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Whole-rock geochemical data for the Bear Lodge alkaline complex, Wyoming: 2009 - 2013

April 17, 2020

This data release contains analytical data from a suite of rock samples collected within the Bear Lodge alkaline complex, northeast Wyoming, which hosts the Bear Lodge rare earth element (REE) deposit. Geochemistry data include major and trace element analytical results for 105 samples including alkaline igneous rocks, carbonatites, and weathered and oxidized samples. Samples were collected from surface exposures, a shallow temporary exploration trench, and drill core. Drill core samples are from exploration holes drilled by Rare Element Resources between 2009 and 2013 and from historical drill core from previous exploration companies. Rare earth element mineralization at Bear Lodge is hosted by carbonatite dikes, veins, and stockworks cutting a diatreme breccia body and adjacent alkaline silicate intrusive rocks. Identified zones within the carbonatite resource include an unoxidized zone, a transitional zone, an oxide-carbonate zone, and an oxidized zone. The deepest zone is characterized by unoxidized carbonatite with mineralized pseudomorphs and veins which contain a REE mineral assemblage of predominantly ancylite and carbocernaite with lesser burbankite, monazite, and Ca-REE fluorocarbonates (Mariano and Mariano, 2012; Andersen and others, 2019). The thin transition zone, generally 6 meters thick, occurs at the top of the unoxidized zone and represents the initial stages of sulfide oxidation and replacement of ancylite by Ca-REE fluorocarbonate minerals (Dahlberg, 2014). The oxide-carbonate zone extends from the top of the transition zone to the base of the oxidized zone. It is characterized by nearly total oxidation of sulfides, partial dissolution of groundmass calcite, and substantial replacement of ancylite by Ca-REE fluorocarbonates. The upper part of the deposit primarily consists of intensely oxidized dikes and veins termed FMR dikes (Fe oxide-Mn oxide-REE). Within the oxide zone, sulfides are completely oxidized, and groundmass carbonate is completely leached. The REE mineral assemblage consists of Ca-REE fluorocarbonates (bastnasite and parisite), with variable, but subordinate, phosphates and cerianite. The oxidized zone is as much as 150-m-thick and may result from hydrothermal and/or supergene processes. The 40Ar/39Ar age determinations of biotite and orthoclase phenocrysts constrain the age of carbonatite magmatism to between 51.45+/-0.08 and 51.89+/-.14 Ma (Andersen and others, 2019). Areas enriched in heavy REEs occur peripheral to the main zone of light REE mineralization at Bull Hill (Andersen and others, 2017). Mineralogical studies identify Ca-REE fluorocarbonates, monazite, xenotime, and rhabdophane-group minerals in areas peripheral to Bull Hill (Dahlberg, 2014; Andersen and others, 2016, 2017). Additional information and geochemical data from the Bear Lodge complex can be found in Staatz and others (1983), Mariano (1989), Moore and others (2015), Verplanck and others (2016), and Olinger (2019). All samples were analyzed by laboratories contracted by the U.S. Geological Survey. Major and trace element concentrations were determined by a variety of methods including inductively coupled plasma-atomic emission spectrometry (ICP-AES), inductively coupled plasma-mass spectrometry (ICP-MS), wavelength dispersive X-ray fluorescence (WDXRF), lead-fusion fire-assay (FA), and ion selective electrode (ISE). Additionally, a subset of samples was analyzed for total sulfur and carbon by combustion and infrared detection, ferrous oxide by titration, and essential and non-essential water by gravimetric methods. For each method outlined above, an acceptable criteria for the data has been identified based on 1) if recovery of each element is within a designated percentage at five times the lower limit of determination and 2) the calculated relative standard deviation (RSD) of duplicate samples is no greater than that percentage. The reported laboratory percentages for the acceptance criteria are as follows: +/- fifteen percent for ICP-AES and ICP-MS, +/- five percent for WDXRF, +/- twenty percent for FA, +/- twenty percent for ISE, +/- fifteen percent for total sulfur, +/- fifteen percent for carbon, +/- fifteen percent for ferrous oxide, and +/- fifteen percent for gravimetric methods. Data are reported in a comma-separated values (CSV) file that lists the samples that were analyzed, drill hole identification number and depth (if applicable), latitude/longitude location information, and brief sample descriptions. All column headings and abbreviations are explained in the accompanying data dictionary. References cited above: Andersen, A.K., Clark, J.G., Larson, P.B., and Neill, O.K., 2016, Mineral chemistry and petrogenesis of a HFSE(+HREE) occurrence peripheral to carbonatites of the Bear Lodge alkaline complex, Wyoming: American Mineralogist, v. 101, p. 1604-1623. https://doi.org/10.2138/am-2016-5532 Andersen, A.K., Clark, J.G., Larson, P.B., and Donavan, J.J., 2017, REE fractionation, mineral speciation, and supergene enrichment of the Bear Lodge carbonatites, Wyoming, USA: Ore Geology Reviews, v. 89, p. 780-807. https://doi.org/10.1016/j.oregeorev.2017.06.025 Andersen, A.K., Larson, P.B., and Cosca, M.A., 2019, C-O stable isotope geochemistry and 40Ar/39Ar geochronology of the Bear Lodge carbonatite stockwork, Wyoming, USA: Lithos, v. 324-325, p. 640-660. https://doi.org/10.1016/j.lithos.2018.11.030 Dahlberg, P.S., 2014, Pre-feasibility study report on the reserves and development of the Bull Hill Mine, Wyoming, NI 43-101 technical report, prepared for Rare Element Resources, Ltd.: Sandy, Utah, Roche Engineering, Inc., October 9, 587 p., accessed March 16, 2020, at http://www.rareelementresources.com/ Mariano, A.N., 1989, Economic geology of rare earth minerals, in Lipin, B.R. and Mckay, G.A., eds., Geochemistry and mineralogy of rare earth elements: Washington D.C., Mineralogical Society of America, Reviews in Mineralogy, v. 21., p. 309-338. Mariano, A.N., and Mariano, A.N., Jr., 2012, Rare earth mining and exploration in North America: Elements, v. 8, p. 369-376. https://doi.org/10.2113/gselements.8.5.369 Moore, M., Chakhmouradian, A.R., Mariano, A.N., and Sidhu, Ravinder, 2015, Evolution of rare-earth mineralization in the Bear Lodge carbonatite, Wyoming - Mineralogical and isotopic evidence: Ore Geology Reviews, v. 64, p. 499-521. https://doi.org/10.1016/j.oregeorev.2014.03.015 Olinger, D.A. 2019, Carbonatite whole-rock and calcite geochemistry from the Bear Lodge alkaline complex, Wyoming and Mountain Pass mine, California: U.S. Geological Survey data release, https://doi.org/10.5066/P9634NRU Staatz, M.H., 1983, Geology and description of thorium and rare-earth deposits in the southern Bear Lodge Mountains, northeastern Wyoming: U.S. Geological Survey Professional Paper 1049-D, 52 p., 2 plates. https://doi.org/10.3133/pp1049D Verplanck, P.L., Mariano, A.N., and Mariano, A.N. Jr, 2016, Rare earth element ore geology of carbonatites, in Verplanck, P.L., and Hitzman, M.W., eds, Society of Economic Geologists Reviews in Economic Geology Vol. 18, p. 5- 32. https://doi.org/10.5382/REV.18