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.

硅质岩 (siliceous rock) 由自生石英形成，记录了盆地古地理、古气候与古构造的关键地质信息。芦草沟组 (Lucaogou Formation) 中的硅质岩广泛发育于细粒沉积岩构成的致密油储层中，以纹层、透镜体与条带的形式产出。但目前针对该区域硅质岩的研究较少，对其形成机制与过程的认知仍存在不足。研究选取两类硅质岩开展岩石学、矿物学、原位 (in situ) 主量与微量元素以及硅同位素分析，以探究其物质来源与成因。其中，硅质透镜体由分布于具软沉积变形构造的凝灰质页岩中的他形微晶石英组成，指示其为同沉积成因。由石英、白云石与蒙脱石构成的硅质斑晶呈砂级棱角状、不规则形态，赋存于具有斑状结构与正粒序层理的凝灰质白云岩中，表明其成因与作为火山碎屑流的水下喷发过程相关。硅质透镜体表现为正铕异常 (Eu/Eu*=1.29~1.65)，且δ³⁰Si值分布范围较广（相对于NBS28标准品，偏差为-0.81‰~-0.06‰），指示其为热液流体的化学沉积产物。同时，硅质斑晶的δ³⁰Si值分布范围较窄（0.30‰~0.69‰），呈现弱正铕异常 (Eu/Eu*=0.84~1.42) 与弱负铈异常 (Ce/Ce*=0.87~1.02)，被解释为热液交代作用的产物。幔源岩浆流体带来的高温高压与高pH碱性湖泊环境，是控制硅质岩形成的关键因素。芦草沟组硅质岩的成因模式为研究区硅质岩的起源提供了新的解释，也为重建盆地古环境奠定了良好基础。芦草沟组中的硅质岩兼具火山与热液混合成因，其形成受控于高pH碱性湖泊环境与幔源岩浆流体带来的局部高温高压条件。硅质岩的沉积机制可为重建整个盆地的古环境提供关键信息。