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).