Abstract

Numerous bands of Lower Permian marine sedimentary phosphorite have been discovered in a cyrogenic diamictite as well as in a late glacial fissile shale sequence in the Natal part of the Great Karoo basin in South Africa. These thin (1-30-cm) bands are extensive, range in P 2 O 5 content from 3 to 29 percent, and average 12.5 percent. The bands are presumed to have formed authigenically by direct precipitation of micron-sized "francolite" (carbonate-fluorapatite, microsphorite) that is optically pseudoisotropic and contains an average of 1.1 percent CO (super -2) 3 that replaces PO (super --3) 4 in its structure. Lithic associates include quartz, clay minerals, and dropstones of varied composition in both the fissile shale and the diamictite. The phosphorite bands formed within 60 degrees to 75 degrees south paleolatitude in a restricted epicontinental basin at the onset of a marine transgression, following the melting and recession of continental ice sheets. The occurrence of phosphorite within the glacial diamictite as well as in the fissile shale sequence, in which scattered dropstones indicate continued glacial influence, suggest polar to cold climatic conditions during phosphorite deposition. Phosphogenesis therefore seems to have taken place in an environment which is characteristically different in its climate and latitude setting from those described for major marine sedimentary phosphorite deposits. Of special interest is the banded morphology, which is not characteristic of post-Cambrian deposits.

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