Mixed volcanic and hydrothermal origin of siliceous rocks in a volcanic lake in the Santanghu Basin, NW China

The siliceous rock is formed by authigenic quartz and records crucial geological information on the basin’s paleogeography, paleoclimate and paleotectonics. The siliceous rocks in the Lucaogou Formation are widely developed in tight oil reservoirs composed of fine-grained sedimentary rocks and occur as lamina, lenses and bands. However, there is little research on the siliceous rocks in this area, and the understanding of their formation mechanism and processes is still lacking. Two types of siliceous rocks were selected for petrological, mineralogical, <i>in situ</i> major, and trace, and silicon isotopic analysis to explore their source and origin. The siliceous lenses consist of anhedral microcrystalline quartz distributed in tuffaceous shales with a soft-deformation sedimentary structure, indicating a syn-depositional origin. The siliceous phenocrysts, which are composed of quartz, dolomite and smectite, are sand-sized, angular and irregular-shaped occurring in tuffaceous dolostones with a porphyritic texture and normal graded bedding suggesting an origin of the subaqueous explosion as pyroclastic flow. The siliceous lenses show positive Eu anomalies (Eu/Eu* = 1.29–1.65) and a wide range in δ³⁰Si values (–0.81 to −0.06‰ deviation from the NBS28 standard), indicating chemical precipitation of hydrothermal fluids. At the same time, the siliceous phenocrysts have a narrow spread in δ³⁰Si values (0.30–0.69‰) with weak positive Eu anomalies (Eu/Eu* = 0.84–1.42) and weak negative Ce anomalies (Ce/Ce* = 0.87–1.02), which are interpreted as hydrothermal metasomatism. A high-pH alkaline lake environment with high temperatures and pressures brought by mantle-derived magmatic fluids are the controlling factors for the formation of siliceous rocks. The genetic model of siliceous rocks in the Lucaogou Formation provides an alternative interpretation of the origin of siliceous rocks in the study area and lays a good foundation for reconstructing the basin’s paleoenvironment. The siliceous rocks in the Lucaogou Formation have a mixed volcanic and hydrothermal origin. The siliceous rocks are formed by a high-pH alkaline lake environment and local high temperatures and pressures from mantle-derived magmatic fluids. The depositional mechanism of the siliceous rocks provides crucial information for reconstructing the paleoenvironment of the whole basin.