Abstract

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 18 O and delta 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 13 C values up to +8 per thousand (PDB) and delta 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 13 C values (-l9 per thousand to -14 per thousand ) and delta 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 18 O values between +16 per thousand and +20 per thousand , and delta 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 18 O values between +12 per thousand and +22 per thousand and delta 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 13 C values between -7 per thousand and +3 per thousand and delta 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 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 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 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 18 O-depleted pore water.

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