Primary and diagenetic controls of isotopic compositions of iron-formation carbonates

Mineralogic, chemical, and isotopic compositions have been determined for 97 carbonate microbands in five core segments from the early Proterozoic (2.5 Ga) Dales Gorge Member of the Brockman Iron Formation, Hamersley Basin, Western Australia. Samples were obtained both from banded iron-formation (BIF) macrobands 9-12 at Paraburdoo, on the southern margin of the basin, and from BIF macroband 13 at Wittenoom, 130 km NW. At Paraburdoo, oxygen-isotopic compositions of coexisting chert and magnetite microbands were measured and indicated final equilibration temperatures ranging from 60-160 degrees C. This range is consistent with observed mineral assemblages and indicates a considerable temperature gradient across the basin (cf. T approximately 300 degrees C at Wittenoom; BECKER and CLAYTON, 1976). Carbon-isotopic compositions of carbonates are near -7% vs. PDB at Paraburdoo and -10.5% at Wittenoom, but the greater isotopic depletion at Wittenoom appears related to primary or diagenetic processes, not metamorphism. Contents of organic carbon are consistently low. Isotopic depletion is roughly correlated with iron abundance and, together with petrographic observations and chemical balances, is consistent with the model of BIF deposition introduced by BEUKES et al. (1990): primary siderite (delta approximately -5%) precipitated from an anoxic water column depleted in 13C; additional depletion of 13C is associated with coprecipitation of iron oxides and organic carbon. Oxygen-isotopic abundances of microbanded carbonates are similar to those under- and overlying massive marine carbonates, ranging from 17.6 to 21.0% vs. SMOW (-9.6 to -12.9% vs. PDB). Millimeter-scale variations in abundances of 13C and 18O are associated with diagenetic replacement of primary siderite by secondary ankerite and/or magnetite. It is shown that these isotopic variations cannot result from mineral-dependent fractionations, metamorphism, or the influence of large volumes of water in an open system.