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.