Depositional setting and deformation of massive sulfide deposits, iron-formation, and associated alteration in the Manitouwadge greenstone belt, Superior Province, Ontario

Geological Survey of Canada, Continental Geoscience Division, Ottawa, ON, Canada
Western Carolina University, United States

The base metal deposits of the Manitouwadge greenstone belt provide a classic example of the difficulties encountered in petrogenetic studies of mineralization in complex metamorphosed and polydeformed terranes. The belt comprises a mafic to felsic volcanic succession in which the felsic rocks are interleaved with iron-formation and massive sulfide deposits. A synvolcanic trondhjemite was a magma source for felsic volcanism and heat source for hydrothermal activity. The belt was affected by four phases of ductile deformation, including D ₁ faulting that dissected the area of known economic mineralization into two tectonic blocks and repeated part of the mineralized sequence. The regional map pattern is determined by D ₃ folds, including the Manitouwadge synform. Based on Cu-Zn-Pb proportions, style of mineralization, and relationships to iron-formation and alteration zones, the orebodies can be divided into three main types: (1) Cu-rich stockwork-disseminated orebodies which are the lowest in the stratigraphy, hosted by the orthoamphibole-garnet-cordierite gneiss and, in the Geco mine area, by sillimanite-muscovite-quartz schist surrounding the main orebody; (2) massive and semimassive Zn-Cu-(Pb) orebodies on iron-formation horizons at middle stratigraphic levels--these include the two largest orebodies of the bolt; and (3) massive and semimassive Zn-Pb-(Cu) orebodies, interpreted as highest in the stratigraphy, which are hosted by iron-formation. Type 1 orebodies represent subsurface conduits for mineralizing fluids that supplied deposits higher in the stratigraphy. Type 2 orebodies are sea- floor precipitates or near-sea-floor replacement deposits, formed during the peak of hydrothermal activity and base metal deposition. Type 3 orebodies formed during waning hydrothermal activity. All rocks have been metamorphosed to upper amphibolite facies, including two zones of synvolcanic hydrothermal alteration. Orthoamphibole-garnet-cordierite gneiss forms a strata-bound sheet of regional extent, mantling synvolcanic trondhjemite in the stratigraphic footwall (structural hanging wall) to mineral deposits. The second alteration unit, sillimanite-muscovite-quartz schist, occurs in close proximity to massive sulfide deposits, mostly in the stratigraphic hanging wall (structural footwall). The precursors of altered units were mafic and felsic volcanic reeks. Despite the unusual extent and concordance of alteration zones at Manitouwadge, geochemical trends from least to intensely altered are similar to those recorded in alteration pipes in the Abitibi camp. Extensive orthoamphibole-bearing gneiss at Manitouwadge may be partly a function of high metamorphic grade, allowing orthoamphibole-hornblende coexistence in relatively calcic bulk-rock compositions. Bulk-rock compositions that produced orthoamphibole-bearing assemblages at Manitouwadge might, at a lower metamorphic grade, be considered incipient alteration. The strata-bound character of alteration suggests that hydrothermal activity was focused on aquifer horizons consisting of permeable, poorly consolidated volcaniclastic deposits.

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