THE PETROLOGY AND MINERAL CHEMISTRY OF SELECT LAMPROPHYRE DIKE INTRUSIONS IN WESTERN KENTUCKY

The Illinois-Kentucky Fluorspar District (IKFD) of western Kentucky and southeast Illinois is one of the most productive fluorspar districts in the world. The IKFD includes a series of northwest trending ultramafic dikes that are classified as lamprophyres. The dikes have been hydrothermally altered to varying degrees by interaction with surrounding carbonate strata. This study was carried out to determine the primary mineralogy and mineral chemistry of the dikes and compare them to known mantle–derived lamprophyres. Samples were drill cores from the Hutson, Minner and Lady Farmer mines in western Kentucky. The Hutson dike is ±450 feet thick, although it is thought to be two parallel dikes. Primary mineralogy is biotite and clinopyroxene with accessory perovskite, Ti-magnetite and apatite. No primary olivine was observed, but serpentine pseudomorphs after olivine were present in the least altered samples. Petrographic analysis of least altered samples from the center of the dikes reveal a primary assemblage of subhedral coarse poikilitic biotite with inclusions of clinopyroxene and subhedral coarse to fine grain clinopyroxene. Secondary minerals are serpentine and leucoxene. Increasing alteration produced greater modal abundance of serpentine followed by dolomite. Most altered samples contain biotite, serpentine and dolomite. Samples from the dike margins contain medium to fine grain subheadral biotite. Clinopyroxene is coarse to medium grained with replacement by serpentine then dolomite. Dike samples show increased CPX replacement toward the margin rim of the dike. Wavelength dispersive electron microprobe analysis indicates CPX is diopside with Mg/(Mg+Fe) = 0.843 to 0.998. Biotite is Ti-phlogopite with Ti atoms/22 O = 0.048 to 0.571, and Mg/(Mg+Fe) = 0.799 to 0.926. Opaque phase is magnetite-spinel solid solution with minor Ti and Cr. These features will be compared to other alkalic ultramafic dikes and diatremes from the southwestern U.S.