Abstract

A disseminated copper deposit occurs in a lens of Archean felsic schist of probable volcanic origin in the Timmins area of northeastern Ontario. The Cu-Mo-Au mineralization, alteration, and alteration-mineralization zoning are comparable in some respects to characteristic features of younger porphyry copper deposits. However, structural, petrographic, and geochemical data suggest that the deposit, as it now exists, may not have evolved in a manner similar to most porphyry copper deposits. The host felsic schist was emplaced during early Archean volcanism, suffered two periods of deformation, and was then altered and mineralized probably during the late Archean. The porphyroidal character, present only in certain parts of the schist, was not a primary feature of the host rock but resulted from metasomatism accompanied by mineralization occurring either during or after the final stage of deformation. The timing of the mineralization-alteration event with respect to host rock emplacement differs from that in many porphyry systems. In addition, the host rock is probably volcanic rather than intrusive and was not the source of the metasomatising fluids. No intrusive source can be identified.

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