Sphalerite, principal ore mineral of zinc

--- from the former Long Lake mine, southeast Ontario, Canada, and from the nearby Balmat mining camp, northwest New York, U.S.A.

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Figure 1. Semi-massive, granular, lustrous brown sphalerite crystals predominate in this zinc ore from the old Long Lake mine, a 1970s zinc producer near the city of Kingston. Rich zinc sulphide ore, with scattered pyrite, calcitic carbonate and fine-grained, dark silicates in the cryptic matrix. A polished thin section and polished mount were prepared from this ore. The estimated volume proportions of minerals in thin section are: diopside and secondary talc (48%), sphalerite (35%), high-relief granular isotropic grossular garnet (5%), (?) chondrodite (5%), pyrite (3%), calcite (2%), marcasite formed by alteration of pyrite (1%), hematite (1%) and traces of galena and chlorite. Wolff (1982a,b) also reported traces of pyrrhotite, chalcopyrite and molybdenite. The sphalerite is deep red in transmitted light and most grains lack obvious chemical zonation, unlike the sphalerite of many other deposits, such as the former Nanisivik mine on Baffin Island.

This material, like the sample from Balmat (Fig. 2, below) has become crumbly after 15-20 years in storage. Sphalerite is generally quite stable in storage, unlike many specimens of iron sulphides, and the reasons for this disintegration are unclear. By analogy with unusual ultramafic rocks with briny fluid content, which may disintegrate by chronic expansion of stored drill core, it may perhaps be due to chloride content, in both fluid inclusion and mineral forms, which are recorded in mineralogical studies at Balmat. GCW sample 2205.

"Rock of the Month #215, posted for May 2019" ---

Sphalerite, main ore mineral for zinc:

is a familiar cubic sulphide with ideal formula ZnS although, as we shall see, Fe and other elements often substitute for Zn, generating both opportunities and challenges for smelters of the zinc ore.

Long Lake zinc deposit

The Long Lake mine, worked sporadically in the 20th century, is hosted by calcitic metasediments of the Central Metasedimentary Belt of the Grenville subprovince of the Canadian shield, intruded by metagabbro of the Mountain Grove pluton, hence the skarn aspect of the mineralogy (grossular, diopside...). Mafic bodies (more generally gabbroic to dioritic rather than ultramafic) are widespread in this region of the Grenville province, see, e.g., this nearby if unusual example. The zinc deposit is located in a geologically and metallogenically diverse region between Sharbot Lake and Kingston, home to resources of feldspar, mica, barite, graphite, corundum and metals such as Pb (galena) and Zn (sphalerite: Harding, 1951). The mine has been described by Wolff (1982a, pp.54-64; 1982b; see also Easton and Fyon, 1992).

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Figures 2-4. Three samples from the Balmat mining camp near Gouverneur, upstate New York. Left-right, we have a) sphalerite, b) pyrite, and c) the pinkish-mauve manganese- bearing tremolite, an amphibole variety known as hexagonite. The U.S. one-dollar coin is 26 mm (one inch) in diameter.This area, on the northwest flank of the Adirondack massif, is of Grenville age (like the Long Lake deposit and the Adirondack High Peak region), the ores and carbonate host sediments strongly deformed and metamorphosed. The Long Lake ore was sent to Balmat (a short journey around the east end of Lake Ontario) for processing. GCW samples 1647, 1648, 1705.

