An aqueous suspension of clay, dolomite, quartz, and organic matter was sedimented through a saline solution containing trace amounts of copper and zinc and accumulated on an organic‐rich layer where sulfide was being generated by sulfate‐reducing bacteria. A control system was set up in which the organic matter was omitted and the sulfide provided by diffusion of a sodium sulfide solution from beneath the sediments. In the organic system the copper and zinc were separated and fixed in different sedimentary layers, whereas in the control system they precipitated together. Metal distributions in the former system indicated that copper forms soluble metal‐organic complexes that keep the copper in solution in the presence of sulfide. Zinc also forms metal‐organic complexes, but these are apparently not resistant to sorption and precipitation processes.