The Trace Element Distribution Patterns of Ediacaran-Early Cambrian Black Shales and the Origin of Selenium in the Guangning Area, Western Guangdong Province, South China |
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| Authors: | TIAN Xinglei LUO Kunli and Andrew V ZUZA |
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| Institution: | 1 Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China
2 University of the Chinese Academy of Sciences, Beijing 100049, China,1 Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China and 3 Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557, USA |
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| Abstract: | The Ediacaran and early Cambrian black shales are widespread across the South China Craton (Yangtze and Cathaysia blocks). However, the trace element distribution patterns of the Ediacaran and early Cambrian black shales in the Cathaysia Block are still unclear. In this study, thirty-four black shale samples in the Lechangxia Group (Ediacaran) and thirteen black shale samples in the lower Bacun Group (early Cambrian) from Guangning area, western Guangdong Province, South China, were analyzed for major and trace elements concentrations. Compared to the upper continental crust, the Ediacaran black shales exhibit strongly enriched Se, Ga, and As with enrichment factor values (EF) higher than 10, significantly enriched Bi and Rb (10>EF>5), weakly enriched Mo, Ba, Cs, V, In, Be, Tl, and Th (5>EF>2), normal U, Cr, Cd, Sc, Pb, Cu, and Li (2>EF>0.5), and depleted Ni, Zn, Sr, and Co. Early Cambrian black shales display strongly enriched Se, Ga, and As, significantly enriched Ba, Bi, and Rb, weakly enriched Mo, Cs, Cd, V, U, Be, In, and Tl, normal Sc, Th, Cr, Li, Cu, Ni, and Pb and depleted Co, Zn, and Sr. Moreover, Se is the most enriched trace element in the Ediacaran and early Cambrian black shales: concentrations vary from 0.25 to 30.09 ppm and 0.54 to 5.01 ppm, and averaging 4.84 and 1.72 ppm, with average EF values of 96.87 and 34.32, for the Ediacaran and early Cambrian shales respectively. The average concentration of Se in the Ediacaran black shales is 2.8 times higher than that of early Cambrian black shales. Se contents in the Ediacaran and early Cambrian black shales exhibit significant variation (P = 0.03). Provenance analysis showed that Se contents of both the Ediacaran and early Cambrian black shales were without detrital provenance and volcanoclastic sources, but of hydrothermal origin. The deep sources of Se and the presence of pyrite may explain the higher Se contents in the Ediacaran black shales. Similar with the Se-rich characteristics of the contemporaneous black shales in the south Qingling Mountain and Yangtze block, the Ediacaran and early Cambrian black shales in Guangning area, Cathaysia, are also enriched in Se, which may provide a clue for looking for the selenium-rich resources in western Guangdong Province. |
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| Keywords: | black shale selenium Ediacaran and early Cambrian Guangning area Cathaysia block |
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