Enrichment mechanism of Se in black shale of the Gufeng Formation in Enshi Prefecture, South China

• Gufeng Formation black shale was deposited in anoxic-eunxic marine environment. • Se enrichment was driven by organic matter adsorption and redox conditions. • Selenium primarily sourced from ambient seawater. • Selenium is predominantly hosted in pyrite through isomorphism. Selenium (Se) is commonly enriched in black shale, making them potential targets for selenium resource exploitation. However, the enrichment mechanism of Se in black shale remains elusive. The Permian Gufeng Formation black shale in Enshi Prefecture, South China, is notably rich in Se, hosting several independent Se deposits. This study employs mineralogical, major and trace element geochemical, electron microprobe, micro-X-ray fluorescence mapping, and sulfur isotope analyses to investigate the Se-rich black shale of the Gufeng Formation from the Shuanghe and Xintang-Hongtu regions in Enshi Prefecture. The Gufeng Formation black shale is primarily composed of quartz, clay minerals, and organic matter. It displays high SiO 2 contents ranging from 41.07 to 84.24 wt% and total organic carbon levels between 10.78 and 48.54 wt%. The black shale exhibits relative enrichment in elements such as V, Cr, Ni, Cu, Se, Mo, and Sb, while being relatively depleted in Mn, Co, Zn, Sn, Ba, and Pb. A distinct negative Ce anomaly is observed, with Ce/Ce* ratios varying from 0.42 to 0.88. The middle lithological unit of the Gufeng Formation contains the highest Se content, accompanied by associated elements like V, Mo, and Ni. Selenium is primarily hosted in fine-grained pyrite through isomorphous substitution, with Se concentrations in Se-rich pyrite ranging from 0.52 to 6.06 wt%, and δ 34 S V-CDT values spanning between −38.9 ‰ and −14.5 ‰. The Gufeng Formation black shale was deposited in an anoxic to eunxic environment along a passive continental margin, with the middle unit exhibiting more reducing conditions compared to the upper and lower units. Selenium in the black shale is predominantly derived from seawater. The absorption and reduction capabilities of biological organic matter play a crucial role in the Se enrichment process, effectively catalyzing the conversion of higher-valent Se species (Se 4+ and Se 6+ ) into the lower valence state (Se 2+ ). This reduced Se then combines with Fe 2+ to form Se-rich pyrite, which precipitates and concentrates around organic matter.