MIOCENE WEATHERING AND THE STABLE CARBON ISOTOPE GEOCHEMISTRY OF GOETHITE IN CID OF WESTERN AUSTRALIA

Miocene chemical weathering produced goethite/hematite-rich ooids that were deposited in meandering Miocene rivers to form the economically important channel iron deposits (CID) of the Hamersley province in Western Australia. Information on the weathering environment that produced the oolitic Fe(III) oxides is archived in the geochemistry of those minerals. In particular, the mineral goethite (FeOOH) contains small amounts of occluded CO₂ which is present as an Fe(CO₃)OH component in solid solution. The mole fraction (X) and δ¹³C values of Fe(CO₃)OH can provide information on carbon cycles in oxidative weathering systems and are measured by incremental isothermal vacuum dehydration of goethite. X and δ¹³C of the Fe(CO₃)OH are related, respectively, to the concentration and δ¹³C of CO₂ in the environment of crystallization. The oolitic goethites of this study are from a core that sampled Miocene CID of the Robe Formation. The site was an iron mine on Mesa J, and core samples were provided by the CSIRO, Western Australia. Among these goethites, plateau δ¹³C values range from -24.2‰ to -22.5‰. Corresponding plateau values of X range from 0.0082 to 0.0334. A plateau mole fraction of 0.0334 is the highest observed to date in natural goethites. Values of X ≥ 0.0082 imply subsurface CO₂ concentrations in the ancient weathering environment that were ≥ 40,000 ppmV. If crystallization of the oolitic goethite occurred in the vadose zone of a paleosol during Fickian-type, steady-state CO₂ transport, the δ¹³C values of the Fe(CO₃)OH suggest that the ancient soil CO₂ had a δ¹³C value of about -31‰ to -29‰. However, if the goethite crystallized in a phreatic environment, the δ¹³C of ambient CO₂ would have been about -27‰ to -25‰. In either case, the negative δ¹³C values and high concentrations inferred for CO₂ in the subsurface of those weathering Miocene landscapes are consistent with high rates of in situ oxidation of organic matter -- implying biological mediation and therefore a biologically active weathering environment. If so, biological processes in those oxidizing Miocene environments might have enhanced the rates of weathering and promoted cycles of mineral dissolution and precipitation that contributed to the relative enrichment of iron and production of the large amounts of ore-grade Fe(III) oxide accumulated in the CID.