Partial melting and melt segregation in footwall units within the contact aureole of the Sudbury Igneous Complex (North and East Ranges, Sudbury structure), with implications for their relationship to footwall Cu–Ni–PGE mineralization

Abstract

We performed detailed field and drill core mapping of partial melting features and felsic rocks (footwall granophyres, FWGRs) representing segregated and crystallized partial melts within the contact aureole of the Sudbury Igneous Complex (SIC) in the 1.85 Ga Sudbury impact structure. Our results, derived from mapping within the North (Windy Lake, Foy, Wisner areas) and East Ranges (Skynner, Frost areas) of the structure, reveal that partial melting was widespread in both felsic and mafic footwall units up to distances of 500 m from the basal contact of the SIC. Texturally and mineralogically, significant differences exist between rocks formed by partial melting within and between localities. In general, however, melt bodies are dominated by different quartz-feldspar intergrowths (e.g. granophyric, graphic) and miarolitic cavities up to 5 cm in diameter. Major and trace element compositions of Wisner and Frost FWGRs imply that they crystallized from melts dominantly derived from partial melting of felsic Levack Gneiss and Cartier granitoid rocks, as well as from gabbroic rocks only at Frost. These results accord with our observations on in situ partial melting features and crystallized melt of microscopic scale in both felsic and mafic rocks. We conclude that partial melting occurred at a pressure of 1.5 ± 0.5 kbar and at temperatures up to 750°C in the Wisner area and up to 900°C in the Frost and Windy Lake areas. Segregations of partial melt into veins and dikes are present in all localities, and were promoted by deformation of the Sudbury structure in the Penokean orogeny as indicated by dominant strike directions. Whereas veins and dikes reflect brittle conditions during melt migration, sheared melt pods in the Sudbury breccia matrix indicate ductile conditions during their crystallization. Our results suggest a close genetic association of partial melting, melt segregation, and hydrothermal processes responsible for remobilization of Cu–Ni–PGE sulphides into and within the SIC footwall.