When the country rock, which is invaded by a plutonic mass, is of a kind that permits extensive penetration by the magmatic vapours and gases, metamorphism may result for a considerable distance from the intruding igneous rocks. Among the new minerals which are generated along the contact zone, metallic ores may occur in sufficient quantity to be economically important. The minerals in question may be deposited in the interstices of the wall rock, or may replace that rock bodily for a greater or less distance from the actual contact. Ore bodies formed in this manner are usually characterized by the presence of garnets, oxides and sulphides of iron in association, and by fluorite and other minerals containing fluorine and boron. As we have seen (p. 409) there is a difference of opinion among geologists as to how far new mineral substances can be carried into the walls of country rock, but such a case as the Dolcoath mine in Montana, more than half a mile from the contact with the granite, which has been the chief agent in metamorphosing the district, is highy suggestive. "The ore-bearing stratum of the mine was originally a bed of impure limestone, which has been metamorphosed to garnet and pyroxene, with spots of calcite.

Associated with these gangue minerals are sulphide and telluride of bismuth, containing gold." (Spurr).