THE "CALAMINE" AND THE OTHERS: THE GREAT FAMILY OF SUPERGENE NONSULFIDE ZINC ORES

Economic nonsulfide zinc ores are less common than sulfide deposits. Nonsulfide minerals of current economic importance in supergene deposits are smithsonite, hydrozincite, hemimorphite, and sauconite (Zn-smectite). Zinc is also hosted in chlorite-type clays, in Fe- and Mn-(hydr)oxides and in Zn-rich dolomite. However, the latter concentrations are generally uneconomic. The occurrence of supergene willemite is a still a debated subject.

Supergene nonsulfide deposits have been characterized by stable isotope analyses of their carbonates (smithsonite-cerussite-calcite). Oxygen isotope variation of the individual carbonate minerals within a deposit is relatively small, indicating constant formation temperatures and a single, meteoric fluid source. Carbon-isotope values are highly variable, suggesting isotopically distinct carbon sources.

The economic value of nonsulfide Zn ores is dependent not only on the geological setting of each deposit, but also on its mineralogy. Host-rock composition can also influence the mineralogy (and hence metallurgy) of nonsulfide deposits. Those in limestone and dolomite (the “Calamine”) tend to be dominated by smithsonite and hydrozincite, whereas the deposits hosted in siliciclastic rocks tend to contain hemimorphite- and sauconite-bearing assemblages. However, even within similar host rocks, the mineralogy can vary substantially: it can be relatively simple (smithsonite, hemimorphite, hydrozincite), as in the oxidation products derived from low-temperature sulfide deposits (MVT, carbonate-hosted SEDEX), or far more complex when derived from the weathering of high-temperature ores (VMS, CRD). Most “Calamine” deposits have higher grades and good extraction recovery. Beside the old European “Calamines”, good examples are Angouran (Iran), Jabali (Yemen), and Accha (Peru). Other kinds of deposits as Skorpion (Namibia) are mineralogically more complex, but their processing challenges have been successfully overcome. Similar orebodies (Yanque, Peru) may have also a good chance of success.

Because the differences in dissolution rates of the Zn minerals have strong implications for the production strategies and metallurgical requirements, it is advisable to conduct detailed mineralogical studies early in the exploration process.