6. Conclusion
Ooids in oolitic grainstones of the Nieniexiongla Formation contain significant proportion of hematite which makes ooids reddish. These hematites exhibit mainly two habits: euhedral crystals and aggregated spheroids (framboids). Euhedral crystals are generally found in the calcium nuclei of ooids, and usually range from several to tens of microns. Aggregated spheroids randomly occur in the whole ooids, the diameter of single amorphous spheroid is around or lesser than 1 μm.
EPMA examination revealed that the hematite is pseudomorph after pyrite. According to the analysis of sedimentary and diagenetic environment, pyrite formed after the deposition but before the lithification of the rocks. Under anoxic conditions, the decomposition of the microorganisms within ooids by sulfate-reducing bacteria provides HS-irons. Then HS- irons reacted with Fe2+ brought by the intrusion of pore water to deposit pyrite.
Larger euhedral pyrites in nuclei may be transferred from framboid aggregates through continuous growth of the constituent microcrystals, since the space of nuclei is an open system and large enough. The research of reddish ooids in oolitic grainstones of the Nieniexiongla Formation demonstrates that sulfate-reducing bacteria would utilize organic matter within ooids after sedimentation, resulting changes in the mineral composition of ooids.