The Depositional Mechanism of Hydrothermal Chert Nodules in a Lacustrine Environment: A Case Study in the Middle Permian Lucaogou Formation, Junggar Basin, Northwest China

Abstract

Although chert deposits are limited in geological distribution, their geological and geochemical characteristics can provide important information to reconstruct paleoenvironmental and diagenetic processes. For the Permian period, cherts are utilized to trace global silicon cycles and hydrothermal activities in relation to the Permian Chert Event. In Northwest China, Permian chert nodules have recently been discovered in both the southeastern and northwestern margins of the Junggar Basin. We conducted an analysis of the mineralogy, petrology and geochemistry of chert nodules of the Lucaogou Formation in the southeastern margin of the Junggar Basin to identify silicon sources and determine the precipitation mechanism of chert nodules. As evidenced by petrology, the chert nodules were mainly composed of crypto-microcrystalline silica (94.33% on average), with development of a soft-sediment deformation structure, indicating the synsedimentary deposition of silicon. Proven by trace elements, high Eu/Eu* ratios (average 2.14), low total rare earth element content (average 6.03 ppm), low LaN/YbN ratios (average 0.17) and low Y/Ho ratios (average 25.25) in chert nodules supports the hydrothermal source of silicon. The wide distribution of authigenic metal-bearing minerals and the significant positive Eu anomalies observed suggest that the chert depositions in the Lucaogou Formation intermittently received high-temperature (>250 °C) hydrothermal fluids, likely associated with the initiation of the Bogda Rift in the middle Permian. Following rapid cooling down and differential compaction, siliceous sediments dehydrated and deformed, finally forming chert nodules.