TRACE AND RARE EARTH ELEMENT ANALYSIS OF FLUORITE FROM THE ILLINOIS-KENTUCKY FLUORSPAR DISTRICT AND ITS IMPLICATIONS FOR THE ORIGINS OF MINERALIZING FLUIDS

The Illinois-Kentucky Fluorspar District (IKFD) is a fluoritic sub-type of a Mississippi Valley Type (MVT) deposit characterized by the district’s abundance of fluorite compared to minor Pb-Zn mineralization. Found along the axis of the district are a host of Permian ultramafic dikes, pipes, and diatremes, and Hicks Dome (HD), a crypto-explosive structure formed by the release of magmatic volatiles from a carbonatite complex intruded into Precambrian basement. The carbonatite complex is enriched in REEs that are principally contained in secondary phosphate and fluorocarbonate minerals. Decreasing fluid inclusion temperatures and changes in fluorite chemistry from HD to the fringes of the district can be interpreted as magmatic input from HD into a regional MVT system, and previous studies support a model for magmatic gases lowering fluid pH, supplying fluorine, and brecciating country rocks. Fluid inclusions in fluorite across the district show the presence of two distinct mineralizing fluids: a lower temperature/higher salinity fluid in paragenetically early fluorite, and a higher temperature/lower salinity fluid in later fluorite. We analyzed the REE and other selected trace element chemistry of 270 fluorite samples from 32 locations across the district using LA-ICP-MS. Chondrite normalized REE patterns of early fluorite are depleted in LREEs, have nearly flat to gently negative sloping HREEs, and can have a small positive Gd anomaly. REE patterns of late fluorite are similarly depleted in LREEs, show small MREE enrichment, more steeply negative HREEs, and a positive Eu anomaly. REE concentrations are similar between the two patterns, typically 10x chondrite or less. We found an overall decrease in REE concentration with increasing distance from HD. Neither early nor later fluorite has REE patterns similar to fluorite from a mineralized fracture within the ultramafic Sparks Hill diatreme, characterized by LREE enrichment and a pattern sloping from La to Lu. This pattern is similar to whole rock REE patterns of the Sparks Hill diatreme and other intrusive rocks within the IKFD. Our data suggest little REE input of magmatic fluid into the regional MVT system.