Nature and Origin of the Sudbury Structure (Canada) and the Ni-Cu-PGE Ores

ABSTRACT
The world-class magmatic Ni-Cu-PGE sulfide deposits of the
Sudbury Structure have been mined for over 100 years. The structure
consists of the Sudbury Basin, the Sudbury Igneous Complex
(SIC) which surrounds the basin as an elliptical collar (58 x 28 km)
and an outer zone of scattered breccia bodies. The basin fill is comprised
of breccias and clastic sedimentary rocks which host Zn-Pb-
Cu sulfide mineralization. The SIC consists of the following units,
from bottom to top: Contact Sublayer (xenoliths in norite matrix),
norite, quartz gabbro and granophyre. Radiating and concentric
“offset” dikes, related to the SIC, intrude the footwall. The Sublayer
is locally underlain by a xenolith-bearing granoblastic-textured rock
known as Footwall Breccia. The Sudbury Structure is the product of
a meteorite which impacted a tectonically active and mineralized
segment of the crust (at 1850 Ma). The structure was further shaped
by later deformation and erosion.
There are four main types of sulfide deposits associated with
the SIC. 1) SIC-footwall contact deposits (e.g. Creighton, Little
Stobie) occur in depressions or terraces within larger, >kilometerscale,
down-dip plunging embayments in the SIC- footwall contact;
the Sublayer, a discontinuous unit, is thickest in the embayments.
Mineralization grades from disseminated to massive pyrrhotite +
pentlandite + chalcopyrite, with Ni>Cu and occurs both in the
Sublayer and the underlying Footwall Breccia. 2) Footwall vein
deposits (e.g. McCreedy West, Strathcona Deep Copper) occur as
massive chalcopyrite + pentlandite ±cubanite ± millerite ± bornite
veins, rich in Cu + Pt + Pd + Au and hosted by thermally metamorphosed
Sudbury Breccia. The deposits are located below mineralized
embayments and are within 700 m from the base of the SIC.
They are locally contiguous with SIC-footwall contact deposits. 3)
Offset deposits occur within radial quartz diorite dikes (e.g. Copper
Cliff) or within melt bodies, similar in composition to the quartz
diorite dikes, which are in major, concentric Sudbury Breccia bodies
(e.g. Frood-Stobie). Offset deposits resemble SIC-footwall contact
deposits in mineralogy and texture but tend to have Ni/Cu =1.
4) The sheared deposits (e.g. Falconbridge) comprise massive sulfides
with deformed inclusions. They are depleted in Cu, Pt, Pd and
Au when compared to other Sudbury deposits.
SIC-footwall contact and offset-hosted deposits were precipitated
as an immiscible sulfide liquid from the sulfur-saturated SIC
melt. The footwall vein deposits are fractionated derivatives of SICfootwall
contact ore where fluids entered fractured zones of Sudbury
Breccia. The sheared deposits represent faulted and remobilized
SIC-footwall contact deposits and are largely restricted to the east
end of the South Range.