Critical elements in nonsulphide Zn deposits

Nonsulphide Zn (NSZ) ore is a general term, describing a group of supergene and hypogene deposits mainly consisting of oxidised Zn-bearing minerals (Hitzman et al. 2003). Sulphide deposits that commonly are progenitors of nonsulphide mineralisation contain sphalerite, which is a known host for gallium, germanium and indium. The aim of the present study is to understand the mobility of Ge during sulphide oxidation and the formation of secondary nonsulphide ores. To determine the mineral residence of Ge in nonsulphide Zn deposits, we conducted a focused program of sampling and analysis on specimens from the Natural History Museum (NHM) collections in London, and on material we collected personally in selected mines and prospects. The most relevant results have been obtained on a batch of Kabwe specimens from the NHM Ores collection, and on drillcore samples from the Rio Cristal (Bongará) prospect in Peru. Kabwe (=Broken Hill, Zambia) is a carbonate-hosted Zn–Pb sulphide deposit, where sphalerite, galena, pyrite, chalcopyrite and accessory Ge-sulphides are patchily replaced by the hydrothermal Zn-silicate willemite (Kamona & Friedrich 2007). The entire assemblage is overprinted by minerals typical of the supergene environment: smithsonite, cerussite, hemimorphite, sauconite, pyromorphite, Pb-vanadates and Fe-oxy-hydroxides. The Rio Cristal prospect (Bongarà, Peru) is a typical smithsonite-rich nonsulphide deposit, derived from a deep weathering process of an original MVT sphalerite-bearing orebody. In the analysed drillcores, several gossanous samples contain high amounts of goethite and hemimorphite. Whole rock chemical analyses and laser ablation on Zn silicates and Fe-oxy-hydroxides from the two deposits have shown that Ge at Kabwe is only hosted in willemite, and not in typical supergene phases. At Rio Crystal, instead, the highest Ge amounts are associated with hemimorphite and goethite. From the present dataset, it appears that from the analysed NSZ deposits, only Rio Cristal contains significant concentrations of Ge in the secondary phases. This is a notable result, because even though the occurrence of Ge in silicates and Fe-oxy-hydroxides has been predicted from theoretical studies, it has been seldom directly observed in nature (Höll et al. 2007; Saini-Eidukat et al. 2009). It is also important to evidence the difference to Kabwe, where the supergene mineral phases are essentially Ge-free, despite the primary sulphide deposit being very enriched in Ge. Looking at the solubility of Ge and Si in aqueous fluids, it is possible to observe that they behave in a similar fashion. It is well known that under humid-tropical climatic conditions under conditions of near-neutral pH, silica can be more efficiently leached from silicate rocks than Al or Fe. The specific conditions of the Rio Cristal area (e.g. the presence of weathered sulphides creating an acid environment, together with the humid climate and high precipitation rates increasing the Si availability) have probably favoured the formation of hemimorphite over smithsonite, thus enhancing the possibility to form Ge-bearing stable secondary phases.