Constraints on the source and evolution of mineralising fluids in the Norrbotten Fe oxide-Cu-Au province, Sweden

Abstract

Norbotten County, Sweden, is well known for the iron oxide-apatite deposits of the Kiruna and Malmberget areas. These are spatially associated with iron oxide Cu (+-Au) deposits and a genetic link between the two has been suggested as a part of the IOCG spectrum of deposits. Here we report the results of a fluid inclusion microthermometry and bulk crush leach halogen and chlorine stable isotope study, carried out to test that hypothesis. Quartz veins from late stage veins from Fe-oxide-apatite deposits and Cu-(Au) deposits contain hypersaline brine inclusions, with a salinity range of 32 to 38wt.% NaCl eq. in Fe-oxide bodies, and of 38 to 47wt.% NaCl eq. in Cu-Au deposits. In Cu-(Au) deposits these are sometimes accompanied by CO2-rich inclusions. The Cl/Br ratio of the fluid inclusion leachates is consistent with magmatic fluid compositions, and ranges between 859 and 9597, with no distinction between the Fe-oxide samples and the Cu-mineralizing brines. d[delta]37Cl(SMOW) values range from -0.99 to -5.63 0/00. The data are far removed from both mantle (d[delta]37Cl=+4.7 0/00) and crustal (d[delta]37Cl=0 0/00) values and must represent fractionation of the isotopes during the mineralizing process. We suggest that this fractionation occurred during the formation of Cl-rich mineral phases commonly found associated with the deposits e.g. scapolite, biotite and amphibole.