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The fossil sulfide and carbonate breccia of Decaturville, Missouri, is described and the structural position of the breccia mass discussed. The large and the small carbonate rock fragments show remnant textures typical of marine carbonates. The isotopic compositions of the carbonate fragments and the “flow” carbonates of the breccia compare well with those of sedimentary carbonates from the host sequence of Mississippi Valley-type stratiform lead-zinc deposits of southeastern Missouri. Therefore, an involvement of deep-seated (mantle) carbon in the process of breccia formation is not indicated.

As suggested by the title of this volume, there is a widespread suspicion that certain types of Precambrian sedimentary mineral deposits may in part owe their origin to the activity of contemporaneous microorganisms. If this view is indeed correct, it is of both economic and academic importance.

Reduced carbon, commonly a residuum of biological activity, has been a conspicuous constituent of sedimentary rocks since the start of the rock record 3.8 × 109 yr ago. The 36,000 Corg assays presently available for Phanerozoic sediments indicate that the average organic carbon content of sedimentary rocks has oscillated around a mean of 0.5 – 0.6% during the last 600 million years. Lack of detailed correlation between observed Corg variations and the isotope age curve of Phanerozoic carbonates raises doubt as to whether all of the variations reported are real. However, the higher organic carbon content of Carboniferous and younger rocks seems to be reflected by more positive levels of the δ13Ccarb which is to be expected from mass balance considerations. Corg assays for Precambrian rocks fall within the scatter of the Phanerozoic data, suggesting that the organic carbon content of Precambrian sediments does not differ significantly from that of geologically younger formations. The constancy of the isotopic fractionation observed between reduced and oxidized carbon throughout the record is best interpreted as the signature of biological activity during the past 3.5 × 109 yr (or possibly 3.8 × 109 yr).

Carbonate constituents from 36 samples of spilitized basalts, dolerites, possible lamprophyres, pyro-clastic tuffs and hydrothermal vein fillings which occur in the sedimentary fram of the Almaden mercury deposit show consistently low δ13C values of — 6.6 ± 2.2 %o vs. PDB, and a corresponding δ18O average of + 17.9 ± 2.5%o vs. SMOW. With the δ13C values of the rare intercalations of sedimentary limestones and dolomites lying in the normal range (- 0.2 ± 0.3%o vs. PDB), the igneous carbonate constituents are unlikely to be of sedimentary pedigree, but rather represent genuinely “hypogenic” material derived from primary magmatic sources.