The Sudbury Impact Structure - New Insights Into the Origin and Emplacement of the Basal Onaping Intrusion and the Parkin, Trill and Foy Offset Dykes of the North Range

The 1.85 Ga Sudbury impact structure is considered a remnant of a multi-ring basin with an estimated original diameter of 150-200 km. The so-called “Basal Onaping Intrusion” form discontinuous sheets between the Granophyre of the Sudbury Igneous Complex (SIC) and the Sandcherry Member of the Onaping Formation and had been considered part of the complex breccia series of the Onaping Formation. Based on the investigation of core and field samples from the North Range we conclude that the Basal Onaping Intrusion, in fact, are the roof rocks of the SIC and, thus, may represent the initial bulk composition of the SIC. It should no longer be considered part of the Onaping Formation, but rather the uppermost member of the SIC, and we recommend “Upper Contact Unit” as a new name. The Offset Dykes are radial and concentric dykes around the SIC and are composed of the so-called Inclusion-rich Quartz Diorite (IQD) and Inclusion-poor Quartz Diorite (QD). Metabreccia (MTBX), an enigmatic and overlooked lithology observed within some dykes in the North Range had been sparsely studied in detail. This investigation of MTBX from the Parkin, Trill and Foy Offset Dykes indicates that MTBX originates from Footwall Breccia (FWBX) that was ripped off during the outward emplacement of the Offset Dykes, which is in contrast to previous studies. The fragments, clasts and blocks were included into the dyke melt and subsequently thermally metamorphosed resulting in an intensive recrystallized fabric within MTBX. A genetic relationship between MTBX and FWBX is supported by whole rock geochemical analyses and similarities in Ni-Cu-PGE mineralization. Oscillatory, compositionally zoned pyrites containing traces of Platinum-Group-Elements (PGEs) are observed within MTBX but not in QD or IQD. Additional PGEs are hosted within Pt-Pd-bismuthotellurides, with MTBX containing a higher amount of Te and Pt than QD/IQD. This study provides new insights into the field relationships, petrology, geochemistry and Ni-Cu-PGE sulfide mineralization of MTBX from the Parkin, Trill and Foy Offset Dykes in the North Range of the Sudbury impact structure compared to QD, IQD and FWBX