Geochemistry and sedimentary environments of the Palaeozoic siliceous rocks in western Hubei

The thin-bedded siliceous rocks are developed in the Upper Ordovician Wufeng Formation, the lower part of the Lower Silurian Longmaxi Formation, Lower Permian Gufeng Formation and Upper Permian Dalong Formation in western Hubei. The nodular and/or stratoid cherts are observed in the limestones of the Permian Qixia, Maokou and Wujiaping Formations. The geochemical signatures based on the Fe-Al-Mn triangular diagram, U-Th discriminant diagram, Cu, Ni, Co, Cr and Zr correlation diagram, the elements Ba, As and Sb and REE distribution patterns all suggest that the bedded siliceous rocks in the Wufeng and Longmaxi Formations may be assigned to the biochemical and chemical sediments deposited from normal sea water. Although the bedded siliceous rocks in the Gufeng and Dalong Formations and chert nodulars in the Maokou Formation are not characteristic of hydrothermal sediments, they are affected by the hydrothermal deposition. This case is especially noticeable for the siliceous rocks in the Gufeng Formation. The sedimentary environments of the siliceous rocks in the Wufeng and Longmaxi Formations are represented by relatively stagnant and deep-water basins surrounded by ancient continents, island chains and carbonate platforms. The siliceous rocks in the Gufeng and Dalong Formations, western Hubei were deposited in the reduced environments of the basins and/or platform trenches within carbonate platforms created by faulting and rifting. The ratios of CaO/(Fe+CaO) and (MgO/Al₂O₃)×100 show that the sea water during the deposition of the siliceous rocks in the Wufeng, Longmaxi and Dalong Formations might be slightly freshened. The bioclastic limestones with nodular or stratoid cherts in the Qixia, Maokou and Wujiaping Formations lay down in the reduced environments caused by rapid transgression and upwelling currents.