"Rock of the Month # 12, posted June 2002" --- Sample Nan-13,
courtesy Ross Sherlock, Canada-Nunavut Geoscience Office, Iqaluit.
Sphalerite is zinc sulphide, ideal formula ZnS, the principal ore mineral of zinc. A cubic mineral, specific gravity near 4.08, sphalerite is an economically- important sulphide which is generally translucent to transparent, unlike other opaque metallic ore minerals. Sphalerite may display a wide range of colours, from pale yellow or green to rich brown and black. This variability signals a diverse chemistry, which often incorporates high levels of iron (Fe), in some cases well over 10 wt.%. Other elements that often exceed 0.1 wt.% include cadmium (Cd), manganese (Mn) and silver (Ag). Also locally present at trace levels are gallium (Ga) and germanium (Ge), tin (Sn), lead (Pb) and mercury (Hg). The sulphur (S) content is quite constant, near 33 wt.%, thus the additional metals replace zinc in the crystal structure.
This sample, Nan-13, a sphalerite-rich banded zinc ore, contains approximately 85% sphalerite and 15% pyrite by volume. This image displays roughly 90% of a 40x23-mm polished thin section, roughly 30 microns (0.03 mm) thick, viewed in transmitted light, and captured by electronic scanning in "transparency mode". The sphalerite has complex concentric zoning, in which grains may develop dark and light zones to variable degrees. Fe contents drop from 6.3 wt.% to 0.1% from dark red cores to clear rims. Cd mirrors Fe, averaging about 0.2%. Ag is appreciable, averaging over 300 ppm, and there are erratic values of Ga, Ge and Sn. These elements were analysed in small crystals and individual zones using electron and proton microprobe techniques (see, e.g., Wilson et al., 2002).
The sample is from the west end of the Main Lens of the Nanisivik deposit, a rich deposit of lead and zinc at the northwest end of Baffin Island, in Canada's Nunavut Territory. It is an example of a "Mississippi-Valley Type" (MVT) lead-zinc-silver deposit, hosted by carbonate sediments of middle Proterozoic age, which is unusually old for MVT deposits. These sediments were deposited approximately 1200 million years ago, an upper bound on the age of the actual mineralization. The deposit lies in the Borden basin, in dolomites of the Society Cliffs Formation, within the compass of a Proterozoic rift structure (Olson, 1984; Arne et al., 1991; Ghazban et al., 1992; Kah et al., 2001; Leach et al., 2001; Sherman et al., 2002).
The ores of Nanisivik are mineralogically quite simple, but notable for the coarse grain size and spectacular crystal habit often exhibited by minerals such as pyrite, and for this reason specimens are prized by collectors (Gait et al., 1990). The Nanisivik mine recently reached the close of its economic life. However, its significance is such that a project to re-interpret the regional geological setting of the deposit, orchestrated by the Canada-Nunavut Geoscience Office, Iqaluit, will soon lead to fresh interpretations of the genesis of the famous lead-zinc deposit.
ARNE,DC, CURTIS,LW and KISSIN,SA (1991) Internal zonation in a carbonate-hosted Zn-Pb-Ag deposit. Nanisivik, Baffin Island, Canada. Econ.Geol. 86, 699-717.
GAIT,RI, ROBINSON,GW, BAILEY,K and DUMKA,D (1990) Minerals of the Nanisivik mine, Baffin Island, Northwest Territories. Mineralogical Record 21 no.6, 515-534.
GHAZBAN,F, SCHWARCZ,HP and FORD,DC (1992) Multistage dolomitization in the Society Cliffs Formation, northern Baffin Island, Northwest Territories, Canada. CJES 29, 1459-1473.
KAH,LC, LYONS,TW and CHESLEY,JT (2001) Geochemistry of a 1.2 Ga carbonate-evaporite succession, northern Baffin and Bylot islands: implications for Mesoproterozoic marine evolution. Precambrian Research 111, 203-234.
LEACH,DL, BRADLEY,D, LEWCHUK,MT, SYMONS,DTA, De MARSILY,G and BRANNON,J (2001) Mississippi Valley-type lead-zinc deposits through geological time: implications from recent age-dating research. Mineralium Deposita 36, 711-740.
OLSON,RA (1984) Genesis of paleokarst and strata-bound zinc-lead sulfide deposits in a Proterozoic dolostone, northern Baffin Island, Canada. Econ.Geol. 79, 1056-1103.
SHERMAN,AG, JAMES,NP and NARBONNE,GM (2002) Evidence for reversal of basin polarity during carbonate ramp development in the Mesoproterozoic Borden basin, Baffin Island. CJES 39, 519-538.
WILSON,GC, RUCKLIDGE,JC, CAMPBELL,JL, NEJEDLY,Z and TEESDALE,WJ (2002) Applications of PIXE to mineral characterization. Nuclear Instruments and Methods in Physics Research B 189, 387-393.
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