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Summary
A geochemical and petrographic examination of onland phosphorites from the south-western Cape is made to elucidate their origin and to compare them with the known offshore occurrences. The investigation shows that some of the massive phosphorites and phosphatic pebbles on land have probably formed as phoscretes in a similar manner as calcretes, whereas some of the pelletal deposits originated by disaggregation of phosphorite rock, which was derived by interstitial precipitation. This mechanism is in contrast to the mode of formation suggested for the offshore phosphatic deposits where replacement of calcium carbonate by calcium phosphate is the major diagenetic process. A model which takes these two modes of formation into account is discussed briefly.