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

摘要

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 33-37 wt. % NaCl eq., and those in IOCG-type deposits from 41-54 wt. % NaCl eq. Minimum trapping conditions for these inclusions are ~200-300MPa and 200-300°C in the Fe oxide-apatite bodies, and 250->300MPa and 300-500°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 ~150MPa. 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 5000ppm Cu, 900ppm Ag) than in the most saline brines (up to 2297ppm Cu, 837ppm 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.