BROMINE IN SCAPOLITE-GROUP MINERALS AND SODALITE: XRF MICROPROBE ANALYSIS, EXCHANGE EXPERIMENTS, AND APPLICATION TO SKARN DEPOSITS

Abstract

Application of an X-ray fluorescence (XRF) microprobe for the analysis of single grains (80 to 1,000 μm in diameter) of Cl-rich minerals for Br has been evaluated for fluorapatite, chlorapatite, scapolite-group minerals (marialite and meonite) and so dalite. A calibration curve based on the Br contents in four international reference materials has been confirmed by measurements on Br - bearing standard solutions and by agreement with the results of Cl-rich minerals from instrumental neutron-activation analyses. Absolute errors associated with individual XRF microprobe analyses ( i.e., counting statistics alone) are less than 5%, and the calculated limit of detection in the analysis of single mineral grains is ~1 ppm Br. Matrix and grain-size effects are shown to be negligible. Experiments at 1 atmosphere and 800 to 1000°C yield the following distribution coefficients for Br-Cl exchanges between marialite or sodalite and hydrous NaCl-NaBr melts: K Dmarialite-melt = 0.97 ± 0.08 and KDsodalite-melt = 0.9 ± 0.1. Therefore, the Cl/Br values in marialite and sodalite closely reflect the halogen proportions of their coexisting melts or fluids. The diffusivity of Br in sodalite follows an Arrhenius relation: D Br = 6.5 * 10^-7 exp(-270 ± 10kJ/mol/RT) m2/s, over the temperature range from 800 to 1000°C. DBr in marialite is 1.7 ± 0.3 * 10^-19 m2/s at 800°C. The Cl/Br weight ratios of marialite in the Tieshan Fe skarn deposit, China, cluster around 650 ± 40, supporting an origin involving hydrothermal brines from associated evaporites. Scapolite- group minerals in the exoskarns of the Nickel Plate Au skarn deposit, British Columbia, have Cl/Br from 560 to 570, higher than those (130 to 180) of their counterparts in the endoskarns and vuggy cavities. This variation is attributable to an increased involvement of magmatic water from distal to proximal zones. Similarly, scapolite-group minerals in the Grenville U-Th-Mo- REE pegmatite-skarn-vein deposits vary widely in Cl/Br, from 80 to 380, indicative of mixed sources of hydrothermal fluids. This study shows the potential of Br analysis of Cl-rich silicate minerals for constraining the sources and evolution of hydrothermal fluids.