Geochemical characteristics of Late Permian sediments in the Dalong Formation of the Shangsi Section, Northwest Sichuan Basin in South China: Implications for organic carbon-rich siliceous rocks formation Hui Chen a , Xinong Xie a, ⁎, Chaoyong Hu b , Junhua Huang b , Hongjing Li a a Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan, 430074, China b State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China abstract article info Article history: Received 10 December 2010 Accepted 17 June 2011 Available online 23 July 2011 Keywords: Hydrothermal contribution Paleo-depositional environment TOC content accumulation The major processes responsible for the interplay of hydrothermal influence and paleo-depositional environments (paleoproductivity and plaeoredox condition) on the accumulation of organic matters can be better understood by integrating lithological and geochemical characteristics of samples from the well exposed Late Permian sediments in the Dalong Formation of the Shangsi Section, Northwest Sichuan Basin, South China: 1) The content of total organic carbon (TOC) ranging from 0.16% to 14.6% is closely related with the host lithology and high TOC contents prone to occur in intervals of siliceous rocks, dark shales, and laminated limestones that developed in deepwater intra-basin environments; 2) Non-detrital component elements and ratios (after Ti-normalized), e.g. Cu xs ,V xs /(V xs +Ni xs ), for all samples and their correlations with TOC contents imprint the control of paleoproductivity level and paleoredox condition on the accumulation of TOC contents; 3) Characteristics such as original contents of Al, Fe, Mn, Th, U and their non-detrital origin values, non-biogenic contribution values (TiO 2 -normalized) of various elements, and the North America Shale Composite (NASC)-normalized Rare Earth Element (REE) Patterns of chert and shale samples are analyzed and compared to that of world average shales, classic hydrothermal and biogenic cherts, indicating hydrothermal contribution to siliceous rocks formation in the Dalong Formation of the Shangsi Section. Together with the discovery of volcanic ashes and bentonites in the Dalong Formation of the Shangsi Section, a new model has been proposed to explain the relationships of the hydrothermal activities and the organic matter accumulation process in the siliceous rocks in the Dalong Formation of the Shangsi Section: Hydrothermal contribution could cause a bio-thriving in surface water by providing nutrient elements and lead to a high primary productivity; With the organic matters sinking down, the anaerobic and anoxic conditions would control the organic matter accumulation during deposition and early diagenesis stage, which can imprint the organic carbon-rich siliceous rocks formation and the origin of high quality marine source rocks for hydrocarbon. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Under sea water environments, some chemical elements enrich or deficit due to special biorelated condition and depositional environ- ments. Thus major and trace-element geochemistry can be taken as proxies for reconstruction of paleo-depositional conditions, such as paleoproductivity, paleoredox condition and influence of hydrother- mal (hot water) activities (e.g., Algeo and Maynard, 2004; Bostrom et al., 1973, 1979; Crerar et al., 1982; Nameroff et al., 2004; Pen et al., 1999; Tribovillard et al., 2006). Nutrient-type elements such as Ni, Cu, Zn, Ba, Cd, P, and Fe might enrich due to organic matter enrichment (McManus, et al., 1999; Piper and Perkins, 2004; Riquier et al., 2005; Rutsch et al., 1995; Sageman et al., 2003; Tribovillard et al., 1996, 2005, 2006). Elements values of V, Mo, U and ratios of V/(V + Ni), V/Cr and Ni/Co can be changed significantly with the redox condition (Algeo and Maynard, 2004; Hatch and Leventhal, 1992; Jones and Manning, 1994). The effect of hydrothermal emanation can be ex- amined on the Al–Fe–Mn and Th–U diagrams as well as the North America Shale Composite (NASC)-normalized Rare Earth Element (REE) Patterns (Adachi et al., 1986; Bostrom et al., 1979; Bostrom and Peterson, 1969; Crerar et al., 1982; Marchig et al., 1982; Murray et al., 1990; Rona, 1984). The chemical composition of detrital and hydro- thermal components can be examined with the TiO 2 -normalized values of various elements (Sugisaki, 1984; Yamoto, 1986), excluding the biogenic contribution. The well exposed Shangsi Section in Guangyuan Area in Northwest Sichuan Basin (Northeast Sichuan Province), South China, was proposed as one of the four Global Boundary Stratotype Journal of Geochemical Exploration 112 (2012) 35–53 ⁎ Corresponding author at: Faculty of Earth Resources, China University of Geosciences, Wuhan, 430074, China. Tel./fax: + 86 27 67883063. E-mail address: xnxie@cug.edu.cn (X. Xie). 0375-6742/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2011.06.011 Contents lists available at ScienceDirect Journal of Geochemical Exploration journal homepage: www.elsevier.com/locate/jgeoexp