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Carbonatite-related rare-earth mineralization in the Bear Lodge alkaline complex, Wyoming: Paragenesis, geochemical and isotopic characteristics

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dc.contributor.supervisor Chakhmouradian, Anton (Geological Sciences) en_US
dc.contributor.author Moore, Meghan
dc.date.accessioned 2014-09-05T21:56:01Z
dc.date.available 2014-09-05T21:56:01Z
dc.date.issued 2014 en_US
dc.identifier.citation Moore, M., Chakhmouradian, A.R., Mariano, A.N., and Sidhu, R. (2014) Evolution of rare-earth mineralization in the Bear Lodge carbonatite, Wyoming: Mineralogical and isotopic evidence. Ore Geology Reviews, in press en_US
dc.identifier.uri http://hdl.handle.net/1993/23991
dc.description.abstract The Bear Lodge alkaline complex in northeastern Wyoming (USA) is host to potentially economic rare-earth mineralization in carbonatite and carbonatite-related veins and dikes that intrude heterolithic diatreme breccias in the Bull Hill area of the Bear Lodge Mountains. The deposit is zoned and consists of pervasively oxidized material at and near the surface, which passes through a thin transitional zone at a depth of ~120-183m, and grades into unaltered carbonatites at depths greater than ~183-190m. Carbonatites in the unoxidized zone consist of coarse and fine-grained calcite that is Sr-, Mn- and inclusion-rich and are characterized by the presence of primary burbankite, early-stage parisite and synchysite with minor bastnäsite that have high (La/Nd)cn and (La/Ce)cn values. The early minerals are replaced with polycrystalline pseudomorphs consisting of secondary rare-earth fluorocarbonates and ancylite with minor monazite. Different secondary parageneses can be distinguished on the basis of the relative abundances and composition of individual minerals. Variations in key element ratios, such as (La/Nd)cn, and chondrite-normalized profiles of the rare-earth minerals and calcite record multiple stages of hydrothermal deposition involving fluids of different chemistry. A single sample of primary calcite shows mantle-like δ18O V-SMOW and δ13C V-PDB values, whereas most other samples are somewhat depleted in 13C (δ13C V-PDB ≈ –8 to –10‰) and show a small positive shift in δ18O V-SMOW due to degassing and wall-rock interaction. Isotopic re-equilibration is more pronounced in the transitional and oxidized zones; large shifts in δ18O V-SMOW (to ~ 18‰) reflect input of meteoric water during pervasive hydrothermal and supergene oxidation. The textural relations, mineral chemistry, and C and O stable-isotopic variations record a polygenetic sequence of rare-earth mineralization in the deposit. With the exception of one Pb-poor sample showing an appreciable positive shift in 208Pb/204Pb value (~39.2), the Bear Lodge carbonatites are remarkably uniform in their Nd, Sr and Pb isotopic composition: (143Nd/144Nd)i=0.512591-0.512608; εNd=0.2-0.6; (87Sr/86Sr)i=0.704555-0.704639; εSr=-1.5-2.7; (206Pb/204Pb)i=18.071-18.320; (207Pb/204Pb)i=15.543-15.593; (208Pb/204Pb)i=38.045-39.165. These isotopic characteristics indicate that the source of the carbonatitic magma was in the subcontinental lithospheric mantle, and modified by subduction-related metasomatism. Carbonatites are interpreted to be generated from small degrees of partial melt that may have been produced via interaction of upwelling asthenosphere giving a small depleted MORB component, with an EM1 component likely derived from subducted Farallon crust. en_US
dc.publisher ElSevier en_US
dc.subject Rare-earth element en_US
dc.subject carbonatite en_US
dc.title Carbonatite-related rare-earth mineralization in the Bear Lodge alkaline complex, Wyoming: Paragenesis, geochemical and isotopic characteristics en_US
dc.degree.discipline Geological Sciences en_US
dc.contributor.examiningcommittee Camacho, Alfredo (Geological Sciences) Clark, James (Applied Petrographics) en_US
dc.degree.level Master of Science (M.Sc.) en_US
dc.description.note October 2014 en_US


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