United Reef | Sudbury Geology

Regional Geological Setting, Sudbury Structure. view

Sudburry Mining Camp

To date, the Sudbury mining camp has produced in excess of 16 billion pounds of nickel, 15 billion pounds of copper, 85 million ounces of silver, 17 million ounces of platinum, and 3 million ounces of gold and remains, to this day, Canada's principal producer of platinum. Currently, there are 35 producing mines in the Sudbury Camp.

Geology of the Sudbury region

The Sudbury region is dominated by a large 60 km long by 30 km wide elliptical depression known as the Sudbury Structure which lies at the junction of three unique geological-structural provinces: the granitic & gneissic basement rocks of the Archean Superior Province to the north, the supracrustal metasediments and metavolcanics of the Early-Proterozoic Southern Province to the south, and the Middle-Proterozoic Grenville Front Tectonic Zone.

Geology of the Sudbury Structure. view

Geology of the Sudburry structure

The Sudbury Structure constitutes the largest known concentrations of nickel-copper-PGE bearing sulfide minerals in the world. Due to its economic importance, the structure is one of the most intensively studied and documented regions of the Canadian Shield.

Stratigraphically, from top to bottom, the Sudbury Structure consists of the Whitewater Group of sediments, the underlying Sudbury Igneous Complex (“SIC”), and brecciated footwall rocks surrounding the SIC.

The Whitewater Group

Infilling the central depression of the Sudbury Structure are the Whitewater Group sediments that consist of, from top to bottom, the Chelmsford Formation greywacke, the Onwatin Formation manganese-rich slate, and the Onaping Formation volcaniclastic/breccia sequence.

The Sudbury Igneous Complex

Underlying the Whitewater Group is the Sudbury Igneous Complex (“SIC”). The SIC consists of a lower zone of augite-bearing norite; a thin middle layer (Transition Zone) consisting of norite grading upwards into quartz gabbro; and an upper zone of micropegmatite/granophyre. At the base of the lower zone is a discontinuous zone of inclusion and sulfide-rich norite-gabbro commonly known as the Contact Sublayer (“Sublayer”). The Sublayer occurs as gently dipping sheets or irregular lenses along the base of the SIC or as small bodies in radial depressions or troughs in the base of the SIC called embayments, and as steeply dipping dikes called offsets which intrude into the adjacent footwall. The Sublayer is typically gabbroic in the base of the SIC and in the embayments, and typically quartz-diorite in the offset dikes. All Ni-Cu-PGE deposits of the Sudbury Structure are contained within the Sublayer and related structures such as the offset dikes.

There are two types of offset dikes: 1. radial, which appear to stem directly from the Sublayer and intrude into the footwall rocks radially away from the SIC. 2. concentric dikes, which are thought to be related to ring faults and may be connected to the Sublayer at depth or represent accumulations of melt rock associated with pseudo-tachylyte formation.

Origin of the Sudbury Structure

Existing evidence for the origin of the Sudbury Structure supports a meteorite impact. This includes: the irregular and dike-like bodies of pseudo-tachylyte breccias (Sudbury and Footwall breccias) up to 70 km from the margins of the structure; shatter cones in rocks marginal to the structure; the 1.8 km-thick volcaniclastics/breccias of the Onaping formation (interpreted as fallback breccia); and shock deformation lamellae in quartz and feldspar in country rock inclusions within the Onaping formation. The following summarizes the evolution of the Sudbury area.

