THE HUTSON NORTH INTRUSION, WESTERN KENTUCKY: COMPLICATING THE PIPE DREAM OF CRITICAL MINERALS IN THE ILLINOIS-KENTUCKY FLUORSPAR DISTRICT

Across the Illinois Kentucky Fluorspar District (IKFD), Mississippian carbonates are intruded by ultramafic lamprophyre dikes and “crypto-explosive structures” and “intrusive breccias” both of which underlie small structural domes. The Hutson North Intrusion (HNI), located in Livingston, Co., western Kentucky, in the Burna 7.5-minute geologic quadrangle, is a unique intrusive pipe largely composed of alnӧite; drill core through the HNI shows the pipe to have a diameter of at least 140 m. These igneous rocks have become targets for rare earth elements (REE), Nb, and other critical minerals.

Fluorite mineralization in the IKFD has been previously characterized as a fluoritic-Mississippi Valley type (MVT) deposit. Interaction between regional basinal brines and F-bearing fluids and/or gases expelled from crystallizing magmas has been proposed to account for fluorite deposits; fluorite is more abundant than sphalerite, galena, pyrite, and barite. The deposits occur as bedding replacements and along normal faults of the Reelfoot Rift-Rough Creek Graben.

The primary igneous assemblage in the HNI is olivine, clinopyroxene, melilite, phlogopite, spinels, and perovskite, and aggregates of groundmass carbonate. The HNI is cross-cut throughout by centimeter- to decimeter-scale veins of carbonate + akermanite + apatite, serpentinite, and carbonate + combinations of MVT minerals. Olivine megacrysts throughout the HNI record complex patterns of serpentine and carbonate alteration and are embayed by poikiolitic phlogopite. Within the HNI, there are decimeter- to meter-scale patchy occurrences of lamprophyre rocks with varying mineralogy (some with chilled margins between them), zones of ocelli, and olivine phoscorite xenoliths; these features are interpreted as evidence of magma mingling in the HNI largely between different lamprophyre melts but with carbonatite magma, too.

HNI alnӧite is enriched in light REE but total REE concentrations are less than 500 ppm, like the dikes across the District. The HNI lacks economic REEs but records interactions among lamprophyre and carbonatite magmas, post-magmatic MVT fluids, and the possible influence of carbonate-bearing meteoric fluids. All these interactions could have played a role in the redistribution of REE throughout the region, thus complicating the assessment of critical minerals in the IKFD.