Balmat, New York

The Balmat mining district includes the famed Balmat-Edwards mine, near the town of Gouverneur, situated in a belt of highly metamorphosed Grenville bedrocks on the northwest margin of the Adirondack massif, in upstate New York, close to the east end of Lake Ontario and the Canadian border. The Zn and Pb mineralization is hosted by siliceous dolomitic Grenville marbles, and contains pyrite, sphalerite and minor galena. The ores extend over 600 m, vertically, implying a long-lived hydrothermal system, depending on the dynamics of dewatering of sedimentary basins (Whelan et al., 1984). The Balmat- Edwards- Pierrepont Zn mining district has total metric past production plus reserves of 40.8 MT grading an average 9.4% Zn (deLorraine, 2001). The remobilization of inferred syngenetic ore during regional metamorphism is ascribed by some (deLorraine, 2001) to a sedex precursor mineralizarion. A range of MVT, VMS, sedex and epigenetic replacement theories have been advanced for Balmat ore genesis. A handful of mineral species dominate the ore and host rocks here, but some small zones, such as the unique magnetite-halite occurrence mined in 1991, have other species, both primary and secondary in nature. The deposit is thought by some to have been an exhalative (sedex) deposit formed on a carbonate platform, now heavily metamorphosed and deformed, with a unique development of long-distance (>1 km) remobilization of sulphide ores with common occurrence of host-rock inclusions, so-called durchbewegung ore texture. There are coarse bedded ores and also fine-grained sheared ores. A number of other mineral localities occur in the Grenville rocks in the district, which is situated some 26 miles (42 km) southeast of Ogdensburg.

Much of the ore consists of just a few minerals, namely sphalerite, pyrite and quartz (plus other gangue minerals). Certain zones have their own mineralogy, e.g., coexisting hematite and willemite. A first glance at the literature suggests that Balmat is just as structurally complex as the Grenville ores of the famed Franklin Furnace- Sterling Hill deposits in northern New Jersey, but contains far fewer mineral species (and the principal ore mineral at Balmat is sphalerite, cf. zincite, willemite and franklinite). Some unusual mineral assemblages are preserved at Balmat, such as a zone, discovered in 1991, of exquisite magnetite crystals in halite and talc (Chamberlain et al., 2010). A short review of a few key articles on Balmat, from 60 in MINLIB, yields a partial mineral list of 44 species at the deposit, including 32 in the host rocks and main ore zones and 23 in the magnetite zone, which displayed a number of chlorides, peculiar for Grenville rocks.

Sphalerite Mineral Chemistry

Sphalerite, ideally such a simple mineral (cubic ZnS) can actually show wide variability. It is allochromatic, varying from colourless to black, through many shades of yellow, green, orange, red and brown.

Seven WDS electron microprobe analyses of Long Lake sphalerite are illustrative of the chemical variability of "ZnS". Two points on a late, pale yellow generation in the thin section returned near-pure ZnS: roughly 65% Zn, 0.1% Fe, <0.1% Mn. Five points on the predominant, darker orange to red early generation contain 56% Zn, the balance of the valuable metal replaced by 8% Fe and 1% Mn. Both generations carry a steady 0.15% Cd. Neither Cu nor Ag were present in sufficient quantity to be revealed by electron microprobe analysis, and the balance of the analyses, of course, was sulphur.

Sphalerite from the venerable Freiberg mining district, Germany, occurs in hydrothermal veins of Permian and Cretaceous ages. The Permian veins were deposited from relatively hot fluids at 350 to 230°C. The Cretaceous vein deposits formed by mixing of fluids of variable salinity at low temperature (circa 120°C). The hotter Permian system deposited sphalerite with up to 2500 ppm indium (In, while the cooler Cretaceous veins have less In but more Ge (up to 2700 ppm) and Ga (up to 1000 ppm). Indium is enriched in high-temperature fluids with moderate salinity and a magmatic-hydrothermal component, whereas Ga and Ge are more enriched in lower-T, high-salinity crustal fluids of no obvious magmatic-hydrothermal affiliation (Bauer et al., 2019).

Notes on 18 elements present at percent to ppm levels, in sphalerite grains from diverse geological environments, are provided by Cook et al. (2009). Some of their findings are incorporated into Table 1, representing the extreme values and their geological settings. Note that the extremes for some elements (Mn, Cd, In, Cu, Ag) are 1-2 orders of magnitude above what may be encountered in more "normal" specimens.

