Abstract

The C zone, located in Bannockburn Township approximately 30 km W of Matachewan, Ontario, is one of several komatiite-associated Ni-Cu-(PGE) deposits within the 2710 to 2704 Ma Tisdale volcanic episode of the Abitibi greenstone belt. The sulfides, host rocks, and country rocks are superbly exposed over a strike length of 150 m in glacially polished and hydraulically stripped outcrops and have been intersected in 53 diamond drill holes. The mineralized zone is up to 2.5 m thick and comprises (from base to top) massive to semimassive, net-textured, and disseminated sulfide facies characterized by a pyrrhotite-pentlandite-chalcopyrite-magnetite assemblage. The massive sulfide zone contains an anastomosing network of dextral and sinistral shears that are oriented broadly subparallel to the contacts with the footwall dacite and hanging-wall andesite. Some contacts with underlying dacites are sheared, but others are scalloped and bordered by skeletal-euhedral Fe-rich chromites and appear to be primary magmatic features. The footwall rocks grade from chloritized dacites within 20 to 30 cm of the contact into massive and brecciated plagioclase-phyric dacites farther away from the contact. The host unit is up to 8 m thick and comprises massive olivine porphyritic and ortho- to mesocumulate komatiite in the eastern and central parts and a texturally heterolithic komatiite breccia in the western part. The breccia is composed of subrounded to subangular clasts 1 to 30 cm in length that exhibit mainly fine (<2 mm) olivine porphyritic or fine (<1 cm) random olivine spinifex textures, within a fine-grained, locally spinifex-textured ultramafic matrix. The absence of evidence for a pyroclastic origin and the presence of a spinifex-textured matrix suggest that the breccia is autoclastic. The hanging-wall rocks are barren differentiated (spinifex/cumulate) komatiite flows (total thickness of the lithostratigraphic package ~22 m) and variolitic pillowed andesites (total thickness >80 m). The komatiitic sequence at the C zone is much thinner than most other sequences within the Tisdale volcanic episode and is oriented oblique to the NW trend of the regional stratigraphy, but all contacts appear conformable and all younging indicators are uniformly to the SSE, suggesting that the local sequence is intact. The C zone is similar in many respects to other type I (Kambalda-type) komatiite-associated Ni-Cu-(PGE) deposits in the Abitibi greenstone belt but differs by not being confined within a well-developed footwall embayment and in being partly hosted by komatiitic breccias. The C zone Ni-Cu-(PGE) mineralization appears to be hosted in the eastern part of the stripped exposure by a thin lava pathway and in the western part by breccias formed via lateral breakout and roof collapse.

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