ReviewThe “Calamines” and the “Others”: The great family of supergene nonsulfide zinc ores
Introduction
Despite the widespread occurrence of surficial zinc (lead) oxides (in gossans and soils), economic deposits of these metals in oxidized form, the so-called “nonsulfides”, are far less common than sulfide deposits. “Nonsulfides” is a term, which comprises a series of oxidized Zn(Pb)-ore minerals, but can also characterize a special deposit type, mainly considered as related to weathering of Zn(Pb) sulfide concentrations. Nonsulfide deposits have experienced a significant revival over the recent years, as a consequence of new developments in hydrometallurgical acid leaching, solvent extraction, and electrowinning techniques. The economic focus is generally on zinc nonsulfides, even though most deposits also contain variable amounts of lead. Silver can occasionally occur in selected ore districts (Sierra Mojada, Jabali, etc.).
The economic value of zinc nonsulfide ores is strictly dependent not only on the geologic setting of each deposit, but also on the nature of the ore and gangue minerals. Host-rock composition may have significantly influenced the mineralogy (and therefore metallurgy) of this kind of ores. Since the differences in dissolution rates of the zinc minerals may have strong implications for the production strategies and metallurgical requirements, a detailed mineralogical and petrographic study, already in the early phases of an exploration process, is a must for this kind of ores, even more than in the case of sulfide deposits.
Although it is improbable that production of Zn metal (and by analogy of Pb metal) from nonsulfide sources will ever completely replace production from conventional sulfide ores, nevertheless it is suggested that nonsulfide Zn (and Pb) deposits are growing in economic importance. One of their more conspicuous advantages is the paucity of sulfur in this kind of ores. Medium- to large-sized deposits currently in development or advanced exploitation stage include Skorpion (Namibia), Lan Ping (China), Angouran and Mehdiabad (Iran), Shaimerden (Kazakhstan), Jabali (Yemen), Hakkari (Turkey), Vazante (Brazil), Accha, Yanque and Bongarà (Peru), Torlon Hill (Guatemala) and Sierra Mojada (Mexico). In addition, there are a number of other localities hosting relatively small tonnages of nonsulfide Zn ores in Vietnam, Thailand, Turkey, British Columbia, Morocco, Algeria, and Egypt (Hitzman et al., 2003) (Fig. 1).
Nonsulfide zinc deposits have been distinguished between supergene and hypogene, according to their mineralogy, geological characteristics and genetic setting (Hitzman et al., 2003, Large, 2001). 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. The hypogene deposits consist dominantly of anhydrous zinc silicates and oxides, such as willemite, zincite, franklinite, and other minerals, locally coexisting with variable amounts of sulfides. The most economically important concentrations of this type are the Vazante deposit (Brazil) in Proterozoic carbonates (Monteiro et al., 2006, Slezak et al., 2014) and the cluster of mineralizations in the Franklin (New Jersey, USA) ore belt, which are also associated with strong metamorphic processes (Dunn, 1995). Other occurrences are the Beltana-type deposits in Southern Australia (Groves and Carman, 2003) and of part of the oxidized ores contained in the Neoproterozoic belts of Namibia–Zambia (Boni et al., 2011b, Schneider et al., 2008).
The mineralogy and size of supergene deposits and their location relative to the precursor sulfide bodies reflect the composition of the primary sulfide assemblage and host rock, as well as the hydrologic regimes and climate throughout time. Hitzman et al. (2003) distinguished between “Direct Replacement” (of sulfides) and “Wallrock Replacement” (of host rock) nonsulfide ores. A third, much less common kind is the “Residual and karst-fill” deposit type (Hitzman et al., 2003). According to the extent of replacement of sulfides, the resulting ore is called mixed (a combination of sulfide and nonsulfide ores) or pure nonsulfide (also referred to as “oxide”) ore. Direct replacement deposits, also known as “red ores” because of their high content in Fe-(hydr)oxides, typically contain > 20% Zn, > 7% Fe and variable amounts of Pb or Ag (Reichert and Borg, 2008). Wallrock replacement deposits, also known as “white ores”, consist of smithsonite, hydrozincite and minor Fe-(hydr)oxides. They contain < 40% Zn, < 7% Fe and very low concentrations of Pb (Reichert and Borg, 2008). Nonsulfide ores can be located in proximity to the sulfide protore or up to several hundred meters away (Heyl and Bozion, 1962, Hitzman et al., 2003). Among the above-quoted deposit types, those hosted in limestone and dolomite (the “Calamines”) have a simple mineralogy, good extraction recovery, and higher grades.
A number of review papers on the subject of nonsulfide Zn(Pb,Ag) ores have been published over the last 15 years (Boni, 2005, Boni and Large, 2003, Borg, 2009, Hitzman et al., 2003, Large, 2001, Megaw, 2009, Reichert and Borg, 2008). The focus of this paper will be mainly on Zn nonsulfides, but also a few Pb–Ag-oxidized deposits and a peculiar case of vanadium supergene ores will be briefly considered (Table 1).
In order to show the high variety of this kind of ores, we will first discuss the typical “Calamine” mineralizations that comprehend the old European deposits and those of the Mediterranean realm, followed by a number of carbonate rock-hosted Zn occurrences in the world, some of which are far from being of supergene origin (e.g. Angouran, Iran). We will then consider a few examples of not carbonate rock-hosted deposits, starting with the case of the world class Skorpion deposit (Namibia).
