Supergene Nonsulfide Zinc Ores: State of the Art

Nonsulfide zinc deposits (NSZ) are a relatively uncommon ore type. They were the earliest source for mined zinc, and in the last years have enjoyed a renewed interest due to new processing technologies. Two types of deposits are indicated: hypogene and supergene [1], which can be distinguished according to their geological setting and mineralogical characteristics. Hypogene ores are derived from high temperature fluids in hydrothermal and/or metamorphic environments (e.g. Vazante-Brazil, Beltana-Australia and Franklin-USA). Hypogene NSZ mainly consist of anhydrous zinc silicates and oxides. They are less abundant and economically less significant (with the exception of Vazante) than supergene types. Supergene NSZ are derived from the weathering of primary sulfide deposits: a combination of conditions is needed for the development of economically significant deposits of this type. Key conditions include: (1) pre-existing zinc concentrations, (2) efficient oxidation promoted by tectonic uplift and/or prolonged deep weathering; (3) permeable wall rock to allow for ground-water movement; (4) effective trap sites; (5) hydrogeological environments that do not promote dispersion and loss of Zn-bearing fluids. These deposits contain hydrated zinc silicates and carbonates such as hemimorphite (Zn4Si2O7(OH)2???H2O) and hydrozincite (Zn5(CO3)2(OH)6), or the most common carbonate smithsonite (ZnCO3). The best example of a supergene nonsulfide Zn deposit is the Skorpion operation in Namibia [2]. There are other examples: e.g. Accha [3] and Yanque [4] in Peru, or Hakkari [5] in Turkey. Other deposits, which have been long considered supergene, are instead related to the circulation of low temperature hydrothermal fluids (as Angouran in Iran [6], and -partly- the well-known ???La Calamine??? deposit in Belgium). Recent developments in processing technologies for the treatment of NSZ zinc deposits (acid-leaching, AmmLeach®, pyrometallurgy, electrowinning) have increased the commercial interest for NSZ ores with a revival in exploration throughout the world, blossoming at the beginning of the twenty-first century. Nevertheless, at several mines the hydro-metallurgical plants are underperforming relative to initial expectations, which has resulted in delays to the development of NSZ deposits exploitation. Capital and operating costs, acid consumption and metal recoveries have not completely met the feasibility study expectation, and other oxide resources (Mehdiabad-Iran, Sierra Mojada-Mexico, Torlon Hill-Guatemala, Jabali-Yemen) are still battling with technical and/or political issues. However, many technical problems can be mitigated by a better identification of the mineralogical association of the metallic and nonmetallic minerals. This should be a fundamental step in the exploration of nonsulfide mineral deposits, because the extraction process is highly sensitive to mineralogy. Therefore, before the metallurgical processing methods are chosen, it is necessary to take into account both the physical and chemical properties of the ores and their gangue minerals. References: [1] Hitzman et al. (2003) Economic Geology 98: 685???714. [2] Borg et al. (2003) Economic Geology 98: 749-771. [3] Boni et al. (2009) Economic Geology 104: 267???289. [4] Mondillo et al. (in press) Economic Geology. [5] Santoro et al. (2013) Ore Geology Reviews 53: 244-260. [6] Boni et al. (2007) Mineralium Deposita 42: 799???820.