Abstract: There are two important manganese carbonate horizons in the Middle Ordovician and Early Sinian strata in central Hunan Province.
Manganese marl and manganese dolomite in the Xiangmeng Formation (Early Sinian) in the vicinity of Anhua County have carbon and oxygen isotope values ranging from -2.69 to -4.00‰ and from -6.87 to -14.04‰, respectively. MnCO₃ in the Xiangmeng Formation yields δ¹³C values ranging from -7.4 4 to -13.00‰ and δ¹⁸O values from -2.65 to -13.03‰.
Manganese marl in the black shale of the Modaoxi Formation (Middle Ordovician) shows negative δ¹⁸C values ranging between -1.70 and -6.56‰, and δ¹⁸O values between -11.67 and -14.00‰. Morever, crystalline manganese ores have highly negative δ¹³C values ranging between -4.70 and -17.79‰, and δ¹⁸O values between -11.67 and -17.27‰.
There are three kinds of evolutionary trends for carbon and oxygen isotopes in the manganese carbonate rocks. (1)Stable δ¹³C trend in the manganese limestone of the Early Sinian strata in the Anhua district; (2) sympathetic trend of MnCO₃ in black shale in the Xiangmeng Formation. and (3) strongly antipathetic trend of manganese ores in the Middle Ordovician black shale. Carbon diagenesis has an important effect on carbon and oxygen isotope compositions during the formation of manganese carbonates.
Carbonate source in the Xiangmeng Formation probably comes from the inorganic marine CO₂, and is related to the addition of organic carbon from sulphatereduction (SR) zone to methanogenesis (Me) zone.
The carbonates in crystalline MnCO₂ in the Modaoxi Formation are derived mainly from the decarboxylation (D) zone. δ³⁴S, ranging from -8.00 to -27.49‰, also has an organic source. The manganese contents in the manganese ores show a negative correlation with δ¹³C values.
The evidences here suggest that the manganese carbonate origin is closely linked to the anoxic events in the marine basin and submarine hydrothermal (exhalative) deposits.