THE HUTSON NORTH INTRUSION, LIVINGSTON CO., KENTUCKY RECORDS POLYPHASE HYDROTHERMAL ALTERATION IN THE WESTERN KENTUCKY FLUORSPAR DISTRICT: IMPLICATIONS FOR REGIONAL REE DISTRIBUTION

Billiton Minerals drilled two cores in 1983 a half-mile north of the now defunct Hutson zinc mine located in the 7.5-minute USGS-KGS Burna quadrangle to explore a roughly circular gravity high; the same gravity anomaly is present in a recent geomagnetic survey of the Western Kentucky Fluorspar District (WKFD) (McCafferty and Brown, 2020, USGS data release). Both cores (core hole numbers BHN-1 and BHN-4 in the KGS Rock Core Inventory Search) intersect a vertical pipe-like intrusion of ultramafic lamprophyre which is at least 140 m wide and 260 m tall, here named the Hutson North Intrusion (HNI). Phlogopite from the BHN-4 core yielded an Ar-Ar isochron age of 267.8+1.3 Ma (Fifarek et al., 2001, GSA Abstract, v. 33, p. 2491). Based on changes in grain size and mineralogy, and macroscopic discontinuities observable in core, internal igneous contacts have been recognized; different intrusive phases vary in size from a few 100 m to 1-2 m thick. Olivine megacryst-rich xenoliths with abundant apatite have been entrained along some internal contacts. Parts of the HNI can be classified as fine-grained alnӧite but its mineralogy and textures are complex. Both cores contain abundant olivine megacrysts, breccia horizons, and carbonatite veins up to a few cm thick; these features are characteristic of carbonatites, but no carbonatite intrusions are exposed in the WKFD and the highest REE concentration in whole-rock samples of WKFD dikes and the HNI is less than a thousand ppm. In the BHN-4 core, veins up to a few 10s of cm thick of calcite+serpentine, calcite+brucite, calcite+fluorite, calcite+yellow-green garnet+serpentinized olivine, and calcite+pyrite+sphalerite are common. Features in thin section also record hydrothermal alteration; for example, serpentine in olivine megacrysts is overgrown by calcite, phlogopite overgrows altered olivine, phlogopite is intergrown with talc and brucite. Our observations indicate that dikes in the WKFD and the HNI record multiple phases of hydrothermal alteration. Understanding their geochemical evolution syn- and post-emplacement bears on the REE distribution in them and across the region and will help distinguish geologic environments and processes that promote REE-enriched rocks versus those that were either never enriched or in which later alteration depleted their REE content.