Characterization of fibrous cryptomelane from the todorokitecryptomelane mineral assemblage at the Sebilo Mine, Northern Cape Province, South Africa

A 30-cm hand sample was obtained from the lowermost manganese ore bed, directly below the Kalahari unconformity within the supergene zone, at the Sebilo manganese mine in the Kalahari Manganese Field, Northern Cape Province, South Africa. The sample comprises an intergrowth of rare supergene fibrous cryptomelane and todorokite, as well as traces of presupergene manganese oxides. A chemical analysis of hausmannite concretions within remnant diagenetic manganese ore fragments in the supergene mineral assemblage shows leaching of pre-existing carbonate and silicate compounds from the primary diagenetic ore (Kleyenstüber 1984, 1985, Chetty 2008, Vafeas 2016) via descending meteoric fluids along the Kalahari unconformity. This resulted in the initial development of todorokite, forming the bulk of the matrix, and the coeval development of early-stage calcite within the todorokite pores. Subsequent to this, late-stage calcite precipitated from descending Ca-bearing supergene fluids and is seen to superimpose the todorokite matrix. Growth of the supergene mineral assemblage ended with the formation of rare medium- and coarse-grained acicular cryptomelane crystals that are orientated perpendicular to the host fracture surface.