Geochemical characteristics of the Xuanwei Formation in West Guizhou: Significance of sedimentary environment and mineralization

Abstract

The Upper Permian Xuanwei Formation widely occurs in western Guizhou, unconformably overlying the Emeishan basalts, and mainly consists of black shales. It is ∼170 m thick at Cuyudong Village, Weining County, West Guizhou, China, where the samples of black shale and sandy shale were collected and analyzed. The shales mainly contain SiO₂, 18.9%–44.1%, Al₂O₃, 14.8%–52.8%, Fe₂O₃, 1.0%–41.2%, LOI, 3.2%–21.1%, TiO₂, 1.0%–6.7%, and MgO, 0.2%–2.5%. The contents of all other major elements are lower than 1.0%. It is shown that the black shales have higher contents of Fe₂O₃ and LOI than normal shales. The siderites occurred in the black shales with higher contents of Fe₂O₃, which may be attributed to hydrothermal activities on seafloor. All analyzed shale samples have extremely high Ga, 47.8×10⁻⁶–109.9×10⁻⁶ (70.5×10⁻⁶ on average), higher than the industrial mining standard of Ga Resource Industry Standard. The total contents of rare-earth elements (REE) of 9 black shale samples vary from 213×10⁻⁶ to 1460×10⁻⁶, suggesting that these black shales are enriched in REE. The shale-normalized REE patterns display both positive and negative Ce anomalies (Ce/Ce* from 0.5 to 1.7), revealing that the Xuanwei shales were precipitated under oxic and anoxic conditions. The Rb-Sr chronological diagram of 6 shale samples in the Xuanwei Formation shows an age of 255±12 Ma. Strontium isotopic ratios (⁸⁷Sr/⁸⁶Sr)_t0 range from 0.70635 to 0.70711, suggesting that these Xuanwei black shales might be derived from chemical weathering of the Emeishan basalts.