Identification of Fluid Inclusion Solid Phases Using A Focused Ion Beam Scanning Electron Microscope With Energy Dispersive Spectroscopy: Implications For Interpreting Microthermometric Data and Composition of Fluids In SN (-W-Mo) Deposits

Abstract

Abstract A feature of many granite-related mineral deposits is the presence of highly saline fluid inclusions that contain abundant daughter minerals, the identity and chemistry of which hold important information for the interpretation of the chemistry of mineralizing fluids in such environments. A focused ion beam (FIB) coupled with a scanning electron microscope (SEM) and energy-dispersive spectroscope (EDS) was used to identify daughter minerals within such fluid inclusions in fluorite from the Mount Pleasant Sn (-W-Mo-In) deposits, New Brunswick, Canada. The technique can be used to obtain quantitative compositional information for solid phases in fluid inclusions for which the optical properties have previously been studied, but whose identification remains equivocal. In addition, for solid-rich fluid inclusions, this technique enables identification of solids that are obscured by other solids and are not readily visible using optical microscopy. Methods such as Raman spectroscopy and SEM-EDS analysis of broken surfaces have limited success in mineral identification in these inclusions due to strong fluorescence in fluorite and an inability to correlate solids with their optical properties, respectively. The FIB-EDS analyses of the daughter minerals and precipitates formed by evaporation of the liquid in the inclusions show that the complex inclusions from Mount Pleasant contain brines with high concentrations of Na, Fe, K, Mn, Pb, Zn, and Cl; daughter minerals comprise halite and a variety of Fe-K(-Mn) chloride hydrates, and less frequently, Pb chlorides and Sn-bearing phases. The precipitates contain similar phases, although in general they are more K-rich than the daughter minerals. Unequivocal identification of most solid phases was not possible due to inaccuracies inherent in the FIB-EDS chemical analyses and a lack of Raman spectra for many K-Fe-Mn chloride hydrate minerals. Our data indicate that square (“cubic”) solids in such inclusions are not always halite, and that misidentification of halite could result in erroneous salinity estimates, which could lead to inaccurate interpretations of fluid composition using techniques such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).