A Profound Meteoric Water Influence on Genesis in the Permian Waterberg Coalfield, South Africa: Evidence From Stable Isotopes

Abstract

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 d18O and d13C 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 d13C values up to +8 (PDB) and d18O value between +9 and +15 (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 d13C values (-l9 to -14) and d18O values between +12 and +22. Ankerite is present only in minor amounts (< 1%) and has isotopic values similar to the coexisti g siderite. Calcite lenses conformable with coal and mudrock beds have d18O values between +16 and +20, and d13C values » -12, The oxygen and carbon isotopic values of cleat-filling calcite, which is present only in coal seams, has d18O values between +12 and +22 and d13C values between -15, and -7. Fine-grained calcite, disseminated in carbonaceous mudrock and as inclusions in kaolinite, is present only at the base of the Grootegeluk Formation and has d13C values between -7 and +3 and d18O values » +15. The calcite lenses are interpreted to be synsedimentary and to have precipitated from depositional waters with d18O value of -13. Spherulitic siderite and disseminated calcite precipitated during early diagenesis dominated by anaerobic microbial decomposition of organic matter. Granular siderite formed at higher temperatures (< 100°C) and later than the spherulitic siderite. The uniformly negative d13C values (-12) of the cleat-filling calcite indicate a predominantly organic source for the carbon, but the erratic and wide range of the d18O 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 d18O-depleted pore water.