Ultrastructural and In-Situ Sulfur Isotope Analyses indicate an dual genesis of Pyrite from the early Ediacaran Doushantuo Formation of South China

Framboidal pyrites within the chert nodules (Type I) range from 2.7 to 21.8 μm in diameter, with tightly packed microcrystals (188—1476 nm). Pyrite framboids within the surrounding matrix (Type II) exhibit comparable sizes (2.7—20.0 μm) and crystal dimensions (193—1790 nm).<i>In-situ</i> SIMS sulfur isotope analyses (δ³⁴S_py) reveal distinct signatures for the different pyrite types. Type I framboidal pyrites within chert nodules show heterogeneous but consistently negative δ³⁴S_py values (-28.8‰—-3.8‰; mean: -18.9‰, n=16). Similar isotopic patterns are found in matrix-hosted framboidal pyrite (Type II), ranging from -28.9‰ to -9.2‰ (mean: -19.9‰, n=17). Euhedral pyrites (Type III) display consistently high and homogenous positive δ³⁴S_py values (+24.7‰—+27.4‰, n=27), without obvious inter-crystal variability. Dendritic pyrite aggregates (Type IV) within the surrounding matrix also exhibit significantly positive δ³⁴S_py values (+17.1‰—+33.1‰; mean: +28.1‰, n=11), showing homogeneous isotopic compositions within individual aggregates but variability among aggregates.