RARE EARTH ELEMENT CHEMISTRY LINKS CARBONATED DIKES AND FLUORITE MINERALIZATION IN WESTERN KENTUCKY

Fluorite mineralization in the Illinois Kentucky Fluorspar District (IKFD) has been classified as Mississippi Valley fluoritic subtype and its spatial association with igneous dikes has long invoked a genetic link between magmatic and diagenetic fluids. The mineralogy and texture of alnöite dikes in the Kentucky portion of the IKFD record carbonate alteration, as evidenced by pseudomorphic replacement of olivine and pyroxene phenocrysts, carbonate replacement in the groundmass, and by calcite veins that cross-cut and embay these altered phases. Understanding the carbonate alteration of the dikes may provide a missing link between magmatic and diagenetic processes in the IKFD. We use rare earth element (REE) characteristics of carbonated alnöite dikes and fluorite samples from the IKFD to provide new details on how magmatic fluids and regional basinal brines are linked.

In the carbonated dikes, the average REE + Yttrium (Y) concentration is 208 ppm (n=7), much lower compared to an average of 432 ppm (n=12) in less altered alnöites. In the carbonated dikes, normalized REE (REE_N) patterns have a negative slope (La/Lu)_N = 24), some with a small negative Eu anomaly and some with a small positive Gd anomaly. Maximum light REE_N values are only a few hundred times that of chondrite. Carbonated dikes can be linked to fluorite by similarities in their REE patterns. Fluorite from across the IKFD have the same small negative Eu and positive Gd anomalies, negative slope of heavy REE, but have depleted light REE. Maximum REE_N values are only 10 times that of chondrite and REE + Y concentrations are typically less than 10 ppm. Fluorite from three mines located along a dike 5 km east of the Coefield Magnetic Anomaly in western Kentucky have slight concave-downwards REE_N patterns, and values that do not exceed chondritic values. Textural relations in the dikes suggest the carbonate alteration of olivine and pyroxene was the result of reaction with carbonatite melt or fluid. Calcite veining could have occurred under crustal conditions as a result of dikes intruding limestone country rock. In Kentucky, mixing of fluorine-bearing fluids derived from already-carbonated dikes could have mixed with regional basinal brines leading to fluorite mineralization. Data from Kentucky indicates little REE input into the brines from these altered dikes.