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

Norrbotten 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 CO

2

-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. δ

37

Cl

(SMOW)

values range from_−0.99 to_−5.63%. The data are far removed from both mantle (δ

37

Cl=+4.7%) and crustal (δ

37

Cl=0%) values and must represent fractionation of the isotopes during the mineralizing process. We suggest this fractionation occurred during the formation of Cl-rich mineral phases commonly found associated with the deposits e.g. scapolite, biotite and amphibole.