Elements in solid solution in ZnS can include Cd, Co, Ga and Ge, In, Mn, Sn, As and Tl. Sphalerite has a specific range of Cd (usually 0.2 to 1%) in each deposit: higher Cd is rare, but may reach 5% and above in some types of deposit. In contrast, Pb, Sb and Bi mostly occur as microinclusions in the host. Ag may be in both solid solution and micro- inclusions. Sphalerite can also carry minor As and Se and perhaps Au. Mn (up to 4%) does not seem to facilitate incorporation of other elements. Sphalerite from Toyoha in Japan can contain zones with either up to 6.7% indium or 2.3% Sn (Cook et al., 2009). Indium contents are lowest in ores rich in Ge and Ga. The mineral colour is influenced by impurities, e.g., black "marmatite" contains 6% or more Fe. Sphalerite is far more common than its two polymorphs, hexagonal wurtzite and trigonal matraite (if the latter is a unique mineral, ibid., p.4762). Sphalerite is the chief ore of Cd and usually carries 0.1-0.5% of the element. Ag is not usually high, but has been reported at 650-700 ppm at the Nanisivik mine on Baffin Island. Sphalerite is an important host of Ga (second only to bauxite ores). Sphalerite and wurtzite are easily the most important source of Ge. Other variations are also possible, with outlier chemistries enriched in As or other usually-trace elements. Exceptional sphalerite at Eskay Creek (British Columbia) contains up to 16.35% Hg in perfect inverse correlation with Zn (Cook et al., 2009).

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Figure 5. These chemist's "play bricks" indicate the metals likely to show up in unusually high levels in an ore of zinc with some lead. In fresh ("primary") ores this generally means sulphides, so all these elements are combined with S. Pb is probably in galena, which often encloses Ag (in part in the crystal structure, but typically as small inclusions of silver sulphosalts, within the host PbS). Sphalerite, ideally ZnS, often contains 1 to 10 wt.% of more Fe, and also appreciable Cd (0.1 to 0.5%, rarely as much as several percent). Other elements, typically in sphalerite at levels of tens to hundred of parts per million by weight, include Ga, Ge and sometimes In (indium), Hg (mercury) and other metals and metalloids. Like the bricks? Twenty bricks on the elements, all the periodic table, including the transactinides, comprise one of many sets of wooden blocks, on diverse themes and in various languages, made in Grand Rapids, Michigan by Uncle Goose. Check them out!

More on minor and trace elements in sphalerite.

The following elements (besides zinc and sulphur) are commonly present in sphalerite: most substitute for some of the zinc, which thus drops below its ideal content of 67 weight percent.

Table 1. Sphalerite chemistry.

Sphalerite Chemistry Summarised
Element MVT deposits Extremes... ... in deposit types like ...
Iron_0.01-13.8 wt.% 10-15.8 wt.%Diverse deposit types
Manganese____≤0.29 wt.% ___6.7 wt.%Epithermal
Cadmium _0.03-2.70 wt.% ___7.4 wt.%Skarn
Gallium ____≤3900 ppm __1000 ppmCretaceous veins (Freiberg) *
Germanium____≤1040 ppm __2700 ppmCretaceous veins (Freiberg) *
Indium_____≤124 ppm __5.87 wt.%Epithermal (Toyoha)
Copper____≤0.14 wt.% __3.62 wt.%Epithermal (Toyoha)
Silver____≤1800 ppm __1.20 wt.%Epithermal (Toyoha)

