Combined gas and ion chromatographic analysis of fluid inclusions: Applications to Archean granite pegmatite and gold-quartz vein fluids

Abstract

The first stage in the interpretation of bulk fluid inclusion analytical data involves consideration of the relative amounts and types of fluid inclusions in a sample, in order to determine which inclusion type dominates the volatile and/or ion bulk analytical data. This then permits discussion of processes affecting fluid chemistry such as fluid-mineral equilibria, metasomatism, and phase separation, and leads to constraints on primary source region fluid composition. This interpretive procedure has been applied to data obtained by combined gas and ion Chromatographic analysis of well characterized samples from the Archean Tanco granitic pegmatite, southeastern Manitoba and the Hollinger-McIntyre and Kerr Addison Archean Au-quartz vein systems, northern Ontario. The bulk composition of an homogeneous late stage magmatic fluid in Lower Intermediate Zone vug quartz from the Tanco pegmatite is 90.6 mol% H 2O, 3.35 mol% CO 2, 2.45 mol% Li + , 2.40 mol% Cl −, 1.01 mol% Na +, and other trace species < 1 mol%. The Li + (Li + + Na +) ratio of 0.69 ± 0.008 for this fluid is greater than a published experimental determination of 0.45 ± 0.02 for a comparable system, probably owing to the CO 2-rich nature of the vug quartz fluids. The halogen (Cl −, Br −, I −) and alkali metal enriched nature of this fluid is consistent with the advanced degree of igneous fractionation of the pegmatite. Compared to estimated bulk earth values, the Br − Cl − ratio of 12.9 × 10 −3 (molar) is high and may be the result of igneous fractionation while the I − Cl − ratio (140 × 10 −6 molar) is low, suggesting that another process governs I − behaviour. Tanco quartz zone samples show trace gas depletion trends comparable to those obtained from samples trapping phase separated fluids in low pressure geothermal systems. Trends in the Tanco cation/anion data reflect both fluid-mineral equilibria and phase separation effects. Li + and Cl − show a closely correlated decrease with increasing CO 2 CH 4 ratio while Na + shows no change, consistent with partitioning of Li + and Cl − in favour of the CO 2-rich phase, which could have contributed to the calcite and holmquistite-bearing propylitic alteration zone around the pegmatite. Average bulk compositions for Hollinger-McIntyre and Kerr Addison fluids are similar and consist of 80–90 mol% H 2, 2–15 mol% CO 2, 1–3 mol% Cl −, 2–4 mol% Na +, and trace species < 1 mol%. Hollinger-McIntyre volatiles show clear wall rock reaction effects while Kerr Addison fluids show strong phase separation trends. Br − Cl − ratios of ∼5.5 to 10 × 10 −3 for Hollinger-McIntyre and Kerr Addison fluids are greater than bulk earth while I − Cl − × 10 6 ratios are low, with a range from ∼20–105. Hollinger-McIntyre and Tanco samples dominated by secondary brine inclusions have Br − Cl − × 10 3 ratios of ∼13 and ∼ 15, respectively, showing that these secondary fluids are distinct from shield brines ( Br − Cl − ∼ 3 to 5.5 × 10 −3 ). Both of these samples have distinctive trace hydrocarbon signatures.