Roles of Organic Matter in Shale- and Carbonate-Hosted Base Metal Deposits
Base metals are incorporated into diverse sedimentary rocks as a result of routine biogeochemical processes during primary production, heterotrophic consumption, bacterial diagenesis, burial maturation, and hydrothermal alteration. In most cases, concentrations of organic carbon and base metals in sedimentary strata are below minimum levels for economic development of energy and/or mineral resources. Major ore deposits are linked, in many cases, to unusual basinal conditions that focus metal-bearing solutions into sedimentary facies containing reactive organic constituents inherited from the original sedimentary inventory or enriched by secondary fluid migration. This chapter examines the chemical and physical role of reactive organic matter in base metal deposits associated with carbonate and shale sequences. The discussion of selected well-studied deposits is organized on the basis of the dominant metals and the sedimentary facies hosting the orebody. Three groups of ore deposits are discussed in this chapter: (1) carbonate-hosted lead-zinc-barium deposits; (2) shale-hosted zinc-lead deposits; and (3) shale-hosted copper deposits. Diverse roles of organic materials in the genesis of sediment-hosted base metal deposits are evident from recent publications on the Mississippi Valley Viburnum trend, Canadian Pine Point, French Massif Central, Canadian Selwyn basin, Australian McArthur River region, Alaska Red Dog mine, European Kupferschiefer, and Michigan Nonesuch shale. Although several important types of sediment-hosted base metal ores are not considered in this paper, the selected examples provide a framework for understanding how modern organic geochemical techniques can be applied in an interdisciplinary approach to exploration and development of a wide range of base metal deposits. Additional discussions of shale- and carbonate-hosted base metal deposits can be found in Coveney (2000), Giże (2000), Kettler (2000), Leventhal and Giordano (2000), and Simoneit (2000).