Manganiferous stromatolites from the Precambrian of Botswana

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Abstract
S tromatolitic fragments were collected by one of us (S. P. M.) from the ore stock piles of the Kgwakgwe Hill Manganese working, near Kanye, se . Botswana, southern Africa. Unfortunately the hand cobbing destroyed the external form of the structures, and despite subsequent visits to the locality it has not been possible to find any specimens in situ. The manganese deposits are found in an outlier of Precambrian sedimentary rocks which is correlated ( Boocock 1965 ; Crockett 1970 ) with the Black Reef and Dolomite Series of the Transvaal System (c. 2000 m.y.). Although the sequence is thin and atypical of the standard succession, the manganiferous mudstones are over-lain by an undisputed Waterberg Succession and thus the upper age limit can be established with certainty. The following generalized stratigraphic column gives some idea of the lithologies involved as well as the stratigraphic position of the stromatolite occurrence. The manganese ores of Kgwakgwe Hill are found in two main modes of occurrence. In the first there is the laminated varve-like shale type ore which is variable in composition due to intermixed clays and, or, iron oxides. It is interlaminated with white kaolinitic shale, chert, layers of wad, shale and sandstone. The other consists of discrete nodules of pyrolusite and nsutite set in a light yellowish ironstained mudstone. These nodules, often concentrically laminated, may constitute up to 40 % of the rock and are concentrated along the faint bedding planes of the mudstone. They vary in composition from a high grade nsutite to a ferruginous

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