Hydrothermal fluid evolution and metal transport in the Kiruna District, Sweden: Contrasting metal behaviour in aqueous and aqueous–carbonic brines

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Smith, M., Gleeson, S., Yardley, B.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2013
• Subjects: Kiruna; Norrbotten
• Online Access: https://gfzpublic.gfz-potsdam.de/pubman/item/item_1825891

Iron oxide-copper-gold (IOCG) deposits and Fe oxide-apatite deposits from Norrbotten, Sweden, formed in similar settings, and in some cases IOCG mineralisation overprinted Fe oxide-apatite mineralisation. Fluid inclusions in quartz veins cutting Fe oxide-apatite deposits range in salinity from 33 to 37 wt.% NaCl eq., and those in IOCG-type deposits from 41 to 54 wt.% NaCl eq. Minimum trapping conditions for these inclusions are similar to 200-300 MPa and 200-300 degrees C in the Fe oxide-apatite bodies, and 250 to >300 MPa and 300-500 degrees C in the IOCG deposits. Deformed Cu-Au deposits have similar early fluid characteristics, but contain complex secondary fluid inclusion assemblages including halite saturated (20-30 wt.% NaCl eq.), aqueous-carbonic (3-13 wt.% NaCl eq.; X(CO2) 0.17-0.29) and CO2-rich fluids. The aqueous-carbonic and carbonic inclusions are consistent with aqueous-carbonic fluid immiscibility at similar to 150 MPa. A secondary population, with a high Ca-content occurs in all deposit types. The chemical composition of these inclusions has been determined by crush-leach analysis and LA-ICPMS. Halogen contents indicate a range of salinity sources with possible inputs from both magmatic and halite-dissolution brines. Element ratios suggest the alkali content of the fluid exceeded the buffer capacity of the host rocks. Iron and other transition metal contents correlate strongly with Cl concentrations, with secondary controls on solubility from pH, redox and temperature. Copper and Ag contents are higher in lower salinity aqueous-carbonic brines (up to 5000 ppm Cu, 900 ppm Ag) than in the most saline brines (up to 2297 ppm Cu, 837 ppm Ag). This may reflect differences in metal source between deposit types, but is also consistent with the complexation of Cu by bisulphide in the lower salinity fluids. Late stage aqueous-carbonic fluid flux through the deformed deposits either introduced additional copper to the deposits, remobilised pre-existing copper or both. (C) 2012 Elsevier Ltd. All rights ...