5.3. The formation of pyrite
The formation of pyrite must be under anoxic environments (Berner, 1984;
Wilkin & Barnes, 1997; Suits & Wilkin, 1998; Kraal, Burton, & Bush,
2013). However, the Nieniexiongla oolitic grainstones were deposited in
mid-ramp environment with periodical influences with storm wave (Han,
Hu, Kemp, & Li, 2018). Additionally, a large amount of broken ooids and
the sparry calcite cement would recommend highly agitated sea water.
Therefore, it can concluded that the water column was well ventilated
and thus hardly to be anoxic to facilitate the formation of pyrites.
On the other hand, pyrites are thought to have close connection with
organic matter (Berner, De Leeuw, Spiro, Murchison, & Eglinton, 1985;
Raiswell & Berner, 1986; Rickard, 2012; Wei, Chen, Wang, Yu, & Tucker,
2012). Especially iron-framboids need organic-rich systems suggesting
that organics would make a substantial contribution in their formation
(Wignall, Newton, & Brookfield, 2005; Cavalazzi et al., 2012). As
organic matter decomposition continues after sediment deposition through
sulphate-reducing bacteria, it leads to reduction of dissolved
interstitial sulphate to H2S, which compounds with
active iron to form pyrites. Meanwhile, intermediate products,
H2S and iron sulphides, also need reduced environment,
or they would be oxidatively destroyed (Berner, De Leeuw, Spiro,
Murchison, & Eglinton, 1985).
In the case of Menqu, iron minerals are barely found in cements and muds
filling in the skeletal chambers, which indicate that the pyrites did
not form in the water column. Additionally, the iron minerals are not
concentrated around the cracks filled with sparite calcites, which might
form during the compaction of the sediments. We suggest that neither the
exotic fluids intrusion nor internal remobilized material after
lithification of the oolitic grainstones are responsible for the
formation of pyrites. Therefore, the timing of the pyrite formation was
after the deposition but before the lithification of the rocks. Pore
water under reduced condition could provide active iron and
HS- (under euxinic condition) to form pyrites.
However, the pyrites in Nieniexiongla grainstones are not concentrated
on the surface of the ooids but randomly distributed in the cortex and
nuclei of the ooids, which indicates the materials remobilized from the
ooids played more important roles.
Three principal factors that limit the amount of pyrites are organic
matter, active iron and dissolved sulphate (Berner, De Leeuw, Spiro,
Murchison, & Eglinton, 1985). Organic carbon comes from a variety
source of autochthonous or allochthonous. Allochthonous organic carbon
is terrestrial origin or detrital form, such as plant debris and animal
fragments. Autochthonous organic carbon originates mainly from primary
production of the pelagic organisms (Zaborska et al., 2016). Very few
fossils of marine microfossils or other kinds of organic matters were
observed in the Nieniexiongla granistones. However, more studies on
ancient and modern cases have revealed that the microorganisms are
involved to form ooids (Pacton et al., 2012; Summons et al., 2013;
Barale, d’Atri, & Martire, 2013; Li, Yan, Algeo, & Wu, 2013; Li et
al., 2017). Therefore, we suggest that the decomposition of the
microorganisms within the ooids by bacterial sulfate-reducing (BSR)
provides the HS-. Furthermore, the oxygenation of
organic matters would produce spaces for the intrusion of pore water
with sufficient Fe2+ to facilitate the pyrite
deposition.