ALTERATION IN ULTRAMAFIC LAMPROPHYRES AND ITS CONTROL ON REE DISTRIBUTION IN WESTERN KENTUCKY

The ultramafic lamprophyres and related rocks in the Midwest Permian Ultramafic District encompassing the Illinois–Kentucky Fluorspar District (IKFD) have been considered a potential domestic source of rare earth elements (REEs). Elevated REE concentrations have been documented at Hicks Dome, an enigmatic domal structure within the district in southern Illinois. However, whole-rock geochemical data on 205 samples from the western Kentucky part of the district show that the igneous rocks here have a markedly lower REE content than those in Illinois. A key characteristic of the igneous dikes in Kentucky is that they display evidence of multiple phases of alteration. The goal of our research is to reconstruct the alteration paragenesis characterize the fluids responsible for these alteration events, and to establish the relationship between alteration processes and the distribution of REEs in the dikes in western Kentucky. Preliminary petrographic analysis of 50 thin sections indicates there is a spectrum of alteration intensity. The least altered endmember contains serpentine pseudomorphs after olivine and pyroxene, and phlogopite and opaque phases occurring as rims around the pseudomorphs. The matrix contains abundant altered melilite as well as numerous euhedral opaque phases that were identified as spinel, titanite, perovskite, and celestine using BSE/EDS. The other endmember is characterized by extensive carbonate alteration, with the core of many serpentine pseudomorphs replaced with various carbonate phases and the matrix altered to UV-fluorescent carbonates. Samples that have been extensively altered to carbonate have lower total REE content compared to samples with better preserved primary igneous phases or samples where the primary phases have only been serpentinized without subsequent carbonate replacement. However, samples with abundant primary mineralogy or serpentinized mineralogy do not always have high concentrations of REEs. To identify the REE-hosting mineral phases and to constrain the characteristics of the mineralizing fluids, the composition of fluids in fluid inclusions and their host and other mineral phases will be determined using LA-ICP-MS. The timing of mineralization will be determined by using U/Pb geochronology on the multiple carbonate generations.