USE OF SYNCHROTRON NANO-CT TO DETERMINE THE NATURE OF METAL OCCURRENCE IN FLUID INCLUSIONS: APPLICATION TO THE ILLINOIS-KENTUCKY FLUORITE DISTRICT

Understanding the concentrations of base metals in hydrothermal fluids is fundamental to understanding how the fluids precipitate base metal ore deposits. Fluid inclusions have historically been a key source of information about the base metal content of hydrothermal fluids. However, the information provided by fluid inclusions is often equivocal in that base metal concentrations are commonly inconsistent within cogenetic assemblages of fluid inclusions. A possible explanation for this inconsistency is that the higher apparent base metal concentrations in the fluid inclusions represent accidental mineral inclusions within the fluid inclusions and not dissolved solute. That is, fragments of earlier precipitated base metal minerals may have been transported in suspension in the hydrothermal fluid and become entrained within fluid inclusions in later-formed minerals. Distinguishing between base metals occurring as accidental mineral inclusions versus dissolved solute is critical to interpreting fluid inclusion composition data obtained from methods like LA-ICP-MS and to answering genetic questions about possible precipitation mechanisms, the duration of ore formation, and whether ore-forming fluids are anomalously metal-rich. Synchrotron nano-CT has the potential to resolve this question of the nature of metal occurrence by imaging fluid inclusion at the scale of tens of nanometers. A pilot study of this type was undertaken on fluorite-hosted fluid inclusions from the Illinois-Kentucky Mississippi Valley-type ore district in the U.S. Previous studies of fluorite-hosted fluid inclusions from the district have reported highly variable base metal concentrations ranging over more than two orders of magnitude. A systematic examination of 85 fluid inclusions did not reveal the presence of any accidental mineral inclusions larger than 50 nm in diameter. Thus, if accidental mineral inclusions account for the high apparent base metal concentrations in fluid inclusions of order 1000’s of ppm, then at least 10’s of thousands of accidental mineral inclusions smaller than 50 nm would need to exist in a fluid inclusion 1 µm in diameter. The results point to the need for a larger scope study to see if they hold for other ore deposits for which high base metal concentrations have been reported in fluid inclusions.