Halogen Element and Stable Chlorine Isotope Fractionation Caused by Fluid–Rock Interaction (Bamble Sector, SE Norway)

Abstract

Pervasive interaction between rock and fluid produces regional-scale alteration and/or metasomatism of rock units. The middle and upper crustal rocks from the Bamble sector (SE Norway) show a complex history of metamorphic and metasomatic events, accompanied by ore deposit formation, which provides an excellent context for investigation of regional-scale fluid‐rock interaction. Halogens are important agents of rock alteration and ore metal transportation in such hydrothermal regimes. Here we use F, Cl, Br and I concentrations and stable chlorine isotope ratios (d 37 Cl) to trace the metasomatic evolution of two gabbroic bodies and understand the interplay between localized and pervasive fluid flow in the upper crust. An alteration sequence was sampled from pristine gabbro to an amphibolitic shear zone, showing progressive amphibolitization and scapolitization caused by pervasive fluid ingress. Halogen concentrations and ratios suggest the evolution of a single fluid, causing enrichment of Cl, Br and I in samples nearest the shear zone. Owing to the differences in fluid‐mineral distribution coefficients, fluid‐mineral interaction resulted in either enrichment or depletion of halogens in the fluid as it reacted pervasively with the gabbro. Although scapolite formation leads to progressive desalination of the fluid near the shear zone, increasing Cl concentrations in amphibole near the gabbro reflect an increase in salinity towards the unaltered gabbro owing to desiccation of the fluid. The formation of alteration minerals mainly governs changes in the halogen chemistry of the fluid, showing that halogens do not behave fully conservatively during fluid‐rock interaction. Nevertheless, the evolving fluid has a feedback on the stability of alteration minerals such as scapolite and biotite, which form only close to the shear zone. The Br/Cl and I/Cl ratios of the most highly altered samples are 3 � 10 ‐3 and 25 � 10 ‐6 , respectively, and overlap with the range of ratios measured for marine pore fluids. Remobilization of evaporites would have produced distinctly lower halogen ratios and can be ruled out as a fluid source. The unaltered gabbro has d 37 Cl values near 0% and a similar value is inferred for the infiltrating fluid. Minimally altered samples, noted by the presence of <20 vol. % amphibole, have negative d 37 Cl values (average ¼ ‐0� 6 6 0� 1%). The d 37 Cl values increase (up to þ 1% nearest the shear zone) with increasing evidence of fluid‐rock interaction towards the shear zone. Mixing of different Cl reservoirs (i.e. gabbro and fluid) cannot explain the observed changes in the d 37 Cl values. Instead, isotopic fractionation owing to Rayleigh distillation with a fractionation factor of 1� 0010 can explain the observed negative d 37 Cl values of the alteration sequence by the evolution of a single fluid