Geochemical Characteristics and Constraints on Provenance, Tectonic Setting, and Paleoweathering of Middle Jurassic Zhiluo Formation Sandstones in the Northwest Ordos Basin, North-Central China

Abstract

To further explore the uranium-bearing prospects of the Zhiluo Formation, the petrography, major elements, trace elements and rare earth elements of Zhiluo sandstone samples collected from four boreholes were analyzed in this research to determine the provenance, tectonic setting and paleoweathering of the formation. The results of the analysis reveal that the Zhiluo Formation sandstone comprises primarily feldspar sandstone, with quartz, feldspar, and mica as the main mineral components. The rare earth elements are mainly characterized by enrichment in light rare earth elements and loss of heavy rare earth elements. The ratio of light to heavy rare earth elements (LREEs/HREEs) is 5.55–7.79, with an average of 6.33. The value of (La/Yb)CN is 12.96–22.33, with an average value of 17.41, indicating obvious fractionation of LREEs and HREEs. The chemical index of alteration (CIA) value of this sandstone is 56.30–63.04, with an average of 59.75, which indicates that the parent rock experienced weak chemical weathering in a dry climate. The discrimination diagrams of the source area and parent rock show that the source area of the Zhiluo sandstones had a mixed intermediate-felsic composition, and that the main parent rocks were andesite and granite. The tectonic setting discrimination diagram indicates that the tectonic setting of the source area was the passive margin. Thus, the provenance of the sandstone of the Zhiluo Formation is proposed to be the volcanic-sedimentary rock series developed on the northern margin of the Ordos Basin.