COMBINED GAS AND ION-CHROMATOGRAPHIC ANALYSIS OF FLUID INCLUSIONS - APPLICATIONS TO ARCHEAN GRANITE PEGMATITE AND GOLD-QUARTZ VEIN FLUIDS

The first stage in the interpretation of bulk fluid inclusion analytic al data involves consideration of the relative amounts and types of fl uid inclusions in a sample, in order to determine which inclusion type dominates the volatile and/or ion bulk analytical data. This then per mits discussion of processes affecting fluid chemistry such as fluid-m ineral equilibria, metasomatism, and phase separation, and leads to co nstraints on primary source region fluid composition. This interpretiv e procedure has been applied to data obtained by combined gas and ion chromatographic analysis of well characterized samples from the Archea n Tanco granitic pegmatite, southeastern Manitoba and the Hollinger-Mc Intyre and Kerr Addison Archean Au-quartz vein systems, northern Ontar io. The bulk composition of an homogeneous late stage magmatic fluid i n Lower Intermediate Zone vug quartz from the Tanco pegmatite is 90.6 mol% H2O, 3.35 mol% CO2, 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 determ ination of 0.45 +/- 0.02 for a comparable system. probably owing to th e CO2-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 t o estimated bulk earth values, the Br-/Cl- ratio of 12.9 X 10(-3) (mol ar) is high and may be the result of igneous fractionation while the I -/CI- ratio (140 X 10(-6) molar) is low, suggesting that another proce ss governs I- behaviour. Tanco quartz zone samples show trace gas depl etion trends comparable to those obtained from samples trapping phase separated fluids in low pressure geothermal systems. Trends in the Tan co cation/anion data reflect both fluid-mineral equilibria and phase s eparation effects. Li+ and Cl- show a closely correlated decrease with increasing CO2/CH4 ratio while Na+ shows no change, consistent with p artitioning of Li+ and Cl- in favour of the CO2-rich phase, which coul d have contributed to the calcite and holmquistite-bearing propylitic alteration zone around the pegmatite. Average bulk compositions for Ho llinger-McIntyre and Kerr Addison fluids are similar and consist of 80 -90 mol% H2O, 2-15 mol% CO2, 1-3 mol% Cl-, 2-4 mol% Na+, and trace spe cies < 1 mol%. Hollinger-McIntyre volatiles show clear wall rock react ion effects while Kerr Addison fluids show strong phase separation tre nds. Br-/Cl- ratios of approximately 5.5 to 10 X 10(-3) for Hollinger- McIntyre and Kerr Addison fluids are greater than bulk earth while I-/ CI- X 10(6) ratios arc low. with a range from approximately 20-105. Ho llinger-McIntyre and Tanco samples dominated by secondary brine inclus ions have Br-/CI- X 10(3) ratios of approximately 13 and approximately 15, respectively, showing that these secondary fluids are distinct fr om shield brines (Br-/Cl- approximately 3 to 5.5 X 10(-3)). Both of th ese samples have distinctive trace hydrocarbon signatures.