Metasomatic silicate chemistry at the Bayan Obo Fe–REE–Nb deposit,Inner Mongolia,China: Contrasting chemistry and evolution of fenitising and mineralising fluids |
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| Institution: | 1. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China;2. Department of Earth Sciences, The University of Hong Kong, Hong Kong, Hong Kong, China;3. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;4. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China;1. Camborne School of Mines, University of Exeter, Penryn Campus, Cornwall TR10 9FE, United Kingdom;2. British Geological Survey, Nicker Hill, Keyworth, Nottingham NG12 5GD, United Kingdom;3. NERC Isotope Geosciences Laboratory, British Geological Survey, Nicker Hill, Keyworth, Nottingham NG12 5GG, United Kingdom;1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;2. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China;3. CAS Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;4. Zhejiang Institute of Geological Survey, Hangzhou 311203, China;5. Hebei Institute of Regional Geology and Mineral Resources Survey, Langfang 065000, China;6. CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;3. Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou 510640, China |
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| Abstract: | Fenite aureoles around carbonatite dykes, and alteration associated with Fe–REE–Nb ore bodies at Bayan Obo, Inner Mongolia, China, show alkali silicate assemblages containing aegirine–augite, (magnesio-)riebeckite, (magnesio-)arfvedsonite, and phlogopite, accompanied by varying amounts of apatite, albite and quartz. In both fenites and orebodies simple thermodynamic constraints indicate mineral parageneses are consistent with rock buffered cooling accompanied by the infiltration of a range of externally buffered hydrothermal fluids. Statistical analysis of amphibole chemistry indicates that even in apparently texturally well constrained paragenetic stages wide variations in chemistry occur in both the ore bodies and fenites. Much of this variation is attributable to the Mg and F content of amphibole, and is therefore interpreted as a result of variation in externally controlled variables (P, T, initial fluid composition) rather than internally controlled variables such as protolith composition. Similarities in chemistry exist between fenite and some ore body amphiboles. Thermodynamic analysis of the composition of biotite and apatite allows constraints to be placed on the F-content of hydrothermal fluids, and indicates relatively consistent compositions in fenites and orebodies (log aHF/aH2O = ? 3.8 to ? 3.6 at 300 °C and 1 kbar). Amphibole and biotite associated with niobate mineralization are both enriched in fluorine relative to the rest of the paragenesis, and biotite compositions indicate significantly higher HF activities in the hydrothermal fluid (log aHF/aH2O = ? 2.6 at 300 °C and 1 kbar). The data presented here reinforce previous interpretations of the complex, multistage nature of mineralisation at Bayan Obo, but are still consistent with a direct involvement of carbonatite derived fluids during ore genesis. |
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