The post-deposition modification of ooids by sulfate-reducing bacteria:
Evidence from the Lower Jurassic in the Tethys Himalayas of southern
Tibet
Abstract
Whether ooids are formed through biotic or abiotic progress seems to
become increasingly controversial in recent decades. New studies
indicate that the microbial effect could make important contributions to
the construction, destruction and modification of ooids. Previous
studies mainly focused on the organomineralization of carbonate minerals
in ooids. This paper investigates the formation process of iron minerals
in red oolitic grainstones of the Lower Jurassic Nieniexiongla Formation
in the Tethyan Himalaya of southern Tibet. Petrographic studies and
electron probe microanalysis revealed that the colorful minerals present
in red ooids are hematites. Sulfur was detected in the hematites, which
indicates that they were transformed from precursor pyrites. These
hematites retain euhedral or framboid shapes of the pyrites. Hematite
pseudomorphs distribute randomly in ooids and could not form a whole
concentric layer of the ooids. Euhedral crystals usually have larger
diameters than amorphous spheres and are scattered mostly in nuclei.
Based on the distribution and morphology of hematites, we speculate that
precursor pyrites were formed through metabolism of sulfate-reducing
bacteria. Moreover, the timing of the pyrite formation was after the
deposition but before the lithification. The decomposition of the
microorganisms within the ooids by bacterial sulfate-reducing provides
the HS-, the degradation of organic matters would produce spaces for the
intrusion of pore water with sufficient Fe2+ to facilitate the pyrite
deposition. In nuclei, the framboid aggregates of pyrites were
transferred to euhedral crystals through continuous growth of the
constituent microcrystals. Our study demonstrated organomineralization
of sulfate-reducing bacteria mediated pyrites in carbonate ooids.