The nature, genesis and geochemistry of the supergene vanadium ores of the Otavi mountain land

Abstract

Abstract by author:

Numerous vanadium ore deposits developed in karst solutional features of which Abenab and Berg Aukas are the largest. Apart from the role which water played in the development of karst through solution, it also acted as a depositional agent for the vanadium ore. A clear relationship exists between the groundwater table and the topography. Slopes of the groundwater table in the direction of the large deposits indicate that the vanadium was liberated from the carbonates by erosion and transported as calcium metavanadate to precipitation barriers where it was precipitated as descloizite-mottramite

Most of the vanadium occurrences are associated with sulphide deposits (which acted as precipitation barriers) and the chemistry of the sulphide deposits are reflected in the chemistry of the descloizite-mottramite

The vanadium ore is located in young karst features which developed after the rifting of Gondwana. Strata overlying the Otavi Mountains (Etendeka Formation) had, however, to be denuded before karstification could commence and therefore the Etendeka lavas can be excluded as a possible source for vanadium

The "African" cycle of erosion must have been severe as during this phase, karst features which formed in pre-Karoo times were removed. No evidence of Karoo Sequence rocks were noted in any karst features in the Otavi Mountain Land

From the Tertiary onwards, karstificatfon was greatly enhanced by soil cover whilst the oxidation of sulphides also assisted with the solution of carbonates. The groundwater around the oxidizing sulphide deposits has a low pH and therefore contains substantial concentrations of copper, lead and zinc. Vanadium, in the form of calcium metavana-date was transported to such areas where descloizite-mottramite was deposited

Because of the correlation between the distribution of vanadium in the country-rocks and the distribution of vanadium ore deposits, it is furthermore deduced that the vanadium was leached from the carbonate rocks and to a lesser extent from the shales