Geochemical evidence for submarine hydrothermal origin of the Middle-Upper Permian chert in Laibin of Guangxi, China

Lithologically, two kinds of chert can be recognized in the Middle-Upper Permian from the Tieqiao section in Laibin area, Guangxi, i.e., calcic chert occurring mainly in the Maokou Formation and pure chert mainly in the Wujiaping Formation. Geochemical data show that both kinds of chert contain very low Al2O3 (0-0.23%) and TiO2 (0.001%-0.024%) and low Sigma REE (0.55-19.94 ppm, averaging 9.97 ppm), as well as high ratio of Fe2O3/TiO2 (17-443, averaging 111) and low ratio of Al2O3/(Al2O3+Fe2O3) (0-0.26, averaging 0.09). Given that the average value Ce anomalies in chert deposited in the ridge-proximal environment is about 0.29, and that in the pelagic environment is about 0.60, the low Ce anomalies in the Tieqiao chert (0.24-0.46, averaging 0.35) imply that they were deposited in an ocean basin with influence of submarine hydrothermal fluid and no input of terrigenous materials. The vertical variation of the silica abundance in strata (SAIS) in the Middle-Upper Permian strata, together with the Eu anomalies and the ratios of Sigma REE/Fe, indicates a relationship between the hydrothermal activity and the Emeishan basalt eruption, and that submarine hydrothermal activity was stronger in the Upper Permian (the Wujiapingian Stage) than in the Middle Permian (the Maokou Stage).