5.2. Pseudomorphic transformation from pyrites to hematites
The red color and chemical composition of the ferruginous minerals indicate that they are iron oxides, i.e. hematites (Franke & Paul, 1980; Préat, Mamet, De Ridder, Boulvain, & Gillan, 2000; Mamet & Préat, 2006; Hu, 2013). The significantly viable sizes as well as the euhedral and spheroidal forms of the hematites rule out that the minerals had been transported into the basin to be combined by carbonates to form ooids. Absence of hematite in the matrix of the Nieniexiongla oolitic grainstones further demonstrates that they were not authigenetically formed in the water masses but grew particulate within the ooids.
Sulfur detected from the hematites could indicate that they were transformed from precursor pyrites (Lu et al., 2005; Soliman & El Goresy, 2012). The preservation of framboidal morphology and bridge-like ferruginous minerals at or near spheroid-to-spheroid contacts also indicates complete pseudomorphic transformation from pyrite to hematite (Lougheed & Mancuso, 1973; Mader, 2006; Huang et al., 2019). Framboidal pyrites associated with euhedral pyrite are normally regarded as authigenic pyrites (e.g. Lougheed & Mancuso, 1973; Wilkin & Barnes, 1997; Merinero, Lunar, Somoza, Díaz-del-Río, & Martínez-Frías, 2009; Soliman & El Goresy, 2012; Wang, Huang, Wang, Feng, & Huang, 2013). In many cases, framboidal pyrite aggregates have been observed to completely convert to hematite pseudomorphs (Mader, 2006) or other iron mineral pseudomorphs (Soliman & El Goresy, 2012). Ferruginous minerals at or near spheroid-to-spheroid contacts might be the result of leaching of interstitial hematite, the leaching may exhume the framboidal fabric (Mader, 2006).
The residual sulfur should attribute to leaching and chemical weathering of pyrite which are very common and have been well documented before (Lu et al., 2005; Soliman & El Goresy, 2012). Most of these researches reported that sulfur in pyrite is transferred to sulfuric and sulfate during oxidation and dissolved in pore water (Weber et al., 2004). The low proportion of residual sulfur in our study suggests high conversion ratio of hematite to pyrite, which means intensely leaching and/or chemical weathering. In the last phase of weathering, only a minor part of sulfur remains as sulfate mineral, but the total mass of iron would not be changed considering iron is rather immobile during weathering (Lu et al., 2005).