Abstract

The New Insco copper deposit is an Archean volcanogenic massive sulfide orebody situated on the northwestern edge of the Noranda mining district, in the Superior Province of the Canadian Shield. The ore occurs near the base of a thin unit of pillowed and brecciated basalt that overlies rhyolitic volcaniclastic rocks. Both the basalt and rhyolite are heavily chloritized and sericitized and form a zone of alteration that is depleted in SiO2, Na2O, and CaO and enriched in FeOT, MgO, K2O, Cu, Zn, and S. The units are separated from an underlying massive rhyolite by a diorite intrusion and from an overlying series of pillowed basalts by a fault.The orebody is a pyrrhotite-rich lens of massive sulfide, containing conformable bands of disseminated chalcopyrite and pyrite, that is interlayered with lenses of pyrite–chalcopyrite-bearing siliceous ore. The massive ore is interpreted to have been deposited in a distal environment with pyrrhotite as a primary mineral. It was recrystallized during regional metamorphism, which imparted a polygonal–granular texture to the pyrrhotite and formed overgrowths on pyrite crystals. Following Pleistocene glaciation, pyrrhotite was extensively replaced by pyrite, marcasite, iron oxides, carbonates, and hydroxides.

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