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Hypervelocity impacts frequently result in the formation of dikes in the crater floor and central uplift. Impact melt-bearing dikes in large terrestrial impact basins – such as the Offset Dikes at the Sudbury impact structure – occur at a large scale, often tens of meters wide and several kilometers in length. The Offset Dikes host significant Ni–Cu–PGE deposits, which include several well-known mines, such as Totten and Copper Cliff that have been mined for nearly 100 years. The Offset Dikes typically consist of a clast- and sulfide-rich core and clast-poor margins. Their formation has been a subject of debate for decades. The most widely proposed model is the early emplacement of clast-poor impact melt shortly after the impact event, followed by the later emplacement of clast- and sulfide-rich impact melt in the center of the dike. An alternative hypothesis is that a single pulse of clast-rich impact melt flowed into the fractured target rocks and flow differentiation resulted in a clast-rich core and a clast-poor margin. In this study, we examine field and petrographic relationships of the Foy Offset Dike to better understand its emplacement. Our results show that the characteristics of the Foy Offset Dike – namely the gradational nature of the contact between the clast-rich and clast-poor phases, the alignment of clasts sub-parallel to this contact, the geochemical similarities and the presence of sulfides within both phases of the dike – are more consistent with the single injection and flow differentiation hypothesis.

The Offset Dykes are impact melt-bearing dykes related to the 1.85 Ga Sudbury impact structure. Currently, the dykes extend radially outward from—or occur concentrically around—the Sudbury Igneous Complex, which is the remnant of a differentiated impact melt sheet and the source of the dykes. The recently identified three Pele Offset Dykes intrude into the Archean rocks north of the Sudbury Igneous Complex. In this study, the Pele dykes are characterized for the first time by a combination of fieldwork, optical microscopy, electron microprobe analyses, and bulk geochemical analyses. The Pele Offset Dykes stand out from the other Offset Dykes at Sudbury in two significant ways: (i) All other known Offset Dykes consist of an inclusion-rich lithology in the center of the dyke and an inclusion-poor lithology along the margins. The Pele dykes, however, are only composed of the inclusion-poor phase. (ii) The Pele dykes—particularly the Central and Eastern dykes—have a more evolved chemical composition relative to the other Offset Dykes. These observations suggest that the Pele dykes were emplaced after the other known Offset Dykes during two injection events: the Western followed by the Central and Eastern Pele dykes.

La mine Thayer Lindsley, situee sur le flanc sud du complexe igne de Sudbury (1.85 Ga), fournit un exemple d'une mineralisation en sulfures de Ni-Cu-EGP pres du contact et en sulfures de Cu-Ni-EGP dans la paroi inferieure (EGP: elements du groupe du platine). Ici, nous traitons des zones de minerai pres du contact; elles sont affectees par des zones de cisaillement dans un encaissant recristallise au facies amphibolite inferieur; celles -ci deplacent le contact entre le complexe igne et les roches paleoproterozoiques de la Province du Sud. Le minerai fortement deforme est largement appauvri en Ni et enrichi en Pt et Pd par rapport au minerai non deforme; de plus, la pentlandite se voit remplacee par la pyrrhotite, et la pyrrhotite, par pyrite et marcasite. Quoique les variations dans la teneur en metal de minerai sulfure magmatique soient en general attribuables a des processus primaires, impliquant par exemple le facteur R, la cristallisation fractionnee d'une solution solide monosulfuree, et des variations en fugacite de l'oxygene, f(O 2 ), aucune de ces hypotheses peut rendre compte de la teneur variable en Ni des minerais a la mine Thayer Lindsley. L'appauvrissement en Ni dans le minerai deforme refleterait plutot le remplacement de la pentlandite par la pyrrhotite, accompagne d'une remobilisation du Ni ainsi libere. Le taux de remplacement de la pentlandite est plus marque dans le minerai le plus fortement deforme; il est associe a une forte densite de veines de quartz syntectoniques. Une permeabilite accrue au cours du cisaillement a facilite le remplacement de la pentlandite et la mobilisation ou la perte de Ni grâce aux fluides en circulation dans les assemblages metamorphiques encaissants aux conditions du facies amphibolite.

The Thayer Lindsley mine is located in the South Range of the Sudbury impact structure, near the contact between the 1.85 Ga Sudbury Igneous Complex (SIC) and the Paleoproterozoic Southern Province. Ni–Cu ore zones at the mine are strongly deformed within a southeast-dipping, lower amphibolite-grade shear zone, which offsets the contact between the SIC and Southern Province rocks. Numerous shear sense indicators, including shear bands, drag folds, and δ- and σ-type rotated porphyroclasts, consistently indicate south-over-north, reverse, dip-slip movement parallel to the mineral stretching lineation in the shear zone. The attitude, slip direction, and metamorphic grade of the shear zone are similar to those of the regional northeast-striking South Range Shear Zone that formed during post-impact, northwest-directed ductile contraction of the Sudbury impact structure. The South Range Shear Zone is generally interpreted as a ca. 1.9–1.8 Ga Penokean structure. Anhedral brown titanite grains from the Thayer Lindsley shear zone yield a mean 207Pb/206Pb Penokean age of 1815 ± 15 Ma. These grains are mantled by younger, syntectonic, colourless titanite, which have a mean 207Pb/206Pb age of 1658 ± 68 Ma. This younger age suggests that the South Range and Thayer Lindsley shear zones may have formed during a 1.7–1.6 Ga collisional tectonic event that is recorded along the southeast margin of Laurentia from the southwest USA. (Mazatzal Orogeny), through the mid-continent to Wisconsin, and as far northeast as Labrador (Labradorian Orogeny). 40Ar/39Ar analyses indicate post-tectonic thermal resetting of biotite occurred at 1477 ± 8 Ma during felsic plutonism across the Sudbury area.