GEOCHEMICAL AND MINERALOGICAL CONSTRAINTS ON THE GENESIS OF THE OTJOSONDU FERROMANGANESE DEPOSIT, NAMIBIA: HYDROTHERMAL EXHALATIVE VERSUS HYDROGENETIC (INCLUDING SNOWBALL-EARTH) ORIGINS

Abstract

The stratigraphic position of the Otjosondu ferromanganese deposit in the Neoproterozoic Damara Supergroup in the Central Zone of the Pan-African Damara orogenic belt is uncertain. Comparison of the enclosing host rocks with regional profiles in the southern Central Zone would indicate that it is hosted by the Etusis Formation (Fe-rich feldspar-quartz rocks) of the Nosib Group, in a condensed stratigraphic section of the supergroup, and not the Chuos Formation (diamictite, turbidite and thin iron-formation layers) of the Swakop Group, as previously proposed. The Otjosondu ore comprises ferromanganese silicate-oxide assemblages that occur in a feldspathic-quartzite sequence lacking the mixtite facies typical of the Chuos Formation. Hematitic iron formation containing minor hyalophane, spessartine and barite, and hyalophane-bearing spessartine-quartz rock are present in close spatial association with the Mn silicate-oxide rocks. Barite is a widespread and locally abundant mineral in the Mn silicate-oxide ore. Bulk-rock chemical analyses indicate trace-element associations of Ag-As-Cu-Co-Bi-B-P with Mn and Sr-Mo-W with Ba in the Mn silicate-oxide and spessartine-quartz rocks. These element associations are explained by adsorption/substitution processes in an oxygenated alkaline marine setting. Sulfur-isotope compositions of barite from the Mn silicate-oxide ore vary from +10.1 to +16.7 ‰, a range of δ34S values that shows negative correlation with bulk-rock Fe content and is interpreted as mixing of isotopically light hydrothermal S with heavier sea-water S. The protoliths of the ferromanganese rocks were precipitates deposited in a basin-margin setting as a result of mixing of an exhalative hydrothermal fluid with oxygenated, sulfate-bearing sea water, and varying degrees of detrital addition. These conclusions are in contradiction to the deep-water, anoxic, sulfate-depleted oceanic settings required for formation of glacially associated Rapitan-type iron and manganese formations. The formation of the Otjosondu ferromanganese deposit is therefore unrelated to any Chuos/Sturtian global glacial event. Origins as hydrogenetic deposits are mostly excluded due to the base-metal- and P-poor, but Ba-rich nature of the Otjosondu ores. The baritic ferromanganese, hematite-quartz and spessartine-quartz rocks at Otjosondu may be compared to oxidised baritic portions of the Gamsberg (Broken Hill-type) base-metal deposit in South Africa and may represent the distal or shallow-water oxidised equivalents of sediment-hosted exhalative base-metal mineralisation such as the penecontemporaneous Tsongoari and Rosh Pinah deposits.