TRACING THE ORIGINS OF FLUIDS IN MINERALIZED SYSTEMS THROUGH HALOGEN RATIOS IN SCAPOLITE: TRANSITIONING FROM EXPERIMENTAL STUDIES TO PRACTICAL APPLICATION

Different types of fluids within the Earth's crust exhibit unique halogen compositions. For instance, contemporary seawater typically contains a molar Cl/Br ratio of around 650, while fluids resulting from halite dissolution show ratios exceeding 10,000. Additionally, residual bittern brines tend to be enriched in Br, resulting in Cl/Br ratios lower than those of seawater. Understanding these ratios can help identify fluid sources in metamorphic, magmatic, and hydrothermal systems, shedding light on processes such as ore formation and the origins of mineralizing fluids. Although measuring halogen ratios comes with challenges due to the fugitive nature of these elements in minerals, the scapolite group minerals stand out for their ability to incorporate halogens, sulfur, and carbon to measurable degrees, sometimes up to several weight percentages. By determining the partitioning of these elements into scapolite, scapolite can serve as a valuable tool for tracing fluid origins. Given its prevalence in mineralized systems, ranging from skarns to iron oxide-copper-gold (IOCG) deposits, scapolite can elucidate the evolution of fluid sources within a specific environment. Currently, there is limited experimental data on the Cl and Br partitioning in silicate minerals. Here, we present new exchange experiments examining chlorine and bromine partitioning between scapolite and brine under varying fluid salinities, ranging from 0.1 to 0.65 mole fraction of salt (NaCl + NaBr), conducted at pressures of 1.50 GPa and temperatures of 800°C, as well as at pressures of 1.41 GPa and temperatures of 1000°C. Additionally, we introduce a novel non-destructive approach for measuring Cl and Br on a micrometer scale using electron probe microanalysis (EPMA). By integrating the new partitioning coefficients obtained from the experimental results and this new analytical approach, we will show distinct halite dissolution and residual bittern brine halogen sources within the Mesoproterozoic Belt Supergroup, particularly within the Co-enriched horizons of the Lemhi sub-basin's metasedimentary strata, which are believed to be correlated with the Belt Supergroup.