"Isoclinally folded Precambrian metasediments of the amphibolite facies of metamorphism contain interlayered protore horizons of manganese carbonate and gondite. Under conditions of tropical weathering, the host rocks have been altered to clay to depths of more than 100 m and the protores have been oxidized to form residual lateritic deposits of manganese dioxide within the zone of weathering. Oxide ore formation is accompanied by enrichment of manganese through replacement of the principal impurities of iron, alumina, and silica in the protores. Migration of the manganese continues at present as illustrated by solution cavities filled with colloform layers of cryptomelane and crusts of crystalline pyrolusite."

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