inconsistent with previously proposed models for the Kolari deposits that they are metamorphosed syngenetic iron formations, or skarns related to ca. 1860 Ma monzonite intrusions. Instead, we propose that the Kolari Fe–Cu–Au deposits are of metasomatic replacement-type, and are controlled by the Kolari shear zone structures related to post-peak metamorphic D 3 thrusting event in northern Finland. Our data suggest that the Kolari deposits best fit into the category of epigenetic iron oxide–copper–gold mineralisation.