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 m in diameter) of Clrich minerals for Br has been evaluated for fluorapatite, chlorapatite, scapolite-group minerals (marialite and meonite) and sodalite. A calibration curve based on the Br contents in four international reference materials has been confirmed by measurements on Brbearing 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 D marialite-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 sodalite follows an Arrhenius relation: D Br = 6.5 ϫ 10 -7 exp(-270 ± 10kJ/mol/RT) m 2 /s, over the temperature range from 800 to 1000°C. D Br in marialite is 1.7 ± 0.3 ϫ 10 -19 m 2 /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. Scapolitegroup 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.Keywords: XRF microprobe, Br, scapolite-group minerals, sodalite, exchange experiments, distribution coefficient, diffusion coefficient, skarn deposits. SOMMAIRENous évaluons l'application d'une microsonde à fluorescence X pour l'analyse de grains isolés (de 80 à 1,000 m de diamètre) pour le Br dans les minéraux porteurs de Cl comme la fluorapatite, la chlorapatite, les minéraux du groupe de la scapolite tels marialite et méionite, et sodalite. Le calibrage a été fait au moyen des teneurs en brome de quatre étalons internationaux, et a été confirmé avec des résultats obtenus pour des solutions standards contenant du Br et des minéraux riches en Cl analysés par activation neutronique instrumentale. Les erreurs absolues associées à une seule analyse par microsonde à fluorescence X, dues aux seules erreurs de comptage, sont inférieures à 5%, et le ...

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

“…These ranges are comparable with the compositions of marialites from the Tienshan Fe Skarn deposit, China (Pan and Dong, 2003), the Marcona Magnetite deposit, Peru (Chen et al, 2010), and the Manto Verde IOCG, Chile (Benavides et al, 2007). …”

“…Sample PN19 has the lowest Cl/Br molar ratios of the samples analyzed (271), contrasting with sample PN14 that has the highest (2363), which is the highest amongst all scapolites analyzed in this study and published elsewhere (e.g. Pan and Dong, 2003;Hammerli et al 2013Hammerli et al , 2014 (Table 4).…”

“…Bromine analyses were carried out on an XRF Microprobe (XRFM) at the University of Saskatchewan, using the same instrument and analytical protocols described in Pan and Dong (2003). Briefly, the XRF microprobe consists of a 2.0 kW X-ray generator, a concave (Govindaraju, 1994).…”

“…Briefly, the XRF microprobe consists of a 2.0 kW X-ray generator, a concave (Govindaraju, 1994). The calibration curve was further evaluated by using the Br contents in a suite of Cl-rich minerals (i.e., chlorapatite, scapolite and sodalite) determined by instrumental neutron activation analysis (INAA; Pan and Dong, 2003). Scapolite grains of ~100 µm in diameter for X-ray microprobe analyses were carefully selected under a petrographic microscope to minimize mineral and fluid inclusions.…”

“…The precision of the method is reflected in the relative standard deviation, which was less than 5% for Br concentrations over 10 ppm, and 10% for concentrations below 10 ppm. The calculated detection limit in the analysis of single mineral grains is ~1 ppm Br (Pan and Dong, 2003).…”