Evolution and origins of pyrite in sandstone-type uranium deposits, northern Ordos Basin, north-central China, based on micromorphological and compositional analysis

Abstract Pyrite is an abundant mineral throughout the uranium-bearing Middle Jurassic Zhiluo Formation, northern Ordos Basin. Three different morphologies of pyrite are identified: framboidal, euhedral and cement. Based on textural and geochemical results from optical microscopy, secondary electron (SE) and back scattered electron (BSE) imaging, energy-dispersive spectroscopy (EDS), electron probe microanalysis (EPMA), and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS), three evolutionary models are proposed as follows: aggregation of framboids to form polyframboids as a result of the bacterial sulfate reduction (BSR) processes, the formation of cement pyrite under the addition of thermal fluids with numerous framboids and microcrystals inside, and euhedral pyrite evolving from framboids. The analytical data show that there is a minor increase in major and trace element contents in transformed pyrite, and each pyrite morphology has a distinct sulfur isotopic composition (-9.9‰ to -8.0‰ for framboids, -19.1‰ to -14.2‰ for polyframboids, +18.7‰ to +20.3‰ for euhedral pyrite, and +15.7‰ to +21.2‰ for cement pyrite). This indicates that the sulfur was derived from more than one source, compared with the original framboids. The association between uranium and different types of pyrite indicates that different processes (e.g., BSR and thermogenic sulfate reduction (TSR)) were involved in the precipitation of uranium mineralization and are important to the formation of the precipitation of uranium in the Zhiluo Formation, northern Ordos Basin.