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 (∼1 g) fluid inclusion-bearing samples is a powerful, but simple, means for obtaining integrated fluid concentrations of major and trace, volatile and ionic fluid constituents without using microthermometrically determined salinity for normalization. The methodology, which is described and assessed in detail, involves crushing a carefully cleaned sample at ∼105°C in a stainless steel crusher on-line to a gas chromatograph. After volatile analysis, the crushed sample is removed and leached with deionized water to produce a leachate solution, which is filtered and analyzed by ion chromatography, and other methods. For example, detailed procedures 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 electrochemical, detection. The data are combined (calculation procedure given) to give whole fluid analyses in concentrations of mol% or mmol/l; this procedure 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% for a mean sample mass of 0.97 g, based on 97 determinations. Tests on the efficiency of the leaching process showed that >90% of univalent ions are removed. 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 and 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 technique to well characterized sample sets from diverse settings, including the Tanco Li–Cs–Ta pegmatite, Archean Au–quartz vein systems, the Polaris MVT Pb–Zn deposit, and the ∼3.2 Ga Barberton sea-floor Fe-oxide deposits (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.