Table 1. Some data from a small sampling of the extensive literature on ZnS mineral chemistry, plus some unpublished data. MVT data still in compilation: to be updated through summer 2019. The extreme values were mostly taken from the excellent review by Cook et al. (2009). Some MVT and other data are from Ye et al. (2011). * See Bauer et al., 2019. NOTE that the extreme values are from unusual settings and compositions, and may only be reproducible on the microprobe scale of sampling, microns to mm at most (the same is true of the MVT numbers). In economic terms, a large tonnage of zinc ore which has lower but steady grades of the minor elements may be ideal for processors. The MVT deposits are relatively low-temperature, and so elevated levels of some elements such as indium and copper are not expected. However, MVT sphalerite may have high Fe and Cd and very significant Ga, Ge and Ag values. Again, remember that these values are for individual growth zones, points or small grains, and thus may not reflect the average in a nice fist-sized sample, never mind 1,000 tonnes of mill feed! Frenzel et al. (2016) performed a statistical analysis of a large dataset of sphalerite analyses, and concluded that analyses of (Ga + Ge + Fe + Mn + In) provide an empirical equation for sphalerite formation that can be used as a geothermometer.

These elements often form their own sulphides which are more or less analogous to sphalerite and wurtzite, e.g. alabandite (MnS), greenockite and hawleyite (CdS), cinnabar and metacinnabar (HgS). Divalent cations such as Zn, Cd and Hg substitute for one another in sphalerite-type semiconductors. Mn is another element in this class (Bernardini et al., 2004), as are Fe and Cu (Lepetit et al., 2003; Lusk and Calder, 2003). A rarer addition is indium (Shimizu and Morishita, 2012). Indium is a potentially valuable byproduct of zinc refining. The volcanogenic massive sulphide (VMS) ores of the Brunswick No.12 mine are In-enriched, mostly in the sphalerite, and the Zn concentrate averaged 220 ppm, according to the work of Corbett et al., 1997a,b).

Colour banding is very common in sphalerite, both within individual crystals and in larger masses. This has been described at Nanisivik, as noted above, at Pine Point (Katsev et al., 2001) and at the Navan deposit in Ireland (Gagnevin et al., 2014). In contrast, at the Bleiberg deposit in Austria, microglobular sphalerite, <0.2 mm in size, has been described and attributed to microbial action, much like the more widely observed framboidal (raspberry-like) crystal habit of fine-grained, sediment-hosted pyrite (Kucha et al., 2010).

The trace elements in sphalerite have their own economic uses, and their own key sources. In the case of gallium, the main primary sources are bauxite, Zn ores and coal fly ash. The principal source of Ga is the Bayer liquor generated by the leaching of bauxite, supplemented by the hydrometallurgical residues of zinc ore beneficiation. GaAs has a signal speed 5 times that of Si, thus its importance and likely burgeoning demand from the electronics industry (Lu et al., 2017).

References (n=21)

Bauer,ME, Burisch,M, Ostendorf,J, Krause,J, Frenzel,M, Seifert,T and Gutzmer,J (2019) Trace element geochemistry of sphalerite in contrasting hydrothermal fluid systems of the Freiberg district, Germany: insights from LA-ICP-MS analysis, near-infrared light microthermometry of sphalerite-hosted fluid inclusions, and sulfur isotope geochemistry. Mineralium Deposita 54, 237-262.

Bernardini,GP, Borgheresi,M, Cipriani,C, Di Benedetto,F and Romanelli,M (2004) Mn distribution in sphalerite: an EPR study. Physics and Chemistry of Minerals 31, 80-84.

Chamberlain,SC, Robinson,GW, Lupulescu,M, Morgan,TC, Johnson,JT and deLorraine,WM (2010) Cubic magnetite crystals from Balmat, New York. Mineral.Record. 41, 527-537.

Cook,NJ, Ciobanu,CL, Pring,A, Skinner,W, Shimizu,M, Danyushevsky,L, Saini-Eidukat,B and Melcher,F (2009) Trace and minor elements in sphalerite: a LA-ICPMS study. Geochimica et Cosmochimica Acta 73, 4761-4791.

Corbett,BE, Goodfellow,WD and Luff,WM (1997a) The distribution of indium and tin on the 1000 meter level in the Brunswick No.12 massive sulphide deposit, Bathurst mining camp, N.B. GAC/MAC Abs. 22, 30, Ottawa.

