Exploration for REE and Gold in the Bear Lodge Tertiary Alkaline Complex, Crook County, Wyoming

摘要

Part I. Exploration History, Geology and Resources Rare Element Resources, Inc. is currently developing a world-class, rare-earth-element (REE) deposit in the Bear Lodge Mountains of northeastern Wyoming. More than 20 companies and government organizations have explored the Bear Lodge district for multiple commodities since 1900, with the most significant contributions to the understanding of the geology and distribution of the REE mineralization made by the U.S. Geological Survey, Molycorp, Inc., Hecla Mining Company, Newmont Exploration, and Rare Element Resources. The REE mineralization is hosted in carbonatite bodies and oxidized equivalents that intrude diatremic intrusive breccias and hypabyssal alkalic intrusive rocks of Tertiary age. Significant gold mineralization is distributed peripheral to and partly overlapping the REE mineralization. The Bull Hill mine contains a Measured + Indicated resource of 16.2 million tonnes grading 3.08% TREO (Total Rare Earth Oxide), and an Inferred Oxide-only resource of 31.8 million tonnes at 2.58% TREO. Mining at Bull Hill is planned as a small, conventional truck and shovel open-pit operation that accesses near-surface mineralization. The physical upgrade (PUG) plant is designed to use a combination of crushing, screening, and gravity separation, depending on the ore type being treated, to reduce the physical mass of the ore by reducing gangue and concentrating the rare-earth-bearing fine fraction for shipment to the hydrometallurgical (Hydromet) plant. The Hydromet plant is designed to extract acid soluble metals from pre-concentrated ores using a proprietary counter-current low-temperature leach process that allows more rare earth elements (REE) to be transferred into the chloride solution than base metals. The rare earths are then selectively precipitated as oxalate crystals, which are later calcined (roasted) to produce the high-purity rare earth oxide (REO) powder. The current project life is 45 years, including 38 years of mining and 7 years of low-grade stockpile processing, with an average, life-of-mine, nominal production rate of approximately 6,813 tonnes TREO per year. The Bear Lodge project is now in the Feasibility Study stage. Part II. Exploration Methods Between 2004 and 2013 Rare Element Resources (the Company), along with joint venture partner Newmont Exploration (2006-2010), employed a variety of exploration methods at the Bear Lodge property that led to exceptional success in the definition and expansion of both REE and gold resources in multiple deposits, and to the realization of tremendous upside exploration potential for both deposit types peripheral to the existing resources. From 2009 to 2013, the Company increased its NI 43-101 compliant Inferred REE resource fivefold and moved a third of that into the Indicated and Measured categories. Exploration methods, including geological mapping, soil and rock geochemistry, geophysics and drilling were integrated and used effectively to discover minable reserves of REE and more than 1 million ounces of gold. Geological mapping of lithology, structure, alteration, and mineralization provided a foundation for the exploration of the fifty-square-kilometer igneous complex. Poor rock exposure in the Bear Lodge complex is mitigated by the collection and analysis of more than 9000 soil and 7000 rock samples, which reveal geochemical vectors for both REE and gold mineralization. Geophysical methods employed include airborne and ground magnetics and radiometrics, gravity, NEWTEM (airborne EM) and CSAMT. Specific geophysical signatures are correlated with aspects of lithology, structure, alteration, and mineralization. These varied data continue to play a key role in the delineation of additional mineralization and district-wide zonation of REE and gold mineralization.