Abstract:

The ca. 2711 Ma Levack gneiss complex constitutes the footwall of the 1.85 Ga. Sudbury Igneous Complex in the North Range of the Sudbury impact structure. It consists of supracrustal hornblende-pyroxene quartzofeldspathic gneisses that were intruded by dioritic and granodioritic bodies. The complex has a composite S1/S2 gneissic foliation, which is associated with the development of tight F2 folds and is refolded by younger, open to tight, NW-trending F3 folds.<br/>Sudbury breccia is the most common impact feature in the North Range. Breccia veins and bodies generally cut foliation in the Levack gneiss complex, but they also commonly follow structural planes of weakness, including lithological contacts, foliations, and diabase dykes. Minimal slip displacements were involved in the formation of the breccia, as shown on outcrop by cm-scale offset of markers along breccia veins and at map-scale by the consistency in the orientation of Archean structures across the study area. Clasts within the breccia are cut by healed microfractures and individual quartz and plagioclase grains in the clasts have patchy undulose extinction and are partially replaced by very fine-grained, irregular grains, suggesting that the breccia originated by microfracturing and brittle comminution of the host Levack gneiss. The matrix of the breccia is higher in Fe2O3, MnO, TiO 2, MgO, and CaO and lower in SiO2 relative to enclosing hosts due to preferential brittle comminution and frictional melting of mafic minerals and plagioclase during in situ brecciation of their host rocks. These observations, together with the increase in abundance of breccia in the footwall rocks immediately below the SIC, suggest that the breccia veins and bodies in the study area formed by shear along both new shear fractures and preexisting surfaces during the initial propagation of shock waves and elastic rebound of the floor of the transient crater. This interpretation differs from the more generally accepted "superfaulting" hypothesis that invokes km-scale slip on rim-collapse listric normal faults as the main mechanism for formation of Sudbury breccia.<br/>Two styles of sulphide mineralisation are exposed at surface in the study area: (1) SIC-footwall contact mineralization at the Levack portal, and (2) Cu-PGE-rich footwall mineralization at the Barnet showing. The Levack portal comprises a semi-massive pyrrhotite-pentlandite-chalcopyrite sulphide body from which sulphide veins extend into the footwall gneisses. The Barnet showing consists of Cu-PGE-rich veins hosted by conjugate fractures that cut across both clasts and matrix of a wide zone of Sudbury breccia. The veins were emplaced after the consolidation of the breccia in fractures that likely formed during isostatic readjustment of the floor of the crater below the cooling SIC.