Organic matter in ore genesis: Progress and perspectives

Abstract

Geochemical processes involving living organisms or detrital carbonaceous material have often been invoked to explain the preferential enrichment of some metallic elements in organic-rich sedimentary layers, as well as the common association of various organic constituents in some sedimentary ore deposits. Despite some differences in opinion depending on the metal considered (PbZnCu, U, Au…), the geochemical environment or the geological setting, ideas about the mechanisms of metal concentration involving organic substances have evolved in more or less convergent ways together with genetic models of ore deposition. The origin of sedimentary organic matter and the ability of living organisms to concentrate various metals had first led researchers to emphasize biological accumulation processes. Next, the assessment of the sorptive and ligand properties of humic and marine substances led them to stress metal sequestration mechanisms. The recognition of the limited cation-exchange ability of organic substrates and of the competition of major and trace elements for chelating sites has now placed most attention on reduction reactions. Reduced sulfur species produced from sulfates and organic substrates are largely recognized as major sinks for many metals through direct precipitation of metal sulfides, co-precipitation with metal sulfides or reduction of non-chalcophile elements. Nevertheless, the conditions (substrates, temperature) for the development of sulfate-reduction processes through biological and abiological pathways are still disputed. Also, little is known about the possibility of direct reduction of metals by organic substrates, without any intervening sulfur species. All mechanisms can play a role in supergene enrichment processes and deposition of syngenetic mineral occurrences, long supposed to constitute an important class of sedimentary ore deposits (e.g. Kupferschiefer, carbonate-hosted ZnPb). However, petrographical, isotopic and fluid inclusion data, often also supported by organic geochemical studies, have now produced clear evidence that most “sedimentary” ore deposits are in fact epigenetic, formed with the participation of warm, i.e. hydrothermal, fluids. In such models, organic-rich layers are frequently supposed to act as reducing and/or hydrodynamic barriers. More work is needed to determine the mechanisms and the conditions under which the reduction phenomena may proceed, their dependence on the metal(s) considered and the composition (origin, maturity) of the organic substrate(s).