Date (Ga) billion of years ago	Description of Events
>2.70	The Sudbury region is dominated by Superior Province Archean basement rocks consisting predominantly of felsic plutons and gneisses, with lesser amounts of mafic volcanics and sediments (greenstone).
2.64	Late Archean metamorphism produces the Levack Gneiss Complex and associated anatectic granitoid rocks.
2.49 to 2.45	Proterozoic Huronian sedimentation and volcanism begin. Gabbroic intrusions are emplaced to the southwest and west of what will be the Sudbury Structure (the East Bull Lake and Shakespeare-Dunlop Intrusions). Both intrusions are believed to be cogenetic with the lowermost volcanics of the Huronian.
2.45	The northwest-trending Matachewan dike swarm intrudes the area.
2.20	Huronian sedimentation and volcanism continued to about 2.2 Ga, largely to the south of the Sudbury area. Sediments were derived from the Archean Superior Province to the north. The Nipissing Diabase sill-dike system intrudes the area.
1.85	Sudbury Meteor Impact Event The impact affects a large area both inside and outside the current limits of the Sudbury Structure. Estimates of the original diameter of the impact structure range from 150 to 225 km based on the spatial distribution of shatter cones and shock metamorphic features. The Structure is composed of 3 principal components: The Whitewater Group of sediments The SIC, an intrusion or melt sheet, that is presently exposed in the form of an elliptical collar around the Sudbury Structure. The SIC is interpreted either as an endogenic intrusion or a melt sheet formed by meteorite impact, or a combination of the two. Current thinking generally favors a melt sheet origin whereby the main mass of magma was generated in-situ as a result of the meteor impact (i.e. melting of the continental crust). It is also believed that some component of the melt was also derived from the mantle which mixed with the crustal melt generating an immiscible sulfide-rich magma, which due to its higher density, settled out along the base of the impact crater forming the Sublayer. An outer zone up to 80 km wide consisting of fractured and locally brecciated and partially melted Archean and Proterozoic rocks which have been shock deformed by the impact. The impact also resulted in the formation of a radial and concentric pattern of offset dikes and zones of pseudo-tachylyte within the surrounding Archean and Proterozoic rocks such as the South Range Breccia Belt (SRBB)
1.70 to 1.90	During and after its formation, the Sudbury Structure was affected by the Penokean Orogeny. The orogeny provided a northwest compressional stress that is believed to be responsible for northwest-southeast shortening of the SIC and Sudbury Structure as a whole; contributing to its current elliptical shape.
1.0	Grenville Orogeny obliterates many impact features to the southeast.

Mineralization and deposit types of the Sudburry structure

Sudbury ores are typically zoned. Fractional crystallization of a monosulfide solid solution from a sulfide melt is believed to have given rise to a cumulate phase rich in Fe, Co, Rh, Ru, Ir and Os, (pyrrhotite-rich ores) and a fractionated liquid rich in Ni, Cu, Pt, Pd, and Au (chalcopyrite and PGE-rich ores). In some cases, the liquid phase is then believed to have migrated out from the Sublayer and further fractionated to form Cu and PGE rich footwall ores.

Common Ni and Cu-ore minerals consist of pyrrhotite, pentlandite, chalcopyrite with minor pyrite, and cubanite.

Sudbury Ni-Cu-PGE sulfide mineralization occurs in three deposit settings:

Contact deposits or “embayment” deposits, are located along the lower contact of the SIC in association with the norite-gabbro inclusion-bearing Sublayer. The Sublayer may be up to 100 metres thick. The greatest thicknesses are found in kilometer-size radial embayments within which are smaller, secondary troughs or “terraces”. The highest sulfide concentrations in the Sublayer are found within these embayments where sulfide distribution is further controlled by the terraces. Large concentrations of sulfides and nickel are often found in footwall deposits immediately adjacent to the terraces. Cu/Ni ratios are typically lowest in the Sublayer and increase towards the Footwall Breccia.
The Sublayer constitutes a well defined exploration target and has been a prolific producer over the years. Consequently contact deposits comprise 21 of the 35 mines in the Sudbury area. Contact deposits at the base of the SIC are still currently being mined by both Falconbridge and Inco at the Falconbridge, Garson and Levack mines.
Footwall deposits are zones of sulfide mineralization in the form of stringers, veins, massive sheets and/or disseminated sulfide which appear to have migrated outwards from the Sublayer and/or Footwall Breccia and penetrated deeply into the footwall rocks. The Frood-Stobie Mine, which is estimated to have originally contained a geologic resource of 450 to 500 million tonnes, is the largest and best example of a footwall deposit. This mine lies at the east end of the South Range Breccia Belt and is situated almost 2 km into the footwall.
Offset Dike deposits are associated with radial and concentric quartz-diorite dikes that extend from the Sublayer into the footwall rocks. Mineralization typically occurs as disseminated to massive sulfides within the dikes. The massive sulfide bodies are often rimmed by a halo of disseminated material that is often found along the contacts of the dike. Examples of offset deposits include Nickel Offsets along the Foy Offset dike, and the Copper Cliff North and South mines and the Totten Mine along the Copper Cliff Offset and Worthington Offset dikes, respectively.

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