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Geological Survey of Israel, 30 Malkhei Israel Street, 95 501, Jerusalem, Israel
Humic substances form an appreciable part of the organic matter in many phosphorites of various ages, including some deposited as early as Cambrian times. In some phosphorites the humic substances are primary, having formed during sedimentation or early diagenesis, and seem to have been preserved as such. In others they appear to be secondary and were probably formed by oxidation of kerogen. While the presence of humic substances in phosphorites (or any sediment) does not necessarily indicate thermal immaturity, their characterization (aromaticity, size of particles, etc.) provides valuable information about their environment of deposition (oxic, anoxic) and their geological history (extent of diagenesis, burial depth, thermal history).
The results of the present work show clearly that the humic substances in the Permian phosphorites of the Western US Phosphate Field were derived from late degradation of kerogen, while those in the Eocene phosphorites from Tunisia are primary. The origin of humic substances in other fields is less clear-cut and it is possible that in some instances, for example, in the Late Cretaceous phosphorites from the Negev, Israel, they are polygenetic.
