Metals in Black Shales

Abstract

Shales that are rich in organic matter occur throughout the geologic record, but special conditions are responsible for their occurrence (Tourtelot, 1979). Before considering organic matter- and metal-rich shales, some background information is necessary. A shale is a sedimentary rock that is composed of small (mostly less than 0.1 mm) particles dominated by phyllosilicate (clay) minerals and containing subordinate amounts of quartz, carbonate, and phosphate minerals and, in some cases, organic matter and pyrite [for a discussion, see Spears (1980)]. Usually, the only detrital minerals are quartz and kaolinite; the others are authigenic. Organic-rich shales often show bedding laminations ranging from less than millimeter to centimeter thickness that are the result of size sorting, organic content, or pyrite layers, caused by changes in depositional environment. Shales can be formed in fresh, brackish, marine, or hypersaline water bodies, but most of the examples in this chapter are from the marine environment (Degens, 1965). The organic matter in shales is mainly a macromolecular “geopolymer” called kerogen (Hunt, 1979) that is insoluble (in normal organic solvents and nonoxidizing inorganic acids and bases).