Introduction

The Sudbury Igneous Complex is a 1.85 Ga elliptical rock body that was formed by a meteorite impact event located on the boundary between the Superior Craton and the Proterozoic Huronian Belt. The Complex is located 400 kilometers north of Toronto and is host to world-class nickel sulphide deposits, it rivals as one of the longest continual operating mining camps (Source: Lightfoot, P. C. Advances in Ni-Cu-PGE Sulphide Deposit Models and Implications for Exploration Technologies).

Geological of the Sudbury Igneous Complex
Figure 1: Geological of the Sudbury Igneous Complex, showing the location of the Sublayer and Offset dykes at the outer margin of the Main Mass (Source: Modified after Ames et al, 2006)

Sudbury Igneous Complex: Geology

The Sudbury Igneous Complex is subdivided into a lower sequence of noritic rocks, a central sequence of gabbroic rocks, and an upper sequence of granophyre. The contained nickel sulphide mineralisation is located proximal to the base of the Sudbury Igneous Complex in three principal environments, which are as follows:

  1. The Sublayer, a discontinuous unit of inclusion-rich igneous-textured norite and metamorphic-textured granite located in depressions at the base of the SIC. The mineralisation in the noritic Sublayer is typically disseminated or forms the matrix to a breccia; leucocratic footwall breccias often contain lenses of massive and disseminated sulphides. Mineralisation occurs in both small embayments (~500 m wide, 500 m long, and ~200 m deep) and sometimes as more continuous zones within troughs (~1 km wide, ~1 km deep, and extending for over 3 km), but there is no clear relationship between the scale of these embayment features and the size of the ore deposit.
  1. Mineralisation associated with radial dykes of quartz diorite and in association with quartz diorite in a concentric breccia belt that flanks the southern part of the Sudbury Igneous Complex. These dykes are termed ‘Offset Dykes’. Within the Offset Dykes and Frood-Stobie Breccia Belt the mineralisation occurs in plunging lenses of inclusion-rich quartz diorite distributed at irregular intervals along the Offset Dykes.
  1. The immediate footwall of the Sudbury Igneous Complex is commonly strongly brecciated with the development of pseudotachylite vein systems; these vein systems are cross cut by a Cu-rich vein style of mineralisation that is locally referred to as “footwall mineralisation”. The mineralisation forms sharp-walled veins that cross cut the Archean gneisses for distances of several hundred metres away from the original base of the Sudbury Igneous Complex and as continuous zones for distances of up to 1 km; locally these veins are associated with patches of trace disseminated sulphides that carry elevated precious metal abundance levels.

(Source: Lightfoot, P. C. Advances in Ni-Cu-PGE Sulphide Deposit Models and Implications for Exploration Technologies).

 Stratigraphic and geological relationships in the Sudbury Igneous Complex
Figure 2: a) Stratigraphic and geological relationships in the SIC and their interpretation according to the meteorite impact hypothesis (based on Grieve, 1994 and Grieve et al., 1991). TZQG – Transition Zone Quartz Gabbro. Geology of the main types of Ni sulphide ore deposits at Sudbury. b) Contact type example from Coleman Mine; c) Contact and Footwall Type from McCreedy West Mine; d) Quartz diorite and Sudbury Breccia in the Frood-Stobie Breccia Belt at Frood Mine; e) Offset type mineralisation as represented by the development of mineralisation at primary discontinuities in the Copper Cliff Offset at Copper Cliff North Mine; f) Offset type mineralisation as represented by the Totten Deposit in the Worthington Offset (Source: Modified from Farrow and Lightfoot, 2002).

Sudbury Igneous Complex: Deposits

At Sudbury, there are a large number of deposits which share features of one of these groups; thus ore deposit models for the Sudbury nickel deposits are influenced by the very specific and unique relationships that developed in response to a combination of meteorite-impact, crustal melting, protracted differentiation of superheated sulphide saturated silicate magmas, gravitational accumulation of the sulphides, and remobilization into dilational structures in the footwall. Geological exploration models are supported by deposit-scale surface and borehole geophysical surveys that effectively image the strongly conductive sulphide mineralisation in the absence of significant spurious conductivity due to barren sulphides in the country rock (Source: Lightfoot, P. C. Advances in Ni-Cu-PGE Sulphide Deposit Models and Implications for Exploration Technologies).

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