Integrated cation-anion volatile fluid inclusion analysis by gas and ion chromatography; methodology and examples

Combined gas and ion chromatographic analysis of well characterized, small (similar to 1 g) fluid inclusion-bearing samples is a powerful, but simple, means for obtaining integrated fluid concentrations of major and trace, vo latile and ionic fluid constituents without using microthermometrically det ermined salinity for normalization. The methodology, which is described and assessed in detail, involves crushing a carefully cleaned sample at simila r to 105 degrees C in a stainless steel crusher on-line to a gas chromatogr aph. After volatile analysis, the crushed sample is removed and leached wit h deionized water to produce a leachate solution, which is filtered and ana lyzed by ion chromatography, and other methods. For example, detailed proce dures are given for I- analysis with a low detection limit of 0.5 ppb using a trace anion concentrator column and pulsed amperometric, rather than ele ctrochemical, detection. The data are combined (calculation procedure given ) to give whole fluid analyses in concentrations of mol% or mmol/l; this pr ocedure removes a large systematic error of approximately a factor of 2, if separate volatile and hand crush cation/anion analyses are linked through sample weights. Results indicate a mean crushing efficiency of 82 +/- 6% fo r a mean sample mass of 0.97 g, based on 97 determinations. Tests on the ef ficiency of the leaching process showed that > 90% of univalent ions are re moved. Based on 145 analyses, charge balances are found to be close to 1.0 when all major positive and negative species are included, both analyzed an d calculated (e.g., carbonate species). Analytical errors (coefficients of variation; percentage) for species in moles vary from 1.1 (Br-) to 17.9 (I- ), from 0.02 (H2O) to 0.21 (I-) for species in mol%, and from 13.1 (F-, Br- ) to 22.1 (I-) for species in mmol/l. Application of the combined GC/IC tec hnique to well characterized sample sets from diverse settings, including t he Tanco Li-Cs-Ta pegmatite, Archean Au-quartz vein systems, the Polaris MV T Pb-Zn deposit, and the similar to 3.2 Ga Barberton sea-floor Fe-oxide dep osits (ironstone pods) has demonstrated its utility for constraining fluid sources, identifying fluid types, and determining the effects of processes such as H2O-CO2 phase separation and fluid-wall rock interaction. (C) 1999 Elsevier Science B.V. All rights reserved.