Relationships between uranium occurrence, pyrite and carbonaceous debris in Fuxin Formation in the Songliao Basin: Evidenced by mineralogy and sulfur isotopes

Abstract

Uranium minerals, pyrite and carbonaceous debris in uranium reservoir of Fuxin Formation in Tiefa Depression in the southeastern edge of Sonliao Basin were carefully observed and analyzed by using techniques such as polarizing microscope, scanning electron microscope, electron probe and laser ablation multi-collector inductively coupled plasma mass spectrometer (LA-MC-ICPMS). The study found that the main type of uranium minerals are pitchblende in granular, particulate aggregate, radial and colloidal shapes, closely coexisting with pyrite and carbonaceous debris; The δ34S range of framboidal pyrite and colloidal pyrite are −35.8‰∼−29.7‰ and −29.2‰∼14.9‰ respectively and the fluid environment of multiple sulfur sources combined with biological and abiotic effects resulted in a broad sulfur isotope composition of pyrite; The lower δ34S value of framboidal pyrite, the higher P content of uranium minerals and the close relationship between these two parameters indicated there was the presence of microbial regulation in the process of uranium mineralization, among which radial pitchblende is the result of uranium minerals metasomatizing to siderite of biogenic origin; Uranium minerals around colloidal pyrite with corrosion phenomenon are more enriched, which indicates that the activated Fe2+ and HS- may promote the reduction and precipitation of U6+; Carbonaceous debris in uranium reservoir promotes the continuous enrichment of uranium through adsorption or complexation, and meanwhile, provides the environment and energy for the development of pyrite and microorganisms. Conversly, pyrite and microorganisms impel the reduction and precipitation of the enriched uranium. In this way, the synergy of the above three processes has created the diversity of occurrence state of uranium minerals of Fuxin Formation in Tiefa Depression.

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