Petrography,chemical composition,and Raman spectra of chrome spinel: Constraints on the diamond potential of the No. 30 pipe kimberlite in Wafangdian,North China Craton |
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| Institution: | 1. State Key Laboratory for Mineral Deposit Research, Institute of Geo-Fluids, School of Earth Science and Engineering, Nanjing University, Nanjing 210023, China;2. The Sixth Geological Brigade in Liaoning Province, Dalian 116200, China;1. MLR Laboratory of Metallogeny and Mineral Resource Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China;2. Centre for Exploration Targeting, ARC Centre of Excellence for Core to Crust Fluid Systems, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia;3. Mineral Branch of Hunan Institute of Geological Survey, Changsha, China;1. Graduate School of Education, Shimane University, 1060 Nishikawazu, Matsue, Shimane 690-8504, Japan;2. Department of Earth Sciences, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan;1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. College of Earth Science, University of Chinese Academy of Sciences, Beijing 100049, China;3. Centre for Exploration Targeting, The University of Western Australia, Crawley, WA 6009, Australia;4. MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, CAGS, Beijing 100037, China;1. V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, 3 Koptyug Ave., Novosibirsk 630090, Russia;2. Institute of Geology, Mineralogy and Geophysics, Ruhr-University Bochum, 44780 Bochum, Germany;3. Novosibirsk State University, Pirogova 2, 630090 Novosibirsk, Russia;4. College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, China;5. A.P. Vinogradov Geochemistry Institute, Siberian Branch of Russian Academy of Sciences, Irkutsk, Russia;6. Institute of Diamond and Noble Metals, Siberian Branch of Russian Academy of Sciences, Yakutsk, Russia;1. Department of Applied Geology, Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, India;2. Geological Society of India, No. 1532, 14th Main, Kumaraswamy Layout, Bangalore 560078, India |
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| Abstract: | Conventional diamond exploration seldom searches directly for diamonds in rock and soil samples. Instead, it focuses on the search for indicator minerals like chrome spinel, which can be used to evaluate diamond potential. Chrome spinels are preserved as pristine minerals in the early Paleozoic (∼465 Ma), hydrothermally altered, Group I No. 30 pipe kimberlite that intruded the Neoproterozoic Qingbaikou strata in Wafangdian, North China Craton (NCC). The characteristics of the chrome spinels were investigated by petrographic observation (BSE imaging), quantitative chemical analysis (EPMA), and Raman spectral analysis. The results show that the chrome spinels are mostly sub-rounded with extremely few grains being subhedral, and these spinels are macrocrystic, more than 500 µm in size. The chrome spinels also have compositional zones: the cores are classified as magnesiochromite as they have distinctly chromium-rich (Cr2O3 up to 66.56 wt%) and titanium-poor (TiO2 < 1 wt%) compositions; and the rims are classified as magnetite as they have chromium-poor and iron-rich composition. In the cores of chrome spinels, compositional variations are controlled by Al3+-Cr3+ isomorphism, which results in a strong Raman spectra peak (A1g mode) varying from 690 cm−1 to 702.9 cm−1. In the rims of chrome spinel, compositional variations result in the A1g peak varying from 660 cm−1 to 672 cm−1. The morphology and chemical compositions indicate that the chrome spinels are mantle xenocrysts. The cores of the spinel are remnants of primary mantle xenocrysts that have been resorbed, and the rims were formed during kimberlite magmatism. The compositions of the cores are used to evaluate the diamond potential of this kimberlite through comparison with the compositions of chrome spinels from the Changmazhuang and No. 50 pipe kimberlites in the NCC. In MgO, Al2O3 and TiO2 versus Cr2O3 plots, the chrome spinels from the Changmazhuang and No. 50 pipe kimberlites are mostly located in the diamond stability field. However, only a small proportion of chrome spinels from No. 30 pipe kimberlite have same behavior, which indicates that the diamond potential of the former two kimberlites is greater than that of the No. 30 pipe kimberlite. This is also supported by compositional zones in the spinel grains: there is with an increase in Fe3+ in the rims, which suggests that the chrome spinels experienced highly oxidizing conditions. Oxidizing conditions may have been imparted by fluids/melts that have a great influence on diamond destruction. Here, we suggest that chrome spinel compositions can be a useful tool for identifying the target for diamond potential in the North China Craton. |
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| Keywords: | Chrome spinel Diamond potential No 30 pipe kimberlite Wafangdian North China Craton |
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