CHARACTERIZATION AND DISTRIBUTION OF BRECCIA, WHITETAIL RIDGE, BEAR LODGE, WY: IMPLICATIONS FOR EMPLACEMENT OF CARBONATITE AND REE MINERALIZATION

The Tertiary-age Bear Lodge Alkalic Complex (BLAC) is in Crook Country, 12 miles northwest of Sundance, Wyoming. Rare Element Resources, Inc. (RER) is engaged with exploration and proposed mine development of rare earth elements (REEs) hosted by carbonatite intrusions in the breccias of the BLAC. RER’s investigations are centered on two major geologic structures, the Bull Hill and Whitetail diatremes. Classifying the breccia at Whitetail Ridge is the objective of this study and it is based upon, the clast lithologies, size and shape of clasts, country rock clast distribution in breccia body, the clast: matrix ratio and mutual relationships, layering characteristics, and lithologies and geometries of other intrusive rocks. Examination of a 5,375 feet of core in five drill holes, RES 12-51, RES 12-60, RES 12-64, RES 12-66, RES 12-69, along a 2,132 foot east-west transect through Whitetail Ridge was the bulk of the investigation. Findings from the logging established five lithologies: heterolithic breccia, monolithic breccia, lamprophyre, and two variants of FMR: oxidized carbonatite (OxCa), oxidized (Ox). The breccia is largely matrix supported with sub-rounded clasts, <1mm-100mm size. Autogenous milling, presence of carbonatite intrusions, lack of bedding, absence of volcanic ash, and lack of Phanerozoic spall are all indicative of a diatreme root. SEM and optical petrology confirmed the presence of autogenous milling, REE mineralization, and alteration effects including recognition of bulk orthoclase overprinting. Leapfrog computer modeling displayed brecciated units in a chaotic sequence, with no clear connectivity of breccia units. Paths of carbonatite intrusions are apparent, however. Carbonatite intrusions show a predictable distribution, based on previous reconnaissance and core intercepts consistent with a northwest-trending dike swarm. From east to west, carbonatite expression varies from the massive carbonatite diking through a zone of mixed carbonate veining with sparse dikes and ending with carbonatite flooding of the breccia matrix. This zoning appears related to the clast geometry and matrix abundance, with vein formation coinciding with more angular clast, and OxCa replacement with rounder clasts and higher percentage of matrix.