Evidence of multiple halogen sources in scapolites from iron oxide-copper-gold (IOCG) deposits and regional Na[sbnd]Cl metasomatic alteration, Norrbotten County, Sweden

Abstract

Scapolites from barren regional NaCl metasomatic assemblages (RM), iron oxide-copper-gold deposits (IOCG), scapolite altered metabasic rocks (IOCG-M), and from IOCG-proximal alteration/Na-Skarns (IOCG-PS) from Norrbotten County in Northern Sweden have been analysed for halogen content and Cl stable isotope composition. The aim of the study was to constrain the source of halogens within alteration assemblages, and to investigate the possible fractionation of Cl isotopes between scapolite and the hydrothermal fluid. Scapolite separates were analysed for Cl, Br, and major oxide concentrations using electron probe micro-analysis (EPMA) and micro-X-ray fluorescence (XRFM) spectrometry. Chlorine was extracted from the scapolite separates via pyrohydrolysis and then analysed for their stable Cl isotope compositions by isotope ratio mass spectrometry (IRMS).All samples of scapolite investigated in this study are marialitic in composition. One of the scapolites from the Gruvberget deposit (IOCG-PS) had a Cl/Br molar ratio of 2363, which is the highest amongst all scapolites reported in the literature to date. Cl/Br molar ratios lower than seawater (650), were identified in two IOCG-PS scapolite samples (Cl/Br=554 and 271), as well as in two IOCG-M scapolites (Cl/Br=393 and 565). Three RM scapolites had Cl/Br molar ratios very close to, or slightly higher than, seawater values (639 to 770). Samples with Cl/Br molar ratios less than seawater are inferred to have halogens derived from evaporative residual brines; whereas samples with molar ratios higher than seawater may have halogens derived from fluids that have dissolved halite and/or are from magmatic systems. Considering the wide variation of the Cl/Br molar ratios in the IOCG-PS and IOCG-M scapolites compared to the restricted composition of the regional alteration (RM), it is proposed that the hydrothermal fluids interacted with several different protoliths to generate the IOCG alteration.RM alteration scapolites had δ37Cl values from −0.1‰ to +0.3‰, two IOCG-M scapolites had values of 0.2‰ and IOCG-PS scapolites had δ37Cl values from −0.1‰ to +1.0‰. Using a previously published δ37Cl value from fluid inclusion leachates (−1.7‰) from the IOCG-M mineralisation at Pahtohavare and the δ37Cl value of co-existing scapolite measured in this study (0.2‰), an empirical fluid-scapolite fractionation factor was calculated to be +1.9‰. This large fractionation factor is not supported by previous predictions for monovalent chlorides and, assuming equilibrium, indicates that 37Cl was preferentially accommodated in the A site of the scapolite structure. This indicates that either the stable Cl isotope partitioning between the CaCl2-rich brine and the scapolite may differ from currently available estimates for NaCl brines in equilibrium with silicate minerals, or that the scapolite and brine are not in isotopic equilibrium.Overall, the data in this study suggest that halogens in early scapolites were derived from residual brines and halite, during metamorphism of evaporites linked to the RM alteration. Later in the history of the Norrbotten district components of the RM alteration were recycled and mixed during magmatic and local metasomatic events to varying extents, resulting in the brines associated with IOCG alteration.