Fe and C isotopes constrain the pathways of hematite and Fe–rich carbonates formation in the Late Neoproterozoic Dahongliutan BIF, NW China

Abstract

Unlike most Neoproterozoic BIFs in which Fe occurs almost exclusively in hematite, the Late Neoproterozoic Dahongliutan BIF uniquely contains Fe oxides (hematite), Fe–rich carbonates (siderite and ankerite) and Ca–Mg carbonates (calcite and dolomite). The diversification of minerals presents a unique opportunity to constrain the biotic and/or abiotic pathways for the precipitation of Fe–bearing minerals involved in Neoproterozoic BIF genesis. Here, we present integrated Fe, Nd and C compositions of hematite, Fe–rich carbonates, bulk–rock various facies BIFs, silicified calcic marbles and schists from the Dahongliutan BIF. These data suggest that the aqueous Fe2+ with light δ56Fe (~−1.60‰) at Dahongliutan was sourced from mixed continentally derived Fe (δ56Fe ~ −2‰) that was mobilized by microbial dissimilatory Fe reduction (DIR) and hydrothermally sourced Fe (δ56Fe ~ 0‰) from MORB systems. Lower than 65% of this dual–source mixed Fe occurred oxidation and formation of Fe(OH)3 via limited atmospheric O2, and/or anaerobic photosynthetic Fe2+ oxidation. The majority of these poorly crystalline Fe(OH)3 precipitates undergo dehydration and recrystallization to eventually bury as hematite with light δ56Fe (averaging −0.15‰). δ56Fe (~−0.11‰) and δ13CV-PDB (−4.00‰ to −6.60‰) of the siderite suggest that conversion of a small amount of Fe(OH)3 into siderite occurred via near–complete reduction by DIR. This DIR process is coupled to the oxidation of organic C, in which the ratio of HCO3− derived from oxidation of CH2O (HCO3−org) to total HCO3− (HCO3−total) = 1/4. Petrographic evidence (the common occurrence of hematite inclusions in the siderite and ankerite), coupled with mineral chemistry, support a model that some siderite transforminto ankerite with consistent δ56Fe (~−0.11‰) and δ13CV-PDB values (−4.20‰ to −5.40‰). In summary, our results demonstrate that DIR play an important role in Neoproterozoic BIFs.