Rare earth element characteristics of the carboniferous Huanglong Formation dolomites in eastern Sichuan Basin,southwest China: Implications for origins of dolomitizing and diagenetic fluids |
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| Institution: | 1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;2. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China;3. Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA;1. Guangxi Laboratory on the Study of Coral Reefs in the South China Sea, Coral Reef Research Center of China, School of Marine Sciences, Guangxi University, Nanning 530004, China;2. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T5R 3C8, Canada;3. School of Earth Sciences, University of Queensland, QLD 4067, Australia;4. School of Earth Science and Engineering, Nanjing University, Nanjing 210093, China;1. Department of Earth and Environmental Sciences, Ludwig-Maximilians-University, Munich, Germany;2. Department of Geology, University of Johannesburg, Johannesburg, South Africa;3. Economic Geology Research Institute, School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa;4. Department of Geography and Earth Sciences, Friedrich-Alexander-University of Erlangen-Nürnberg, Erlangen, Germany |
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| Abstract: | Marine dolostones of Carboniferous Huanglong Formation constitute major gas reservoir rocks in eastern Sichuan Basin. However, the investigation with respect to sources of dolomitizing and diagenetic fluids is relatively underexplored. The current study attempts to investigate the REE characteristics of dolomites using seawater normalization standard, and therefore discusses the origins of dolomitizing and diagenetic fluids, on the basis of continuous 47.33-m-long core samples from the second member of Huanglong Formation (C2h2) in eastern Sichuan Basin. Low Th, Sc, and Hf concentrations (0.791 × 10−6, 4.751 × 10−6, and 0.214 × 10−6, respectively), random correlation between total REE concentration (ΣREE) and Fe or Mn abundance, and seawater-like Y/Ho ratios (mean value of 45.612) indicate that the carbonate samples are valid for REE analysis. Based on petrographic characteristics, four dolomite types are identified, including micritic-sized dolomite (type Dol-1), fine-to medium-sized dolomite (type Dol-2), medium-to coarse-sized dolomite (type Dol-3), and coarse-to giant-sized saddle dolomite (type Dol-4). Dol-1 dolomites, characterized by positive Ce anomaly (mean value of 6.398), light REE (LREE) enrichment, and heavy REE (HREE) depletion with mean LREE/HREE ratio of 12.657, show micritic calcite-like REE patterns, indicating seawater origin of their dolomitizing fluids. Dol-1 dolomites were formed in sabkha environment whereas the dolomitizing fluids originated from evaporative brine water due to their micritic crystal sizes and tight lithology. Dol-2 dolomites, particularly subtype Dol-2a barely developing vuggy porosity, also show micritic calcite-like REE patterns, suggesting their dolomitizing fluids were seawater or seawater-derived fluids. This inference is confirmed by low Fe and Mn concentrations, which range from 651 μg/g to 1018 μg/g (mean value of 863 μg/g) and 65 μg/g to 167 μg/g (mean value of 105 μg/g), respectively, whereas homogenization temperatures (Th, mean value of 103 °C) indicate that Dol-2 dolomites were formed under burial environment. Dol-3 dolomites, in form of cements of Dol-2 dolomites, show similar REE patterns to their host minerals (i.e., Dol-2 dolomites), indicating their parent source was possibly derived from Dol-2 dolomites. Dol-3 dolomites have high Fe and Mn concentrations with mean values of 3346 μg/g (ranging from 2897 μg/g to 3856 μg/g) and 236 μg/g (ranging from 178 μg/g to 287 μg/g), respectively, indicating the involvement of meteoric water. Meanwhile, it confirms that the dissolution in Dol-2 dolomites was caused by meteoric water leaching. Positive Eu anomalies (mean value of 1.406) in Dol-4 dolomites, coupled with high homogenization temperatures (mean value of 314 °C), suggest that Dol-4 dolomites precipitated from hydrothermal fluids. High Fe and Mn concentrations (mean values of 2521 μg/g and 193 μg/g, respectively) in Dol-4 dolomites likely results from interactions of hydrothermal fluids with deep burial clastic rocks. |
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| Keywords: | REE Seawater normalization Dolomitization Huanglong formation Eastern Sichuan Basin |
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