ALTERATION OF ULTRAMAFIC LAMPROPHYRES AND ITS EFFECT ON RARE EARTH ELEMENT DISTRIBUTION IN THE ILLINOIS-KETUCKY FLUORSPAR DISTRICT

The ultramafic lamprophyres of the Illinois-Kentucky Fluorspar District (IKFD) have been identified as a potential domestic source of rare earth elements (REEs). The present distribution of the REEs and the role of the carbonate alteration in the distribution of the REEs is poorly understood. Petrographic and geochemical analyses were employed to determine the nature of the mantle source region and timing of the carbonate mineralization. Ratios of 143Nd/144Nd vs. 87Sr/86Sr demonstrate that in addition to the intrusion of primitive magma into the crust, a melt was generated from a mantle source region metasomatized by a phlogopite-rich melt. Fractionation trends in Nb/Ta and Zr/Hf further suggest that the mantle source region for the intrusions most enriched in REEs was metasomatized by the intrusion of a carbonatite melt and the melting of the lower continental crust. The carbonate alteration seen in many of the lamprophyres is the result of listwanitization and ophicalcitization, processes that alter serpentinite to carbonate. Carbonate alteration does not seem to be as strong of a control on REE distribution as mantle metasomatic processes.