Metallogenesis of germanium - A review

Germanium (Ge) is a scarce, but not an extremely rare element in the Earth's crust (about 1.6 ppm Ge crustal average). Principal geochemical substitutions and mineral associations of Ge include Si, C, Zn, Cu, Fe, Sn, and Ag. Most Ge is dispersed through silicate minerals due to the substitution of Ge4+ for the geochemically similar Si4+. Ge is unusual in that it exhibits siderophile, lithophile, chalcophile and organophile behaviour in different geologic environments. Only minor variations in Ge concentrations are known from different igneous rocks, siliceous sedimentary rocks, and their metamorphic equivalents. Carbonates and evaporites show a depletion to the crustal average of Ge. There is a tendency for Ge to be slightly enriched in silicate minerals of late magmatic differentiates (e.g., muscovite granites), rocks that crystallize in the presence of a high volatile concentration (e.g., pegmatites, greisens) and late hydrothermal fluids, accounting for ore deposits. Ge does not form specific ore deposits; rather it occurs in trace and minor amounts in various ore deposit types. Grades of a few tens to several hundred ppm Ge are known in sulphide deposits: volcanic-hosted, massive sulphide Cu-Zn(-Pb)(-Ba) deposits; porphyry and vein-stockwork Cu-Mo-Au deposits, porphyry and vein-stockwork Sn-Ag deposits; vein-type Ag-Pb-Zn deposits; sediment-hosted, massive sulphide Zn-Pb-Cu(-Ba) deposits; carbonate-hosted Zn-Pb deposits, and polymetallic, Kipushi-type Cu-Pb-Zn-Ge deposits. Low-iron sphalerite is the most important of all minerals containing Ge. Other sulphur minerals, e.g., enargite, bornite, tennantite-tetrahedrite. luzonite, sulvanite, and colusite, are significant Ge sources in some deposits. At high S activities, the thiocomplex [GeS4](4-) can give rise to the formation of thiogermanate minerals, e.g., argyrodite, briartite, renierite, and germanite, which can form elevated Ge concentrations, above all in Kipushi-type deposits. Ge concentrations due to sorption processes in iron hydroxides and oxides refer to those in oxidation zones of sulphide ore deposits, especially at the Apex Mine, USA, and Tsumeb, Namibia, as well as to iron oxide ores, particularly in banded iron formations (BIT). Lignite and coal deposits show germanium grades that vary by several orders of magnitude, both regionally and within particular deposits, from levels less than the Ge abundance in the Earth's crust up to a few thousands ppm Ge. This Ge enrichment is effected by chemisorptive processes on relatively stable organo-complexes, e.g., lignin and humic acids. Currently, Ge is recovered as a by-product from sphalerite ores, especially from sediment-hosted, massive Zn-Pb-Cu(-Ba) deposits and carbonate-hosted Zn-Pb deposits, from polymetallic Kipushi-type deposits, and lignite and coal deposits in China and Russia. Figures for worldwide Ge reserves are not available. (c) 2006 Elsevier B.V. All rights reserved.