PETROLOGY, MINERALOGY & GEOCHEMISTRY OF AN ALNÖITE FROM WESTERN KENTUCKY

The Hutson BHN-4 core was produced by drilling through nearly 1,000´ of an ultramafic lamprophyre dike located in Livingston County in the southwestern portion of the Western Kentucky Fluorspar District (WKFD). This dike is one of the very few relatively unaltered dikes in the WKFD and the BHN-4 core is the longest core intersecting the Hutson dike, providing the deepest samples. The Hutson dike is classified as an alnöite, a type of ultramafic lamprophyre, and consists of three lithofacies: an altered, serpentinized margin; a pristine interior; and a highly porphyritic, anomalously altered facies dubbed “leopard rock”. Chemically, these rocks contain <35% SiO₂, are metaluminous and very sodium-poor (hence the misleading label of ultrapotassic). The pristine interior is porphyritic with augite, forsterite-rich olivine, and phlogopite phenocrysts set in a groundmass composed largely of phlogopite and melilite; the grain size of the groundmass varies considerably. Secondary groundmass phases include calcite and apatite; minor phases consist primarily of spinel-magnetite and very fine euhedral perovskite. Alteration in olivine is greater than in pyroxene and this alteration progresses outward from fractures within the crystals indicating that olivine is xenocrystal, fractured due to rapid ascent during intrusion. The dike contains <5% by weight igneous carbonate and secondary calcite is present as an alteration product. The igneous carbonate is coarser and shows distinctive cleavage while the secondary carbonate is nearly aphanitic in thin section and displays a wavy habit. Both primary and secondary calcite occur in the groundmass, while veins consist of only secondary calcite.

The leopard rock facies occurs within the dike interior and consists of very coarse olivine phenocrysts, some with rims of opaque minerals, within a highly altered matrix of calcite and serpentine. Some of the phenocrysts are altered to serpentine and some show no alteration.

The Hutson dike is partly altered but to a much lesser extent compared to other dikes in the WKFD. Melilite is altered to cebollite and olivine to serpentine, and other alteration phases include talc, chlorite, minor sphalerite and pyrite.