"Rock of the Month # 48, posted June 2005" --- Sample GCW 2219. A striking banded calcite vein displays mm-thick bands of a) continuous rich brown sphalerite, b) isolated galena grains, and c) isolated, coarser grains of galena + sphalerite. Overall, the galena probably represents a higher lead content in the bulk ore than zinc, which is present in the sphalerite. The sample of vein material, originally brick-sized, has been broken and sawn: note the weathered surface (upper left), freshly-broken face (lower left) and the slowly-sawn but unpolished flat interior (upper right).

This is a zinc-lead vein, largely composed of coarsely crystalline, snowy-white calcite, with distinctive bands enriched in the common base-metal sulphides galena (PbS) and sphalerite (ZnS). The textures of the vein display some interesting features, including: 1) the host carbonate is generally much coarser than the sulphides; 2) some of the sphalerite forms continuous bands, suggesting plating of sulphide onto the veinlet walls; 3) the banding can be very fine (circa 1 mm) and evenly-spaced, which has implications for the conditions of deposition of the sulphides; and 4) some galena grains and bands display white to pale pinkish deposits, ascribed to subsequent partial alteration of the lead sulphide. As might be expected, the specimen is not appreciably magnetic, but reacts vigourously with dilute hydrochloric acid.

The piece is a rather spectacular example of a banded calcite-galena-sphalerite vein from the old Frontenac lead mine, in central Loughborough township, Frontenac county, Ontario. The mine (also known as the Forbes Galena Mine or Draper Lake Frontenac Lead-Zinc Mines Ltd) lies west and northwest of the village of Perth Road, between Kingston and Sharbot Lake.

The Proterozoic Grenville shield in southeast Ontario and southwest Quebec is home to many hundreds of old abandoned mines, many so small and overgrown as to be hard to locate. This particular mine was the first producing lead mine in Ontario (Udd, 1999). It saw intermittent exploration and production from 1866 to 1958, and three shafts plus a number of pits and trenches were sunk on two calcite-rich vein systems (Malczak et al., 1985). Production occurred in the periods 1866-1882 and 1903-1917. The commodities from this style of carbonate-hosted vein system include lead and zinc with values of other metals such as silver, and also barite (Uglow, 1916; Thomson et al., 1957). In addition to calcite and the two principal ore minerals, other reported minerals include pyrite, chalcopyrite, barite, cerussite and celestite (Sabina, 1983). Older work compiled by Malczak et al. lists the ore minerals, in descending order of abundance, as galena, sphalerite, then pyrite, graphite and marcasite. Similar calcite occurs in other veins in southeast Ontario and upstate New York, notably at the Rossie lead mine in St. Lawrence county (Robinson et al., 2001).

The north vein, accessed by shafts 2 and 3, was 1-44 feet (0.3-13.4 m) wide, averaging 10 feet (3.05 m). The host rock is Grenville marble, with gneisses to the southeast, and minor granite, with some unconformable cap rock of Cambrian sandstone. Both the samples shown here correspond to descriptions of the north vein (ibid., p.88). This vein strikes N70°W and dips 80°N. Most sulphides were apparently found toward the footwall side of the vein. Open spaces might contain celestite and marcasite. The south vein, accessed by the No.1 shaft, which was 313 feet (95 m) deep, was 1 to 22' (0.3-6.7 m) wide.