THE SANCHA-WANGJIASHAN SECTION, HUNAN PROVINCE, CHINA: OCCURRENCE OF SYNGENETIC Fe-Ni-Mo ORE LAYERS NEAR THE BASE OF THE LOWERMOST CAMBRIAN NIUTITANG FORMATION

摘要

Well-exposed sections of Neoproterozoic to Middle Cambrian rock successions are widely distributed within the Wulingshan (Wuling Mountains; Fig. 1), south of Zhangjiajie (formerly Dayong City), Hunan Province. The stratigraphic section at Sancha-Wangjiashan (Fig. 2) is located approximately 15 km southeast of Zhangjiajie, and is surrounded by the beautiful scenery of the Wulingshan. As exposed near the village of Sancha ("Junction of Three Roads"), Hunan Province, the basement succession comprises schists and quartzites of the slightly metamorphosed Banxi Group. The terminal Neoproterozoic comprises tillites of the Nantuo Formation, clastic dolostones and phosphorites of the Toushantuo (Doushantou) Formation, and dolostones of the Tongying (Dengying) Formation. Near to the inferred edge of the Neoproterozoic carbonate platform the thicknesses of the Toushantou and Tongying formations are strongly reduced compared to elsewhere. At the Sancha-Wangjiashan section, the top of the Tongying dolostone succession is partly brecciated, possibly because of paleokarstification or brecciation associated with hydro-thermal solution. Cavities within the Tongying dolostones are filled in places by Lower Cambrian phosphorites of the Niutitang Formation. The Neoproterozoic Tongying Formation is discon-formably overlain by the Lower Cambrian Niutitang Formation.