The origin and geochemical characteristics of Permian chert in the Eastern Sichuan Basin, China

Abstract

Chert is widespread in Permian Maokou Formation and Wujiaping Formation of the eastern Sichuan Basin, China. Field observations in conjunction with major element, REE, and isotope analysis suggest that the chert is both biogenic and hydrothermal in origin. Further analysis indicates that hydrothermal features are more prominent in chert of the Maokou Formation, whereas chert of the Wujiaping Formation contains stronger evidence of biogenic origin. The SiO2 content of chert of the Maokou and Wujiaping formations are 80.09–97.91 and 65.52–97.76 wt%, respectively. The average Al/(Al + Fe + Mn) values are 0.35 (Maokou Formation) and 0.38 (Wujiaping Formation), and most samples are plotted in the Fe-rich section of the Al–Fe–Mn diagram. SiO2 abundance and Al/(Al + Fe + Mn) ratio analysis of chert of the Maokou and Wujiaping formations suggest a hydrothermal origin, although several samples were more indicative of biogenic origin. The average REE content of chert in both formations is low, and average REE is lower in chert of the Maokou Formation than in chert of the Wujiaping Formation. The Ce anomaly is weakly negative in the chert of the Maokou Formation (0.62) and Wujiaping Formation (0.71), whereas the average Eu value is greater in chert of the Maokou Formation than in chert of the Wujiaping Formation. The δ30Si values of chert are 0.4–1.2‰ in the Maokou Formation and 0.7–1.4‰ in the Wujiaping Formation, and these values are close to the δ30Si values of modern radiolarians. The average palaeoseawater temperatures under which the chert formed were calculated as 66 °C for chert of the Maokou Formation and 62 °C for chert of the Wujiaping Formation. These calculated temperatures are significantly higher than estimated ancient sea temperatures. These data and interpretations suggest that the chert of the Permian Maokou and Wujiaping formations were affected by volcanism and fracturing during the Permian. Seawater seeped through fractures and interacted with upwelling hot magma along basement faults, resulting in silica dissolution and the enrichment of waters from hot spots along fractures as well as the enrichment of seawater. This enrichment resulted in the mass reproduction of siliceous organisms, such as radiolarians and sponges, which were eventually deposited to form biosiliceous rocks.