Semi-quantitative electron microprobe analysis of fluid inclusion salts from the Mount Isa copper deposit (Queensland, Australia)

Abstract

Abundance ratios of major and minor elements in single fluid inclusions from the synmetamorphic copper deposit at Mount Isa (Queensland, Australia) have been estimated by electron microprobe analyses of salt residues liberated by thermal decrepitation. X-ray intensity ratios were calibrated using thin mixed-salt standards. Measured cation ratios are within ±20% of the values in the parent fluid inclusion, provided that decrepitation can be achieved below 400°C. Wave-length dispersive spectrometry (WDS) used here is less accurate for major salt components, but more sensitive for minor elements, than energy dispersive spectrometry (EDS) used recently by Haynes and Kesler (1987). Results demonstrate the advantage of the single-inclusion method for studying paragenetically complex ore samples containing several generations of inclusions. Microprobe analyses could be assigned to single fluid inclusions or small groups of inclusions, which were texturally interpreted as primary and secondary, and which were characterized by microthermometric measurements prior to decrepitation. In favourable cases, the combination of microthermometry and microprobe analysis allowed estimation of absolute cation concentrations in individual fluid inclusions. Syntectonic metasomatism at Mount Isa involved highly calcic brines with major KCl and AlCl3 in early dolomitic alteration preceding copper mineralization. Compositionally similar fluids were again trapped on cracks postdating mineralization. The main stage of copper mineralization was deposited by a NaCl-dominated brine with a composition of about 2 m (mol/kg) NaCl, 0.2 m KCl, 0.01 m each of S (as HS− + H2S) and AlCl3, and with MgCl2 and FeCl2 near or below their respective detection limits of 0.002 and 0.01 m.