Mixed Biogenic and Hydrothermal Quartz in Permian Lacustrine Shale of Santanghu Basin, NW China: Implications for Penecontemporaneous Transformation of Silica Minerals
The cycling of various isomorphs of authigenic silica minerals is a complex and long-term process. A special type of composite quartz (Qc) grains in tuffaceous shale of Permian Lucaogou Formation in the sediment-starved volcanically and hydrothermally active intracontinental lacustrine Santanghu rift basin (NW China) is studied in detail to demonstrate such processes. Samples from one well in the central basin were subject to petrographic, elemental chemical, and fluid inclusion analyses. About 200 Qc-bearing laminae are 0.1-2 mm and mainly 1 mm thick and intercalated within tuffaceous shale laminae. The Qc grains occur as framework grains and are dispersed in igneous feldspar-dominated matrix, suggesting episodic accumulation. The Qc grains are bedding-parallel, uniform in size (100 s µm), elongate, and radial in crystal pattern, suggesting a biogenic origin. Qc grains are composed of a core of anhedral microcrystalline quartz and an outer part of subhedral mega-quartz grains, whose edges are composed of small euhedral quartz crystals, indicating multiple episodic processes of recrystallization and overgrowth. Abundance of Al and Ti in quartz crystals and estimated temperature from fluid inclusions in Qc grains indicate that processes are related to hydrothermal fluids. Finally, the Qc grains are interpreted as original silica precipitation in microorganism (algae?) cysts, which were reworked by bottom currents and altered by hydrothermal fluids to recrystalize and overgrow during penecontemporaneous shallow burial. It is postulated that episodic volcanic and hydrothermal activities had changed lake water chemistry, temperature, and nutrient supply, resulting in variations in microorganic productivities and silica cycling. The transformation of authigenic silica from amorphous to well crystallized had occurred in a short time span during shallow burial.
X. Jiao et al., "Mixed Biogenic and Hydrothermal Quartz in Permian Lacustrine Shale of Santanghu Basin, NW China: Implications for Penecontemporaneous Transformation of Silica Minerals," International Journal of Earth Sciences, vol. 107, no. 6, pp. 1989-2009, Springer Verlag, Sep 2018.
The definitive version is available at https://doi.org/10.1007/s00531-018-1583-0
Geosciences and Geological and Petroleum Engineering
Keywords and Phrases
Biogenic; Composite quartz; Diagenesis; Hydrothermal; Penecontemporaneous
International Standard Serial Number (ISSN)
Article - Journal
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01 Sep 2018