Constraining role of organic matter in P-U mineralization: A case study of the Bahuang uranium-rich phosphorus deposit in eastern Guizhou, China

Abstract

Phosphorus and uranium are both critical strategic mineral resources, and the potential for developing uranium-rich phosphorus ore deposits is considerable. However, research in this area remains relatively underexplored. The Lower Cambrian black rock series in Bahuang, eastern Guizhou, China, stands out due to its high concentrations of phosphorus, uranium, and organic matter, making it a representative phosphorus-rich deposit. This study investigates the role of microbial organic matter in the mineralization processes of phosphorus and uranium within uranium-rich phosphorite deposits, using the Bahuang deposit as a case study. A comprehensive geological survey was conducted, integrating petrographic analysis, major and trace element testing, rare earth element analysis, total organic carbon (TOC) content analysis, and gas chromatography-mass spectrometry (GC–MS) to systematically examine the geological characteristics, elemental geochemical properties, and biomarker compounds in the organic matter of the ore-bearing rock series. The results indicate that the primary ore minerals in the Bahuang uranium-rich phosphorite deposit are collophane, pitchblende, and uraninite. Pitchblende is absorbed by organic matter, while uraninite is predominantly found along quartz edges and fractures, suggesting a hydrothermal origin. Organic matter is mainly distributed in zones where collophane is concentrated, closely aligning with the spatial distribution of phosphorus and uranium. This organic matter primarily derives from low-grade eukaryotic aquatic organisms, such as algae, and is at a mature to highly mature stage, exhibiting significant degradation. As a result, the TOC content of the ore is reduced, and its correlation with phosphorus and uranium content is weak. The enrichment of phosphorus and uranium is not solely influenced by organic matter content but also by factors such as depositional environment, hydrothermal activity, and the maturity of the organic matter. The ore-bearing rock series formed under sub-reducing to reducing conditions, with significant influence from hydrothermal processes. The ore-forming materials primarily originated from the extraction of continental rift basalt (alkaline basalt and tholeiitic basalt) by hydrothermal fluids, followed by contributions from normal seawater. The phosphorus and uranium mineralization process unfolded in three stages: submarine exhalative activity, which served as the source of phosphorus and uranium; a rise in sea level, which facilitated the upwelling of these elements; and environmental hypoxia, which enabled the precipitation of phosphorus and uranium. During these stages, organic matter influenced the migration and precipitation of phosphorus and uranium in several ways: phosphorus was absorbed by low-grade marine organisms, such as algae, and migrated as phosphate colloids, which precipitated upon reduction by sulfur bacteria; UO₂²⁺ was adsorbed and transported by organic acid colloids formed during organic matter degradation or formed uranyl complexes through ion exchange with organic colloids; and uranium precipitated through reactions with reducing gases produced during the degradation of organic matter.