FLUORINE CONCENTRATIONS OF ORE FLUIDS IN THE ILLINOIS-KENTUCKY DISTRICT: EVIDENCE FROM SEM-EDS ANALYSES OF FLUID INCLUSION DECREPITATES

The Illinois-Kentucky (IL-KY) district is an anomalous occurrence of Mississippi Valley-type (MVT) mineralization in which fluorite greatly predominates over metal sulfides. A possible hypothesis for this anomaly is that the IL-KY ore fluids were enriched in fluorine compared to typical MVT ore fluids. Support for this hypothesis comes from the unusual abundance of mafic to ultramafic intrusions in the district that may be coeval with the MVT mineralization, and may have imparted the distinctive K, Sr, and ³He rich signature to fluid inclusions in the ore. The purpose of the present study was to test this hypothesis by measuring the fluorine concentration of the IL-KY ore fluids. Primary fluid inclusions hosted by sphalerite were first analyzed by microthermometry and then thermally decrepitated. The resultant decrepitate mounds were then analyzed using SEM-EDS and calibrated to major element concentrations from previously published LA-ICP-MS analyses. All of the fluid inclusion decrepitate mounds analyzed in the present study had detectable quantities of fluorine, whereas none of the sphalerite matrix analyzed around the decrepitate mounds contained detectable fluorine. If these fluorine signals represent aqueous solute rather than fluorite accidentals, then these signals correspond to aqueous F concentrations on the order of 10’s to 10,000’s of ppm. Based on the Ca concentration of the fluid inclusions and assuming saturation with respect to fluorite, these F concentrations imply an ore fluid pH of <3, well below the pH range of 4-5 that is thought to be more typical of MVT ore fluids. If the F concentrations calculated in the present study do in fact represent dissolved aqueous F in the fluid inclusions, then the IL-KY ore fluids would have been significantly enriched in F, which would not only have promoted the precipitation of fluorite but suppressed the precipitation of metal sulfides because of their increased solubility at lower pH.