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Abstract

Introduction The iron oxide-apatite and spatially related copper deposits of Norbotten, Sweden have been suggested to have linked genetic mechanisms, and form part of the iron oxide-copper-gold class (IOCG). Recent developments in the bulk analysis of fluid inclusions for cation and anion chemistry and Cl isotope ratios provide an ideal route for investigating fluid source in this environment. We report the results of an investigation of quartz veins from Fe-oxide bodies and Cu deposits in the Norbotten region, carried out as part of the EU-RDFs Georange program. Fluid inclusions All the samples studied 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 deposits. These are typically accompanied by CO2-rich inclusions. The inclusion assemblage from the Aitik deposit is considerably more complex, with coexisting hypersaline and CO2-rich aqueous inclusions. Pressure estimates for the trapping of the fluids range from 2-3kbars. Halogen and major cation fluid inclusion geochemistry A bulk crush leach study was carried out on samples from both deposit deposits. Overall the analyses suggest that the mineralizing fluids in the Fe-oxide and Cu deposits are NaCl-CaCl2-H2O-CO2 hypersaline brines. The Br/Cl elemental compositions for the bulk of the samples range from 0.0004 and 0.0016 and are broadly consistent with a magmatic origin for the fluids. However, two samples, including the sample from Aitik, are more Br enriched (or Cl depleted) than typical magmatic-hydrothermal fluids and may suggest a component of metamorphic fluids in the mineralising system. Overall, there is no systematic variation in the halogen and most of the cation elemental ratios between the Fe-oxide deposits and the Cu deposits. However, the Fe-oxide mineralizing fluids do appear to have lower Fe/Mn and Ca/Ba ratios than the Cu deposits. Preliminary work suggests evolution from late stage fluids associated with Fe-oxide deposits to more saline fluids associated with Cu-deposits. The Aitik deposit, and possibly other shear zone hosted deposits in the area, formed from distinct fluid types, although aqueous-carbonic immiscibility still appears to be a major process.