Bromine in scapolite-group minerals and sodalite: XRF microprobe analysis, exchange experiments, and application to skarn deposits

Application of an X-ray fluorescence (XRF) microprobe for the analysis of single grains (80 to 1,000 mum in diameter) of Cl-rich minerals for Br has been evaluated for fluorapatite, chlorapatite, scapolite-group minerals (marialite and meonite) and socialite. 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 similar to1 ppm Br. Matrix and grain-size effects are shown to be negligible. Experiments at I atmosphere and 800 to 1000degreesC yield the following distribution coefficients for Br-Cl exchanges between marialite or sodalite and hydrous NaCl-NaBr melts: KDmarialite-melt = 0.97 +/- 0.08 and K-D(sodalite-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 socialite follows an Arrhenius relation: D-Br = 6.5 X 10(-7) exp(-270 +/- 10kJ/mol/RT) m(2)/s, over the temperature range from 800 to 1000degreesC. D-Br in marialite is 1.7 +/- 0.3 X 10(-19) m(2)/s at 800degreesC. 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 An 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 tile 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.