The porphyry of the Porcupine district, Ontario, has been considered pre-Timiskaming by some and post-Timiskaming by others. A detailed study reveals that it is the result of two processes—igneous intrusion and metasomatic replacement—separated by a period of deformation.

Igneous intrusion is indicated by the stock- and dikelike forms crosscutting enclosing rocks and by their content of inclusions. Relative age is shown by the structures; porphyry crosscuts Timiskaming sedimentary beds but in most places contains a linear schistosity. Therefore, the intrusion followed deposition of the Timiskaming rocks but preceded the conclusion of the Timiskaming deformation. A Keewatin-Timiskaming unconformity indicates that the Keewatin rocks were folded and then eroded to a depth of at least 8000 feet before Timiskaming deposition.

Metasomatic replacement of the schistose intrusive is suggested by the lack of orientation of the feldspar and quartz “phenocrysts,” their transection of the schistosity, and almost complete lack of strain. Similar features are common to “phenocrysts” in the porphyry of “ecole” structures, the porphyry fragments in Keewatin agglomerate, and the porphyry pebbles in Timiskaming conglomerate. In all these “fragmental” rocks, feldspar and quartz crystals, identical with those in the porphyry, occur in the matrix. Additional evidence of replacement is the presence in the stocks of lenses of carbonaceous porphyry which usually lie on the strike of carbonaceous schist lenses in the surrounding rocks. It is suggested that after the Timiskaming folding quartz and feldspar “phenocrysts” were deposited from permeating solutions. The Algoman granite is the probable source of the solutions.

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