REE fractionation,mineral speciation,and supergene enrichment of the Bear Lodge carbonatites,Wyoming, USA |
| |
| Institution: | 1. Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia;2. School of Earth Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia;1. Geological Survey of Western Australia, Mineral House, 100 Plain Street, East Perth, WA 6004, Australia;2. Centre for Exploration Targeting, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia;3. CSIRO, Earth Science and Resource Engineering, 26 Dick Perry Av., Discovery Theme, Minerals Down Under Flagship, Kensington 6151, Australia;4. Department of Civil Engineering and Geological Sciences, 156 Fitzpatrick Hall, University of Notre Dame, Notre Dame, IN 46556, USA;5. Department of Geological Sciences and Geological Engineering, Queens University, Kingston, Ontario K7 3N6, Canada;1. Geology Department, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand;2. Department of Mineralogy, British Museum of Natural History, Cromwell Road, London SW75BD, England, United Kingdom;1. UNESP - Universidade Estadual Paulista, Instituto de Geociências e Ciências Exatas, Rio Claro, São Paulo, Brazil;2. UQ - University of Queensland, School of Earth Science, Brisbane, Australia;3. UNIFAL – Universidade Federal de Alfenas, Instituto de Ciências e Tecnologia, Alfenas, Minas Gerais, Brazil;1. Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada;2. Department of Mineralogy, St. Petersburg State University, St. Petersburg, Russia;3. Department of Earth Sciences, The Natural History Museum, London, UK;4. Manitoba Geological Survey, Winnipeg, Manitoba, Canada;5. Laboratory of Orogenic Belts and Crustal Evolution, Peking University, Beijing, China;6. Department of Geology and Pedology, Mendel University, Brno, Czech Republic;7. Central European Institute of Technology, Brno University of Technology, Brno, Czech Republic;8. Department of Geology, Brandon University, Brandon, Manitoba, Canada |
| |
| Abstract: | The Eocene (ca. 55–38 Ma) Bear Lodge alkaline complex in the northern Black Hills region of northeastern Wyoming (USA) is host to stockwork-style carbonatite dikes and veins with high concentrations of rare earth elements (e.g., La: 4140–21000 ppm, Ce: 9220–35800 ppm, Nd: 4800–13900 ppm). The central carbonatite dike swarm is characterized by zones of variable REE content, with peripheral zones enriched in HREE including yttrium. The principle REE-bearing phases in unoxidized carbonatite are ancylite and carbocernaite, with subordinate monazite, fluorapatite, burbankite, and Ca-REE fluorocarbonates. In oxidized carbonatite, REE are hosted primarily by Ca-REE fluorocarbonates (bastnäsite, parisite, synchysite, and mixed varieties), with lesser REE phosphates (rhabdophane and monazite), fluorapatite, and cerianite. REE abundances were substantially upgraded (e.g., La: 54500–66800 ppm, Ce: 11500–92100 ppm, Nd: 4740–31200 ppm) in carbonatite that was altered by oxidizing hydrothermal and supergene processes. Vertical, near surface increases in REE concentrations correlate with replacement of REE(±Sr,Ca,Na,Ba) carbonate minerals by Ca-REE fluorocarbonate minerals, dissolution of matrix calcite, development of Fe- and Mn-rich gossan, crystallization of cerianite and accompanying negative Ce anomalies in secondary fluorocarbonates and phosphates, and increasing δ18O values. These vertical changes demonstrate the importance of oxidizing meteoric water during the most recent modifications to the carbonatite stockwork. Scanning electron microscopy, energy dispersive spectroscopy, and electron probe microanalysis were used to investigate variations in mineral chemistry controlling the lateral complex-wide geochemical heterogeneity. HREE-enrichment in some peripheral zones can be attributed to an increase in the abundance of secondary REE phosphates (rhabdophane group, monazite, and fluorapatite), while HREE-enrichment in other zones is a result of HREE substitution in the otherwise LREE-selective fluorocarbonate minerals. Microprobe analyses show that HREE substitution is most pronounced in Ca-rich fluorocarbonates (parisite, synchysite, and mixed syntaxial varieties). Peripheral, late-stage HREE-enrichment is attributed to: 1) fractionation during early crystallization of LREE selective minerals, such as ancylite, carbocernaite, and Ca-REE fluorocarbonates in the central Bull Hill dike swarm, 2) REE liberated during breakdown of primary calcite and apatite with higher HREE/LREE ratios, and 3) differential transport of REE in fluids with higher PO43?/CO32? and F?/CO32? ratios, leading to phosphate and pseudomorphic fluorocarbonate mineralization. Supergene weathering processes were important at the stratigraphically highest peripheral REE occurrence, which consists of fine, acicular monazite, jarosite, rutile/pseudorutile, barite, and plumbopyrochlore, an assemblage mineralogically similar to carbonatite laterites in tropical regions. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
