Geochemistry of Germanium in the Oxidized Zone of the Tsumeb Mine, South-West Africa^*

The primary ores of the Cu-Pb-Zn deposit at Tsumeb contain germanium in solid solution in relatively large amounts in enargite, tennantite, renierite and germanite. Oxidation of the ores in a limestone environment has given rise to an extensive suite of secondary carbonates, sulfates, arsenates, phosphates, vanadates, and oxides. The chief cations are Cu, Pb, and Zn together with minor amounts of Ca, Mg, Fe, Ba, Cd, Mo, and Co. The germanium contained in the primary minerals is released during oxidation and is found in two valence states in the secondary minerals. It occurs very sparingly as divalent Ge in substitution for Pb in anglesite and cerussite (50–500 ppm) and in a new basic lead sulfate containing 8.18 weight per cent GeO. The bulk of the germanium in the oxidized zone is quadrivalent. It preferentially enters the early-formed arsenates, including mime-tite, olivenite, and in particular bayldonite (with 500–5000 ppm Ge). The Ge⁴ substitutes for As⁵ in the (AsO₄) tetrahedra, with Ge⁴ 0.44 kX and As⁵ 0.47 kX, analogous to the well-known substitution of Ge⁴ for Si⁴ in silicates. The later-formed minerals smithsonite, cerussite, azurite, and malachite, which comprise the bulk of the upper part of the oxidized zone, are low or lacking in Ge.

The Ge² ion is found at the immediate interface between the sulfide zone and the oxidized zone where, in line with the known chemical and Eh-pH relations of this ion, it appears to have formed under weakly acid and reducing conditions. With increasing Eh and pH of the environment the Ge² is converted to Ge⁴ and is housed in early-formed arsenates stable under moderately acid and oxidizing conditions. In the upper part of the zone of oxidation, under alkaline and oxidizing conditions, the germanium apparently is leached as alkali germanates in the absence of hemimorphite and other secondary silicates stable under such conditions and suited to house Ge⁴ in solid solution.