Carbonaceous substances in mineral deposits: implications for geochemical exploration

Abstract

Carbonaceous substances (CS) are intimately linked directly and/or indirectly to the genesis of many types of mineral deposits. Examples of the phenomenon include metal accumulations in living organisms, coals, black shales, crude oils and solid bitumens. CS interact with metals by virtue of their inherent reducing, acidic and chelating characteristics. After kerogen, and across the oil-to-solid bitumen continuum, bitumen is the most important CS that promotes metallization. The most familiar Phanerozoic example is the carbonate-hosted Pb–Zn ores of the Mississippi Valley-type, for which diverse reduction models have been proposed. At Elliot Lake, Canada, and in the Witwatersrand, South Africa, CS including kerogen and derivative bitumen (as petrographically defined) are closely related to the genesis of Precambrian uranium and gold mineralization. Occurrences of solid bitumen have long been recognized as important in petroleum exploration, and specific metal ratios (e.g., V/V + Ni and Mo/Mo + Cr) in viscous crudes and solid bitumens have been successfully applied in oil–source rock correlation studies. Typically, there is an increase in concentration of organometallic moities and complexes from crude oil to solid bitumen. Highly anomalous concentrations of elements (U, Pt, Au, V, and Ni) occur in solid bitumen, and numerous minerals (e.g., Zn, Pb, U, Ti, Au and Ag bearing species, illite, pyrite, REE-phosphates, quartz, barite, carbonates) develop authigenically in association with bitumen. Consequently bitumens are potentially important in the search for, and evaluation of, diverse mineral deposits. Where linked to the hydrothermal history of organic matter in sedimentary rocks, bitumen and organic acids generated during diagenesis help to prepare ground for mineral deposition, and like some oil field brines, are genetically associated with mineral deposits. The mineral-forming constituents are carried in an aqueous phase that accompanies bitumen during its emplacement. The mineral-associated constituents and organically bound metals in solid bitumens provide important clues in exploration geochemistry.