Section snippets
History
In the past zinc was considered only for its importance in making brass (alloy of zinc and copper). Copper–zinc alloys were produced as early as the 5th millenium BC in China, and were widely in use in eastern and central Asia by the 2nd and 3rd century BC. It is likely that the alloys were smelted from zinc-rich copper ores, producing crude brass-like metals (Kharakwal and Gurjar, 2006).
Greek and Roman documents suggest that the intentional production of alloys similar to modern brass, using
Mineralogy of supergene nonsulfides and classification
The supergene nonsulfide deposits are far more widespread, and considered so far to be more economically important than the hypogene ones. The main minerals associated in nonsulfide Pb and Zn deposits and their physical and chemical properties useful in field identification are quoted in Hitzman et al. (2003) and McConachy et al. (2007). In Table 2 we have listed the most common minerals occurring in the nonsulfide deposits quoted in text, with their chemical formulas and the related amounts of
Methods of analysis
Mineralogy and petrography of nonsulfide Zn–Pb deposits are generally investigated using several laboratory methods (optical microscopy, chemical analyses, X-ray diffraction, scanning electron microscopy, microprobe, QEMSCAN®). Recently, spectral reflectance has been measured on several minerals occurring in nonsulfide deposits, with the aim to use the results as an exploration tool (McConachy et al., 2007). Because the reader is certainly familiar with most of the methods used in the
Examples of “Calamines”
Most nonsulfide Zn(Pb) deposits belong to the category of “Calamine”, that is: “concentrations of Zn(Pb) oxidized minerals derived from deep weathering of carbonate-hosted primary sulfide ores”. “Calamine” is a historic name for oxidized zinc ore, derived from the Belgian town of Kelmis (“La Calamine” in French), which was home to an important zinc mine and processing plant.
The mineralized protore can be either a Mississippi Valley-type deposit, or a SEDEX, or a polymetallic CRD. Calamine-type
Skorpion (Namibia) — everybody's dream
The Skorpion nonsulfide zinc deposit is located approximately 40 km north of the Orange River in the southernmost Namib Desert, Namibia (Fig. 13). It comprises a significant nonsulfide ore body (at the beginning of exploitation it consisted of 24.6 Mt @ 10.6% Zn, Borg et al., 2003) and subordinate amounts of primary hypogene base metal sulfide concentrations, which underlie the nonsulfide ores at depth. The mining commenced in October 2001 with the stripping of the overburden and exposure of the
The lead version: Magellan (Australia)
The Magellan Pb deposit is located in an outlier of the Palaeoproterozoic Earaheedy Group overlying the southeast Yerrida Basin (Pirajno et al., 2010) (Fig. 16), and likely represents a new category within the nonsulfide class of ore deposits. Its measured and indicated ore resources consist of 21.4 Mt grading 5.8% Pb; inferred resources are of 7.2 Mt grading 4.6% Pb (SEG Newsletter, April 2006, No. 65, p. 27). The deposit is currently exploited, due to its low cost metal production. The lead
Supergene vs. not-supergene
For a long time, nonsulfide Zn–Pb deposits seemed to belong to a fairly easy-to-describe type of ores, consisting of variable amounts of oxidized minerals considered as derived from the weathering of sulfides and resembling deeper gossans. As stated in the Introduction, the first blow to these over-simplified genetic concepts on the nonsulfide ores was given by the discovery that not all of them were weathering products, but those characterized by willemite as prevailing mineral were instead
Conclusive remarks
In conclusion, from the geological, mineralogical and geochemical point of view there is an entire world comprised in the concept of “nonsulfide Zn(Pb–Ag–V) ores”. Their economic value is highly variable: more than in the case of metallic sulfide deposits, it resides on the mineralogy of the contained metals. Some technical problems encountered during processing can be thus mitigated by a better identification of the mineralogical association of the metallic and nonmetallic minerals. This
Acknowledgments
First of all, one of us (M.B.) would like to thank Duncan Large, for having introduced her to the fascinating world of nonsulfides, which, until 2001, she rather regarded as rubbish and not as real ore. Our research benefitted from the discussion with G. Balassone, C. Allen, V. Arseneau, G. Borg, A. Clegg, V. Coppola, L. DeJonghe, H.A. Gilg, B. Grist, R. Herrington, M. Hitzman, C. Laukamp, M. Joachimski, G. Olivo, S. Paradis, F. Pirajno, G. Rollinson, L. Santoro, P. Schmidt, G. Stanley, B.
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2021, Applied GeochemistryCitation Excerpt :Part of the primary Zn–Pb ore was exposed to weathering and underwent secondary mineralization that resulted in smithsonite, cerussite, goethite, hydrozincite, anglesite, and hemimorphite, which are the main components of Zn–Pb supergene mineralization (Cabala, 2009; Coppola et al., 2009). The most intensive weathering conditions (climatic and tectonic) took place during a period from the Paleogene to the Miocene when part of the primary ore was exhumed to the surface within the Bolesław tectonic horst (Boni and Mondillo, 2015). Some Zn–Pb supergene mineralization occurred within the Pomorzany tectonic graben, where eroded Keuper claystones facilitated hydration and oxidation of sulfide minerals (Coppola et al., 2009).