Abstract:

The C Zone, located in Bannockburn Township ~30 km west of Matachewan, Ontario, is one of several komatiite-associated Ni-Cu-(PGE) deposits within the 2710-2703 Ma Tisdale assemblage of the Abitibi greenstone belt. The ores, host rocks, and country rocks are superbly exposed over a strike length of 150m in glacially-polished and hydraulically stripped outcrops, and have been intersected in 53 diamond drill holes. The mineralized zone is up to 2.5m thick and comprises (from base to top) massive to semi-massive, 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 20-30 cm of the contact into massive and brecciated plagioclase-phyric dacites further away from the contact. The host unit is up to 8m thick and comprises massive olivine porphyritic and ortho- to meso-cumulate komatiite in the eastern and central parts, and a texturally-heterolithic komatiite breccia in the western part of the stripped exposure. The komatiite breccia is composed of subrounded to subangular clasts 1-30 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 ~22m) and variolitic pillowed andesites (total thickness >80m). The komatiitic sequence at the C Zone is much thinner than most other sequences within the Tisdale assemblage and its orientation is 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 AGB, 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 is interpreted to be hosted in the eastern part of the stripped area by a thin lava pathway and in the western part by komatiite breccias formed via lateral breakout and roof collapse.