A profound meteoric water influence on genesis in the Permian Waterberg Coalfield, South Africa; evidence from stable isotopes

The coal-bearing sediments of the Grootegeluk Formation of the Waterberg Coalfield contain siderite, ankerite, and calcite of various forms that precipitated during deposition and diagenesis, and have highly variable delta (super 18) O and delta (super 13) C values. Spherulitic siderite, which is present predominantly as early diagenetic nodules (1-5 mm in diameter) in coal and carbonaceous mudrock in the lower part of the Grootegeluk Formation, has relatively high delta (super 13) C values up to +8 per thousand (PDB) and delta (super 18) O values between +9 per thousand and +15 per thousand (V-SMOW). Granular siderite (< 0.1 mm in diameter) is present in organic-poor mudrock of the upper Grootegeluk and the base of the overlying Beaufort Formation; it has low delta (super 13) C values (-l9 per thousand to -14 per thousand ) and delta (super 18) O values between +12 per thousand and +22 per thousand . Ankerite is present only in minor amounts (< 1%) and has isotopic values similar to the coexisting siderite. Calcite lenses conformable with coal and mudrock beds have delta (super 18) O values between +16 per thousand and +20 per thousand , and delta (super 13) C values nearly equal -12 per thousand , The oxygen and carbon isotopic values of cleat-filling calcite, which is present only in coal seams, has delta (super 18) O values between +12 per thousand and +22 per thousand and delta (super 13) C values between -15 per thousand , and -7 per thousand . Fine-grained calcite, disseminated in carbonaceous mudrock and as inclusions in kaolinite, is present only at the base of the Grootegeluk Formation and has delta (super 13) C values between -7 per thousand and +3 per thousand and delta (super 18) O values nearly equal +15 per thousand . The calcite lenses are interpreted to be synsedimentary and to have precipitated from depositional waters with delta (super 18) O value of -13 per thousand . Spherulitic siderite and disseminated calcite precipitated during early diagenesis dominated by anaerobic microbial decomposition of organic matter. Granular siderite formed at higher temperatures (< 100 degrees C) and later than the spherulitic siderite. The uniformly negative delta (super 13) C values (-12 per thousand ) of the cleat-filling calcite indicate a predominantly organic source for the carbon, but the erratic and wide range of the delta (super 18) O values do not allow distinction between precipitation daring burial or later during uplift of the sediments. Stable-isotope data of the carbonates are consistent with precipitation from meteoric water, but estimation of temperatures of formation are not reliable because burial processes result in highly delta (super 18) O-depleted pore water.