"Rock of the Month # 42, posted December 2004" ---

Pyrite, FeS₂ is the most abundant of sulphide ore minerals. This material is from the famous Nanisivik Mississippi-Valley Type (MVT) lead-zinc deposit hosted by carbonate strata of the Borden basin. It was collected by Ross Sherlock from the soon-to-close mine site in June 2002. The Canadian 1-cent coin has a diameter of roughly 18 mm.

Sample 2364 (right) is a 7x4x4-cm mass of pyrite, displays complex bevelled and striated cubes of pyrite up to 8 mm in diameter on a fine-grained pyrite matrix. Sample 2365 (left) is a spectacular 9x8x5-cm mass of complex pyrite and rounded, highly modified crystals of black, resinous sphalerite (ZnS) on a bed on fine-grained, somewhat vuggy pyrite. Note the equant grains of striated pyrite to 11 mm in diameter, and dark gunmetal-grey sphalerite grains up to 6 mm in size. The samples also contain interstitial white, dolomitic carbonate.

This page is dedicated to Frank W. Beales (1919-2004), a pioneer in Canadian sedimentology and acknowledged authority on basinal brines and the sedimentological controls on the origins of base-metal deposits, particularly MVT deposits such as Nanisivik. A "research bibliography" of Frank and his many students and associates has been posted on this site.

The striking Nanisivik pyrite crystals tell a complex tale. In particular, the well-known pyrite pseudomorphs after marcasite are explained as pyrite replacements after marcasite, modified in turn by an epitaxial overgrowth of equant pyrite crystals (Gait and Dumka, 1986).

Long considered to be of Proterozoic age, like the petroliferous dolostone host rocks of the Society Cliffs Formation, recent geological evidence constrains the deposit to a younger time. It postdates the Franklin intrusions (circa 723 Ma). Hydrothermal alteration identified with the mineralizing event includes alkali feldspar which has been dated, pinpointing the time of ore formation to 461 Ma, in middle Ordovician time (Sherlock et al., 2004).

References

Gait,RI and Dumka,D (1986) Morphology of pyrite from the Nanisivik Mine, Baffin Island, Northwest Territories. Can.Mineral. 24 Pt.4 (Ferguson Volume), 685-688.

Sherlock,RL, Lee,JKW and Cousens,BL (2004) Geologic and geochronologic constraints on the timing of mineralization at the Nanisivik zinc-lead Mississippi Valley-type deposit, northern Baffin Island, Nunavut, Canada. Econ.Geol. 99, 279-293.