Corbett,BE, Goodfellow,WD and Luff,WM (1997b) The form, distribution and origin of indium in the Brunswick No.12 massive sulphide deposit, Bathurst mining camp, N.B. GAC/MAC Abs. 22, 30, Ottawa.

deLorraine,W (2001) Metamorphism, polydeformation, and extensive remobilization of the Balmat zinc orebodies, northwest Adirondacks, New York. In `Proterozoic Iron and Zinc Deposits of the Adirondack Mountains of New York and the New Jersey Highlands' (Slack,JF editor), SEG Guidebook 35 Part I, 25-54.

Easton,RM and Fyon,JA (1992) Metallogeny of the Grenville province. In `Geology of Ontario' (Thurston,PC, Williams,HR, Sutcliffe,RH and Stott,GM editors), OGS Spec.Vol. 4, part 2, 1217-1252.

Frenzel,M, Hirsch,T and Gutzmer,J (2016) Gallium, germanium, indium, and other trace and minor elements in sphalerite as a function of deposit type - a meta-analysis. Ore Geology Reviews 76, 52-78.

Gagnevin,D, Menuge,JF, Kronz,A, Barrie,C and Joyce,AJ (2014) Minor elements in layered sphalerite as a record of fluid origin, mixing, and crystallization in the Navan Zn-Pb ore deposit, Ireland. Econ.Geol. 109, 1513-1528.

Harding,WD (1951) Geology of the Olden-Bedford area. ODM Ann.Rep. 56 part 6, 100pp. plus map 1947-5, 1:63,360 scale.

Katsev,S, L'Heureux,I and Fowler,AD (2001) Mechanism and duration of banding in Mississippi Valley-type sphalerite. Geophys.Res.Letts. 28, 4643-4646.

Kucha,H, Schroll,E, Raith,JG and Halas,S (2010) Microbial sphalerite formation in carbonate-hosted Zn-Pb ores, Bleiberg, Austria: micro- to nanotextural and sulfur isotope evidence. Econ.Geol. 105, 1005-1023.

Lepetit,P, Bente,K, Doering,T and Luckhaus,S (2003) Crystal chemistry of Fe-containing sphalerites. Physics and Chemistry of Minerals 30, 185-191.

Lu,F, Xiao,T, Lin,J, Ning,Z, Long,Q, Xiao,L, Huang,F, Wang,W, Xiao,Q, Lan,X and Chen,H (2017) Resources and extraction of gallium: a review. Hydrometallurgy 174,105-115.

Lusk,J and Calder,BOE (2003) The composition of sphalerite and associated sulphides in reactions of the Cu-Fe-Zn-S, Fe-Zn-S and Cu-Fe-S systems at 1 bar and temperatures between 250 and 535°C. Chemical Geology 202, 319-345.

Shimizu,T and Morishita,Y (2012) Petrography, chemistry, and near-infrared microthermometry of indium-bearing sphalerite from the Toyoha polymetallic deposit, Japan. Econ.Geol. 107, 723-735.

Whelan,JF, Rye,RO and DeLorraine,W (1984) The Balmat-Edwards zinc-lead deposits - synsedimentary ore from Mississippi Valley-type fluids. Econ.Geol. 79, 239-265.

Wolff,JM (1982a) Geology of the Long Lake area, Lennox and Addington and Frontenac Counties. OGS Rep. 216, 76pp.

Wolff,JM (1982b) The Long Lake zinc deposit, description and classification. Econ.Geol. 77, 488-496.

Ye,L, Cook,NJ, Ciobanu,CL, Liu,Y, Zhang,Q, Liu,T, Gao,W, Yang,Y and Danyushevskiy,L (2011) Trace and minor elements in sphalerite from base metal deposits in south China: a LA-ICPMS study. Ore Geology Reviews 39, 188-217.

Graham Wilson, 25-27 November 2018, 05-07 December 2018, last updated 06-19 January 2019, 02 February 2019, minor edits on 20-25 May 2019

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