PETROGENESIS AND RARE EARTH ELEMENT ECONOMIC POTENTIAL OF PILOT KNOB, AN APATITE-RICH, PLIOCENE ALKALINE INTRUSIVE BODY IN THE UPPER WIND RIVER BASIN VOLCANIC FIELD, WY (U.S.A.)

The recently defined upper Wind River Basin volcanic field (UWRB), northwestern WY (U.S.A.) is located ~80-100 km southeast of the Yellowstone Plateau volcanic field and hosts a <ca. 5 Ma suite of intrusives and lavas (Brueseke et al., 2017). Field and geochemical data show that these magmas were either erupted or emplaced along normal fault zones at different locations and range in composition from tholeiitic basalt to calc-alkaline basaltic andesite through dacite, and include Pilot Knob, a kersantite lamprophyre intrusion. The Pilot Knob intrusion was emplaced through Eocene sedimentary strata along a regional normal fault zone and new ⁴⁰Ar/³⁹Ar geochronology yield identical ages constraining its crystallization (whole rock age = 3.9 ± 0.1 Ma; phlogopite = 3.8 ± 0.1 Ma). All Pilot Knob samples we collected are coarse to medium-grained and average ~67.1% plagioclase, 14.5% clinopyroxene, 6.5% phlogopite, 6% orthopyroxene, and 6% apatite. Apatite crystals are commonly poikilitically enclosed within the plagioclase. Bulk rock geochemistry obtained via XRF verifies its transitionally alkaline composition and Pilot Knob is enriched in many incompatible trace elements (e.g., Ba >2000 ppm, Sr ~2000 ppm, Zr ~250-300 ppm, etc.) relative to other UWRB rocks; similar major element differences exist. Pilot Knob also has enriched La, Ce, and Nd concentrations relative to other UWRB rocks (600 ppm total light-rare earth element ore lode with ~149 ppm Nd, ~176 ppm La, and ~338 ppm Ce enrichment). Apatite was found to be the primary REE-bearing mineral via electron microprobe analyses. While it is demonstrated that alkaline intrusive complexes host high REE concentrations, the REE concentrations were not found to be significant enough to denote an economically viable REE ore deposit given current technology and processing methods. Bulk rock Sr-Nd isotope data indicates that Pilot Knob and other UWRB magmas likely formed via melting of ancient Wyoming craton lithospheric mantle. Extension and uplift associated with the Yellowstone hotspot tectonic parabola, coupled with temperature increase due to the hotspot, likely drove UWRB magmatism and led to the emplacement and subsequent exposure of Pilot Knob.