Otjosondu, a Neoproterozoic ferromanganese deposit located about 130 km north-east of Okahandja, Namibia, has a garnet-braunite-barite rock containing ~32 weight % barium. The garnet is intergrown with barite and has end-member proportions as follows: spessartine (48 to 55 mol %), grossular (25 to 35 mol %), andradite (7 to 13 mol %) and, notably, blythite (4 to 9 mol % Mn(II)3Mn(III)2[SiO4]3). Spot measurements for rare-earth elements (REE) and yttrium indicate that the garnet is characterised by a shale-normalised negative anomaly of cerium that is typical of oxic sea water. The garnet has Y/Ho ratios between 18 and 25, a range of values indicative of aqueous fractionation of yttrium from holmium, as observed in marine ferromanganese deposits formed in oxic sea water. The REE patterns of the Otjosondu garnet drastically differ from those found in garnet from Broken Hill-type deposits in Australia, particularly in the absence of cerium anomalies in the latter. The Otjosondu garnet is thought to be metamorphosed hydrothermally derived ferromanganese precipitates in oxic sea water. The abundance of barite and its coexistence with the spessartine-rich garnet suggest that Otjosondu may represent an oxidised variety of Broken Hill-type, sediment-hosted exhalative mineralisation.
