Occurrence, source and enrichment mechanism of silver in black shale-hosted Baiguoyuan Ag-V ore deposit, Hubei Province, China

The Baiguoyuan Ag-V ore deposit in Xingshan County, Hubei Province, is hosted in Neoproterozoic black shale, containing approximately 1860 tons Ag at average grade of 116 g/t and 21.8 Mt. V at average grade of 1.5%. The occurrence, source and enrichment mechanism of silver in this deposit are still a topic of debate. In this study, we applied high-resolution electron microprobe analysis (EMPA) and LA-ICP-MS technique to determine the occurrence of silver and to infer its source and enrichment mechanism. Mineralography observation revealed that pyrite is the main metallic mineral in the ores, which shows three-stage formation histories: (1) sedimentation, (2) diagenesis, and (3) deformation. BSE imaging detected tiny naumannite inclusions in pyrite. EMPA showed high contents of Ag in sedimentary pyrite. LA-ICP-MS analysis displayed both solid solution and tiny naumannite inclusion of Ag in pyrite. A mass balance calculation suggested that Ag occurred mainly as solid solution and minor as tiny naumannite inclusions in pyrite. The high contents of Co and Se, and high ratios of Co/Ni and Se/S in sedimentary pyrite indicated that Ag was probably supplied by submarine hydrothermal venting. The correlation between Ag and Fe or Se in sedimentary pyrite indicated that Ag was enriched through Ag and Se replacing Fe and S in pyrite, respectively.