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.