Pyrites in the Plant Fossils from the Ningming Formation, Guangxi, and Their Depositional Significance

Key words: 
• pyrite /
• plant fossil /
• Oligocene /
• depositional microcondition /
• Ningming Formation

Abstract: The study of pyrites preserved in fossils can provide useful information on the process of organic fossilization and the depositional microcondition of these fossils. Using the light and scanning electron microscopes, we observed pyritization tissues preserved in plant fossils from the Ningming Formation, Guangxi. Pyrite textures are preserved in two forms, which are dissolved crystals and framboidal clusters. Most of crystals are of irregular to spherulitic shapes with pits on the surface and indistinct edge, whereas few are of octahedron shapes with smooth surface and distinct edge. The size of crystals ranges from 0.79-1.58 μm. Pyrite framboids are densely packed. They are spheroidal in shape and 7.23-14.95 μm in diameter. The individual crystal units of pyrite framboids have an S/Fe ratio of 0.16-2.06, with the mean value of 1.54. The ratio of S/Fe is less than the standard value 2 and thus shows a relative sulfur deficiency. The sizes of these pyrite framboids are small and vary within a narrow range. Their mean diameters fall generally within the dysoxic field. The size distribution and S/Fe ratio of pyrite framboids indicate that the intervals between the abaxial and adaxial cuticles of plant detritus had a dysoxic water microcondition during the formation of pyrites. And in this microcondition, framboids were formed rapidly in a very restrictive time span. Numerous pyrite crystals and framboids infilling the intervals between the cuticles of plant detritus also support the conclusion that the formation of pyrite framboids is in association with the decay of organic matter.