Geochemical constraints on the hydrothermal chert of the Kuhfeng Formation in the Middle Permian in the Lower Yangtze and its significance

Key words: 
• Kuhfeng Formation /
• Hydrothermal activity /
• Chert /
• Lower Yangtze /
• Middle Permian /
• Organic matter enrichment

Abstract: [Objective] The organic-enriched black bedded cherts in the Kuhfeng Formation were deposited in the Lower Yangtze during the Middle Permian. The hydrothermal cherts may be the key part for the relationship analysis between chert deposits and the extra-ordinarily high organic matter enrichment. This would be the basic theories for the shale gas exploration in this area. [Methods] This work uses major, minor and rare-earth elements, combined with the petrology, to analyze the origin of chert and their tectonic setting for the Kuhfeng Formation in the Chaohu and Tongling areas. [Results] The black bedded cherts in the Chaohu area contain abundant siliceous sponge spicules and radiolarians. The cinerous cherts in the Kuhfeng Formation in Tongling area contain very rare siliceous fossil, and consist of micro-quartz. Hydrothermal proxies, such as Al/(Al+Fe+Mn), Eu/Eu*, Al-Fe-Mn diagram and LuN/LaN ratios, indicate a hydrothermal origin and a biotic origin for the chert in the Kuhfeng Formation in Tongling and Chaohu areas, respectively. La-Th-Sc diagram and crossplot between La/Sc and Ti/Zr indicate oceanic island arc and active continental margin for the Tongling and Chaohu areas. [Conclusion] The chert in the Kuhfeng Formation in Tongling area was primary deposited by hydrothermal activity. The silicic acid was brought by hydrothermal vent near the slope environments. The black bedded cherts in the Lower Yangtze area are biotic origin, but the source of silicon was related to the hydrothermal activity. The organic matter enrichment in the Kuhfeng Formation in the Lower Yangtze area may be related to the phytoplankton flourishment induced by the nutrient element input from hydrothermal activity.