"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).

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.

This is a rather spectacular example of a banded calcite-galena-sphalerite vein from the old Frontenac lead mine, in Loughborough township, Frontenac county, Ontario. 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 1952, and three shafts were sunk on the calcite-rich vein: in addition to calcite and the two principal ore minerals, other reported minerals include pyrite, chalcopyrite, barite, cerussite and celestite (Sabina, 1983).

References

MALCZAK,J, CARTER,TR and SPRINGER,JS (1985) Base metal, molybdenum, and precious metal deposits of the Madoc - Sharbot Lake area, southeastern Ontario. Ontario Geological Survey Open File Report 5548, 374pp. plus map P2737, pp.84-89.

SABINA,AP (1983) Rocks and Minerals for the Collector: Kingston, Ontario to Lac St-Jean, Quebec. Geological Survey of Canada Misc.Rep. 32 (revised version of GSC Pap. 67-51), 130pp., p.21.

UDD,JE (1999) The Mines of Ottawa: A Guide to the Mineral Deposits of Southeastern Ontario and Southwestern Quebec. CJ Multi-Media Inc., 89 Ridgefield Crescent, Nepean, Ontario, 1st edition, 375pp., p.140.

UGLOW,WI (1916) Part of southeastern Ontario, showing relations of galena- calcite- barite veins to major elements of geologic structure. Ontario Bureau of Mines Map 25c, 1:500,000 scale.