Sphalerite, principal ore mineral of zinc

--- from the former Long Lake mine, southeast Ontario, Canada

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Figure 1. Semi-massive, granular brown sphalerite ore from the old Long Lake mine, a 1970s zinc producer near the city of Kingston. Rich zinc ore, with scattered pyrite and carbonate. This material, like the sample from Balmat (Fig. 2, below) has become crumbly after 15-20 years in storage. 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.

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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.

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 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 a 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., 2018).

Notes on 18 elements present at percent to ppm levels, in sphalerite grains from diverse geological environments, are provided by Cook et al. (2009). 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% or even <10% 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). The mineral colour is influenced by impurities, e.g., black "marmatite" contains 6% or more Fe. In contents are lowest in ores rich in Ge and Ga. 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 cadmium). 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.

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: Iron Cadmium Gallium Germanium Indium Silver Mercury

References (n=xxx)


Bauer,ME, Burisch,M, Ostendorf,J, Krause,J, Frenzel,M, Seifert,T and Gutzmer,J (2018) 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, 26pp., accepted 15 October.

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.

Wilson et al., unpublished data.

Graham Wilson, 25-27 November 2018, 05,07 December 2018

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