The geology of the copper deposits now being developed by Gaspe Copper Mines in the Gaspe Peninsula is summarized in this paper with attention restricted to metamorphism in the host rocks associated with ore replacement. The ore occurs in faulted and gently folded Palaeozoic sediments and seems derived from a buried stock of granite. The best ore-body is a replacement in a limestone bed and lies up-dip from the intrusive. The deeper end of this limestone ore approaches within half a mile of the granite and the upper end is one and one-quarter miles away, with the ore confined to an area of lime-silicate alteration. More siliceous sediments above and below the limestone bed have been bleached and silicified over a two-square mile area. All known ore deposits in these rocks are within the altered zone. The chemical changes accompanying hydrothermal alteration in these ore horizons, which were originally different in composition, are compared. In each case a marked net gain in silica and a corresponding loss in lime is indicated. Quantitatively some 500 tons of silica seem to have been deposited in the host rocks for every ton of copper. In rocks low in calcite, the silicification is widespread and is not a specific guide to the location of ore within the zone of alteration. In limestone rocks alteration is less widespread and is a useful guide in locating ore.

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