The Sudbury Structure

The Sudbury Structure is a unique geological feature located in the Southern Province just to the southwest of the Abitibi subprovince. It is famous for its mineral deposits which have made it a world-scale producer of nickel, copper, cobalt and platinum group elements. Within the Sudbury corridor, LITHOPROBE's objectives were to establish the origin of the structure (which is still debated) by delineating the three-dimensional characteristics of the region; and to establish a framework for geotectonic understanding that will further exploration/development efforts by industry and the Ontario Geological Survey.

The Sudbury Structure is situated at the contact between Early Proterozoic Huronian supracrustal rocks of the Southern Province and Archean basement rocks of the Superior Province. About 15 km southeast of the Sudbury Structure, the Middle Proterozoic Grenville front tectonic zone (GFTZ) truncates the older structural provinces. The Sudbury Structure proper contains the Whitewater Group, which infills a central depression, the underlying SIC, and brecciated footwall rocks around the SIC. The Whitewater Group comprises breccias of the Onaping formation, pelagic metasedimentary rocks of the Onwatin formation, and metawackes of the Chelmsford formation. The SIC includes the Sublayer, Norite, quartz Gabbro, and Granophyre. Footwall rocks consist of Archean granitic and mafic igneous rocks, including granulite facies rocks of the Levack gneiss complex to the north of the Structure, and metavolcanic and metasedimentary rocks of the Huronian Supergroup to the south.

The 1.85 Ga age of the SIC coincides with a major episode of tectonic assemblage in North America. Existing evidence strongly supports a meteorite impact origin and includes: irregular and dyke-like bodies of pseudo-tachylite breccia (Sudbury breccia) in country rocks up to 70 km from the margin of the structure; shatter cones in rocks marginal to the structure; the 1.8 km-thick suevite like breccias of the Onaping formation; and shock deformation lamellae in quartz and feldspar in country rock inclusions within the Onaping formation. The current elliptical shape of the SIC is the result of northwest-trending compression during the 1.8 Ga Penokean orogeny. Deducing the original three dimensional shape of the structure by unraveling the deformation of the southern rim is of critical importance to distinguishing genetic and ore emplacement models.

Due to its economic importance, the Sudbury structure is one of the most intensively studied regions of the Canadian Shield. LITHOPROBE, in association with industry and the provincial government, completed a three-dimensional survey of the structure based on a series of studies from 1991- 1994. Knowledge of the surface and near-surface geology is well developed as a result of exploration activity. Many holes have been drilled, some to as deep as 2.2 km. This data-base enabled an extraordinary degree of three-dimensional control on the LITHOPROBE survey. A newly defined structural model provided strong support for the meteorite origin and has resolved two major enigmatic features, namely the elliptical shape of the structure, and a large hidden high-density mass which is no longer required to explain the gravity data, that had been cited as objections to the crater hypothesis. The preliminary results demonstrated that the ore-bearing norite-granophyre boundary could be imaged seismically, and also revealed complex structural relations in the Southern Ranges of the Sudbury; the key to three-dimensional reconstruction of the Sudbury structure lay in unraveling the geometry of its southern margin. LITHOPROBE participated with the mining industry in a three dimensional reconstruction of strategic areas in the South ranges, and the mining industry have themselves independently commissioned a series of seismic reflection studies of this region.