RARE EARTH ELEMENT CONTENT AND RARE MINERALOGY IN ALKALINE ULTRAMAFIC ROCKS, WESTERN KENTUCKY FLUORSPAR DISTRICT, KENTUCKY

The igneous intrusive complex in the western Kentucky Fluorspar District (WKFD) indicates a diverse and complex mantle-derived magma source. Historically described as alnӧite lamprophyre and mica peridotite, these igneous intrusions contain rare minerals, including garnets, and are slightly enriched in rare earth elements. Minerals in igneous groundmass and phenocrysts in core indicate a fractionated peridotite, carbonatite and nepheline syenite phase of parental magmas. Unusual minerals in dikes, determined by X-ray diffraction, include: a) schorlomite and andradite, b) astrophyllite, c) REE bearing perovskites, d) villiaumite, e) natrite, f) spodumene, e) niobium rutile, f) fluoro-phosphates, g) wὓstite, h) fluoro-tetraferriphlogopite, i) molybdenum and j) fluocerite, a REE-bearing fluorescent mineral. These REE bearing fluoride complexes are within the dikes are usually associated with nepheline syenite and carbonatite in other worldwide occurrences. A zone of oxidized vein mineralogy and fluorite bedded replacement deposits along some dikes suggests a trend toward the late stage, fluorite rich, and volatile carbonatite phase of a differentiated intrusion. Several of these rare minerals have never been described in the WKFD and their occurrence (wὓstite) suggest deep mantle metasomatism.

Whole rock elemental analysis was chondrite normalized and shows some of the dikes are slightly enriched in light REE. Variations in major element content, high titanium, niobium, zirconium values and high La/Yb, Zr/Y, Zr/Hf and Nb/Ta ratios suggest metasomatized lithospheric mantle sourced intrusions. The high La/Yb ratios in some dikes in the titanium garnet facies suggest a magma melt trend toward the carbonation phase of a fractionated peridotite parent magma. Within the family of dikes, some contain La/Yb crossover values suggesting variable metasomatism in the district.

Fluorite mineralization in the district was also analyzed for REE content, one group of fluorite enriched in LREE and another enriched in a MREE, which suggests a bimodal or periodic fluorite emplacement.