Trace element distribution and enrichment patterns of Ediacaran-early Cambrian, Ziyang selenosis area, Central China: Constraints for the origin of Selenium

• Published in: Journal of geochemical exploration Vol. 172; pp. 211 - 230
• Main Authors: Long, Jie, Luo, Kunli
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.01.2017
• Subjects: Hydrothermal activity; Lujiaping Formation; Selenium; South Qinling Mountain; South Qinling Mountain; Selenium; Lujiaping Formation; Hydrothermal activity
• ISSN: 0375-6742; 1879-1689
• DOI: 10.1016/j.gexplo.2016.11.010

Ziyang County is one of the two major regions in Central China exposed to Selenium (Se) poisoning. To understand the origin of anomalous Se enrichment strata of the Ediacaran-early Cambrian period (ca. 635–520Ma) in the area of Ziyang affected by selenosis, major and trace elements were analyzed for 468 samples collected from the Lujiaping Formation in Ziyang County, Central China. Se is the most highly enriched trace element in the Lujiaping Formation of the Ediacaran-early Cambrian period. Se concentrations vary from 0.03 to 303ppm, with an average of 16ppm, or >324 times the concentration found in the upper continental crust. The average Se content of Bed 6 (approximately 63m thick) is the highest, at 49.20ppm (1.45–154ppm). The Lujiaping Formation is highly enriched in Cd (×53), S (×31), Mo (×22) and As (×13), compared to the upper continental crust. It is slightly enriched in V (×8.07), Ba (×7.65), Ga (×7.24), Zn (×7.22), U (×5.43), Cu (×4.02), Bi (×2.55), Tl (×2.16), Ni (×1.75), P (×1.03), and depleted in Pb (0.97), Cr (0.96), Sr (0.81), Li (0.79), In (0.69), Sc (0.69), Be (0.38), Cs (0.36), Co (0.30), Rb (0.29), and Th (0.27). All Eu/Eu* for the forty anomalous Se-enrichment samples (Se=42–303ppm) of the 1st Member (1.09–2.09, average=avg.=1.40), 2nd Member (1.20–1.38, avg.=1.29), 3rd Member (1.12–4.00, avg.=2.38) and 4th Member (2.51–4.48, avg.=3.56), show positive Eu anomalies and an increasing trend upward. However, Eu/Eu* of two samples are 0.90 (HSBY324, Bed 4 of 1st Member) and 0.95 (HSBY401, Bed 6 of 1st Member), which indicate that they are characterized by hydrothermal sediments, except HSBY324 and HSBY401 of 1st Member. Based on these elements' geochemical and petrographic characteristics, we conclude that these samples are of hydrothermal origin. There is also a strong positive correlation between Se and Ba (RSe-Ba=0.738**, n=40). In three point X-ray energy spectrometric analysis of black shale mineral anomalous Se-enrichment samples (HSBY601), the nor.C measurements of Ba (L Series) are 55.50wt.%, 54.94wt.% and 27.20wt.%, respectively, with the corresponding Atom. C of 14.80, 14.50 and 5.72at.%. Thus, the main Se source was derived from hydrothermal fluids, of anomalous Se-enrichment strata of Ediacaran-early Cambrian (ca. 635–520Ma) of South Qinling Mountain. •Se content of black shale (No. HSBY601) is 303 ppm and more than 6064 times of Upper Continental Crust (UCC).•The average Se content of Bed 6 (ca. 63m-thick) are the highest, reaching to 49.20 ppm (1.45-154 ppm).•Average content of Se, Cd , Mo, S and As are more than 324, 53, 31, 22 and 13 times of UCC.•Se source was derived from hydrothermal fluids, of anomalous Se-enrichment strata of Ediacaran-early Cambrian of SQM.