Cores of organic-rich muds from the tropical meteorite crater Lake Bosumtwi, Ghana, contain laminae of authigenic calcite polyhedra and aragonite needles, as well as scattered diagenetic calcite, Mg-calcite spherulites, and aggregates of dolomite crystals. Their respective origins are traced by analyses of oxygen and carbon stable isotopes. Anoxic dolomite and Mg-calcite formed in pore waters reflecting highest degrees of evaporative concentration and high rates of methanogenesis. Carbon dioxide reduction must have been the dominant methanogenic pathway, producing diagenetic carbonates with δ13C values up to ±27‰ PDB. This example shows the inherent problems of interpreting bulk samples with progressive mineralogical sequences of calcite to Mg-calcite to aragonite to dolomite merely in terms of Mg2+/Ca2+ in lake waters. It provides a model for the interpretation of ancient lacustrine carbonates from organic-rich environments.