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-information (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 parts per thousand vs. PDB at Paraburdoo and -10.5 parts per thousand at Wittenoom, but the greater isotopic depletion at Wittenoom appears related to primary to 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 parts per thousand) precipitated from an anoxic water column depleted in C-13; additional depletion of C-13 is associated with coprecipitation of iron oxides and organic carbon. Oxygen-isotopic abundances of microbanded carbonates are similar to those of under- and underlying massive marine carbonates ranging from 17.6 to 21.0 parts per thousand vs. SMOW (-9.6 to -12.9 parts per thousand vs. PDB). Millimeter-scale variations in abundances of C-13 and O-18 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.