The geology of the Rakkurijärvi copper-prospect, Norrbotten county, Sweden

The Rakkurijärvi prospect, Norrbotten, Sweden, is a newly discovered IOCG deposit, approximately 8 km south of Kiruna town. Exploration drilling has found mineralised intercepts including 43 m of 0.83% Cu and 0.05 g/t Au and 40.4 m of 1.41% Cu and 0.33 g/t Au, and the extent of mineralisation is currently still open. The host rocks to the deposit consist of strongly altered conglomerate and trachyandesitic lavas interbedded with thin marble bands and possible beds of pelitic sediment. These are cut by a northeast-trending shear zone which either hosts the ore bodies, or with which the ore bodies are spatially associated, and which is affected by extensive carbonate metasomatism. The ore bodies themselves consist of chalcopyrite-pyrite mineralised magnetite breccias, which grade laterally into magnetitelithic breccias and lithic breccias, with the clasts mainly derived from the volcanic rocks. Alteration consists of early albitisation associated with actinolite and magnetite, overprinted by sodic-potassic (biotite-scapolite) and potassic (biotite-K feldspar) and finally propylitic (hematite-epidote-muscovite-chlorite-calcite) alteration. Sulphide mineralisation is associated with the propylitic stage. Both textural and chemical constraints indicate sulphide mineralisation post-dates brecciation and magnetite mineralisation. Preliminary mineralogical constraints indicate early alteration from 500 to 600°C, and chloritisation/carbonatisation from 250 to 350°C. Portions of the deposit underwent intense weathering resulting in the formation of native copper in places. Geochronological constraints (Re-Os molybdenite, U-Pb titanite and allanite) indicate sulphide mineralisation took place at around 1860 to 1850 Ma, with some LA-ICPMS analyses of allanite suggesting initial mineralisation at around 1890 to 1880 Ma, and Pb-loss from titanite via metamorphism at ~1800 Ma. The origin of fl uids responsible for Fe mineralisation is currently poorly constrained, but C and O isotope analyses of calcite are consistent with a magmatic or magmatic equilibrated origin for copper-stage fluids.