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ABSTRACT: The stratiforrn sedimentary manganese ore deposit at Orkf~t, Hungary contains primary ore that is exclusively manganese carbonate. It shares many geological, sedimentological, mineralogical, and geochemical characteristics with other organic-rich, sedimegt-hosted, marine manganese d posits. Samples from two stratigraphic sequences through the maflstone-hosted Mncarbonate ore at Urkfit were studied. Stable isotope compositions ofboth Mn-rich and Mn-poor carbonates were analyzed and are discussed, together with the geologic background of the deposit, rock and mineral chemistry, and x-ray diffraction mineralogy. The ~3C values of carbonates (- 1.24 to-30.78%0 show a negative linear correlation with Mn contents and a negative exponential correlation with total organic arbon contents. These relations uggest that the 0rkfit deposit was produced by early diagenetic precipitation of manganese carbonate. Data are consistent with models for bacterial metabolic pathways. Thus, mineralization was a consequence of bacterially mediated diagenetic reactions that involved Mn reduction via one or two coupled reactions: the oxidation of organic matter with Mn oxyhydroxide r duction, or the oxidation of FeS produced as a byproduct of seawater sulfate reduction with Mn oxyhydroxide r duction. Oxygen-deficient water in the depositional basin acted as a carrier and reservoir for Mn +2 before its deposition