CRITICAL MINERAL POTENTIAL OF ULTRAMAFIC LAMPROPHYRES ASSOCIATED WITH THE REELFOOT RIFT, WESTERN KENTUCKY

Ultramafic lamprophyre (UML) intrusions in western Kentucky occur near the junction of the Reelfoot Rift and Rough Creek Graben. UMLs in this region have been the focus of intense study due to their potential to host rare earth elements (REEs). Earlier studies suggest that this rift system formed during the Late Precambrian to Early Cambrian in conjunction with the breakup of Rodinia. Similar ultramafic intrusions in the region have been grouped together as the Midwest Permian Ultramafic District (also referred to as the Wauboukigou Igneous Province) and are inferred to have been emplaced during the Permian reactivation of this rift system associated with the earliest phases of the breakup of Pangea. UMLs in western Kentucky are highly altered and have been classified as alnoites and aillikites, although their intense alteration makes their classification difficult. The least altered rocks contain partially preserved primary igneous silicates, such as olivine and clinopyroxene. The most abundant silicate mineral is phlogopite. Widespread serpentinization resulted in pseudomorphic replacement of silicate phenocrysts. In the more intensely altered samples, serpentine pseudomorphs are partially to completely replaced by carbonates. The carbonates are typically Mg-calcite, dolomite, and Fe-dolomite. Opaque minerals are abundant, represented by rutile, ilmenite, magnetite, Cr-Ti spinel, and perovskite. The groundmass is fine-grained and is typically made up of carbonates and opaque minerals with melilite, talc, chlorite, apatite, fluorite, and barite. Preliminary melt inclusion analysis identified perovskite, apatite, and Mg-calcite in clinopyroxene host minerals, whereas a residual olivine was found to host phlogopite, clinopyroxene, Mg-calcite, barite, and epidote. Whole-rock geochemical data suggest that the most intensely carbonated samples generally have lower total REE contents compared to samples with higher silicate content. Preliminary in-situ geochemical analysis confirmed the lower REE content of the carbonates compared to the silicate minerals. Apatite is the most likely host mineral for REEs; however, it is a minor phase in the UMLs of western Kentucky. Further in-situ geochemical analyses are underway to help decipher the geochemical and mineralogical evolution of the UMLs of western Kentucky and their control on REE distribution.