Iron-rich rocks of Orosirian Period in the Chilpi Group on the northern margin of the Bastar Craton, Central India, contain an association of hematite-magnetite-greenalite-chamosite-quartz in oxide-silicate facies. Additionally chert (quartz) and siderite occur in chert and carbonate facies. Presence of these mineral assemblages was investigated to infer the redox state of the depositional basin. The results have indicated formation temperature variation of 116–255 °C (average 198 °C) and log P_(O2) between – 37 and – 60 (average –44). A ferruginous state of the shallow water depositional environment, having oxygen content of 10^–2 to 10^–5 times the present atmospheric level, is inferred. The variations in composition of greenalite-chamosite association indicate development of the mineral phases from the reaction involving kaolinite-illite and magnetite-siderite as end-members. Thermodynamic requirements for the formation of the rare association of magnetite-greenalite-cronstedtite indicate the precipitation of the mineral phases from seawater with enhanced activities of Fe²⁺, Al, Si, Mg and C compared to the level in the present day seawater. The results indicate a steep fall in the atmospheric oxygen content immediately after the Great Oxidation Event of 2400–2000 Ma.

印度中部巴斯塔克拉通北缘 Chilpi 群的奥罗西纪富铁岩石含有氧化硅酸盐相中的赤铁矿-磁铁矿-绿钠石-铜锰矿-石英组合。此外，燧石（石英）和菱铁矿出现在燧石和碳酸盐相中。研究这些矿物组合的存在以推断沉积盆地的氧化还原状态。结果表明，地层温度变化为 116–255 °C（平均 198 °C），log P _(O2)在 – 37 和 – 60 之间（平均 – 44）。据推测，浅水沉积环境为铁质状态，氧含量为当前大气水平的10 ^–2至10 ^–5倍。绿钠石-菱铁矿组合的组成变化表明，矿物相是由高岭石-伊利石和磁铁矿-菱铁矿作为端元的反应形成的。形成稀有的磁铁矿-绿钠石-锰锰矿缔合体的热力学要求表明，与当今海水中的水平相比，海水中矿物相的沉淀具有增强的 Fe ²⁺ 、Al、Si、Mg 和 C 的活性。结果表明，2400-2000 Ma 大氧化事件之后，大气中的氧含量立即急剧下降。