Eogenetic siderites in lacustrine oil shales from Queensland, Australia, a stable isotope study

Tertiary lacustrine oil shales from the Lowmead and Duaringa Basins in Queensland, Australia, contain siderite as a ubiquitous minor phase and in ironstone bands within the sequence. There are two distinctive types of siderite in the ironstone bands: sphaerosiderites and finely crystalline siderites. Petrological evidence indicates that the sphaerosiderites in the ironstone bands formed eogenetically by growing displacively within the soft sediment. The geological setting of the basin and the sedimentological evidence indicate a lacustrine environment. The deposition of siderite is therefore considered to be controlled by processes in the lacustrine water body and sediments. The maximal burial depth is estimated to be 1400 m and consequently the oil shales have not undergone any advanced thermal alteration. Stable oxygen and carbon isotope analysis of the siderites show a wide compositional range: δ¹⁸O = -12.8 to - 2.4% vs. PDBG and δ¹³C = -5.5 to + 12.9% vs. PDB for the Lowmead Basin: δ¹⁸O = -9.6 to 1.2% vs. PDB and δ¹³C = -18.6 to + 16.4% vs. PDB for the Duaringa Basin. The oxygen isotope compositions indicate that the siderite formed in a to + 16.4% vs. PDB for the Duaringa Basin. The oxygen isotope compositions indicate that the siderite formed in a freshwater environment but generally show a range more negative than the expected equilibrium composition with coeval meteoric waters. The carbon isotope composition may reflect deposition within the bacterial oxidation and fermentation zones. The shallow-lake setting with high productivity results in siderite precipitation under high pH either at the water-sediment interface or within the sediment. The δ¹³C of the dissolved carbonate will be elevated by intense photosynthesis. Under these conditions of CO₂-depletion/high-pH isotopic disequilibrium during siderite formation will result in δ¹³C and δ¹⁸C lower than the equilibrium composition, in agreement with the recorded isotopic data.