Organic material incorporated in an ore or a sediment may represent the resistant parts of organisms, organic compounds adsorbed on inorganic minerals, organic precipitates from chemical reactions in basin waters, or detrital grains eroded from older carbonaceous rocks. In addition, hydrocarbons may be generated elsewhere in a basin before migrating into and being retained in the rocks under consideration. During diagenesis and metamorphism under non-oxidising conditions, the chemical composition of organic matter from all sources progressively changes as volatile compounds (including carbon dioxide, water, and hydrocarbons) are evolved. All types of carbonaceous source material give graphite as the final solid product, but in various amounts, depending on their initial composition and subsequent history. Bacterial or aerial oxidation of solid organic materials is most rapid at low ranks and initially causes an increase in oxygen/carbon ratios before complete removal of carbonaceous matter is achieved. At high temperatures, oxidation by water can completely convert graphite to carbon dioxide. Data are given on the insoluble organic matter isolated by demineralisation of samples from the Red Sea, Julia Creek, McArthur River, Mount Isa, Broken Hill, Cobar, Woodlawn, Kambalda, Rum Jungle, Alligator River (Australia) and the Witwatersrand (South Africa). Most of these samples contain material, probably of algal origin, that is now at the graphitic stage of metamorphism. Organic matter from recent sediments in the Red Sea and ore-related rocks from McArthur River, Mount Isa, and Broken Hill show a progressive change in rank consistent with a syngenetic theory of ore formation. The presence (or even the absence) of carbonaceous material in an ore or sediment is a valuable parameter in reconstructing the chemical and biological environment of genesis and subsequent geological changes.
