Framboidal pyrite in Dongsheng sandstone-hosted uranium deposit, northern Ordos Basin: Implications for fluid evolution and uranium mineralization

Abstract

Occurrence characteristics, internal morphology patterns, size variations and trace elements contents for framboidal pyrite in the sandstone of middle Jurassic Zhiluo Formation hosting U deposit in the northern Ordos Basin collectively demonstrate three types of framboids: (i) Py1 is the synsedimentary origin, characterized by the close association with clay minerals, equal-sized microcrystals with globular or octahedral shape, the diameter of framboids and microcrystals ranging from 2.1 to 9.3 µm (mean = 5.9 µm) and from 0.1 to 1.4 µm (mean = 0.5 µm), respectively, and relatively low trace element contents (mean = 0.83 wt% of Co, Ni, As, Mo in total); (ii) Py2 is the early diagenetic origin, characterized by equal-sized pyritohedral microcrystals while other substance also occurs within the framboid, or non-uniform size and morphology of microcrystals with the diameter of framboids and microcrystals ranging from 7.3 to 26.3 µm (mean = 11.8 µm) and from 0.3 to 1.9 µm (mean = 0.8 µm), respectively, and similar trace element compositions with Py1; and (iii) Py3 is the U ore-stage origin, characterized by equal-sized cubic microcrystals in the loosely packed pattern with U-bearing minerals filling between microcrystals, the diameter of framboids and microcrystals ranging from 7.6 to 27.4 µm (mean = 15.9 µm) and from 0.7 to 3.2 µm (mean = 1.5 µm), respectively, and the highest trace element contents (mean = 1.07 wt% of Co, Ni, As, Mo and Se in total). During mineralization, Py1 and Py2 are dissolved by the interlayer oxidation fluid, providing S source and trace elements for the precipitation of Py3 with larger diameter. Its strong reducing ability, large specific surface area and abundant internal porosity facilitate the fixation of U in the interstices of Py3 through reduction or adsorption. Negative sulfur isotope compositions suggest biogenic redox processes for U mineralization. The formation and transformation relationship of different stages of framboidal pyrite indicates the complex evolution process of synsedimentary, early diagenetic and U ore-forming fluids.