The Calamines and the Others: The great family of supergene nonsulfide zinc ores

Nonsulfides is a term, which comprises a series of oxidized Zn(Pb)-ore minerals. It has also been used to define a special deposit type, mainly considered as derived from the weathering of Zn(Pb) sulfide concentrations. However, nonsulfide zinc deposits have been distinguished between supergene and hypogene, according to their mineralogy, geological characteristics and genetic setting. The supergene deposits formed by weathering and oxidation at ambient temperatures, whereas the hypogene ones are considered hydrothermal, or associated with metamorphic processes on primary sulfide ores. In this review paper, a comparison between a number of several nonsulfide deposits has been carried out: typical Calamines, peculiar Calamines and Others. The whole group comprises deposits of typical supergene origin, mixed supergene-hypogene mineralizations, and oxidized concentrations characterized by different metals only locally associated with zinc. The Zn-Pb nonsulfide concentrations hosted in carbonate rocks, which are mainly attributed to wall-rock replacement and direct-replacement supergene processes, are the typical Calamines (Liege district, Belgium; Iglesias district, Italy; Silesia-Cracow district, Poland). Peculiar Calamine deposits are those mineralizations that have been generally considered as supergene, but which are instead genetically related, at least partly, to hypogene processes (e.g. Angouran, Iran; Jabali, Yemen), though mineralogically and texturally similar to supergene nonsulfide deposits. The Others are prevailingly supergene nonsulfide zinc deposits not hosted in carbonate rocks (Skorpion, Namibia; Yanque, Peru), or characterized by other metals as main commodities, like lead (Magellan, Australia), silver (Sierra Mojada, Mexico; Wonawinta, Australia) or vanadium (Otavi Mountainland, Namibia). Minerals of current economic importance in most Calamine deposits are smithsonite, hydrozincite, and cerussite. This mineralogical association is generally simple but, when the Calamines are dolomite-hosted, one of the consequences of the wall-rock replacement process is the generation of a series of economically useless Zn- and Mg-bearing mixed carbonate phases. Secondary deposits hosted in silicatic (sedimentary or volcanic) rocks mainly contain hemimorphite and/or sauconite. Lead-, Ag- and V-rich nonsulfide ores are characterized by a more complex mineralogical association: mixed Pb-carbonates, Pb-sulfates, Pb-phosphates, Pb-arsenates, various Ag-sulfosalts, and Zn-Pb-Cu-vanadates. Carbon and oxygen stable isotope studies allow distinguishing between supergene and hypogene nonsulfide deposits, evaluating the effects of oxidative heating and even gaining indirect paleoclimatic information. The 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, thus suggesting several isotopically distinct carbon sources. Periods of paleoclimatic switch-overs from seasonally humid/arid to hyperarid have been considered as the most favorable conditions for the formation and preservation of supergene nonsulfide deposits. However, while several recent nonsulfide deposits throughout the world are positioned between 15 degrees and 45 degrees N latitude, thus pointing to a warm and humid weathering climate, others have been deposited in sub-Arctic regions. The economic value of the nonsulfide Zn(Pb-Ag-V) ores is highly variable, because more than in the case of metallic sulfide deposits, it resides not only on the geological setting, but also on their mineralogy that can directly influence processing and metallurgy. (C) 2014 Elsevier B.V. All rights reserved.