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Gabbroid-associated Nickel, Copper, PGEs subtype 12.2.A: Layered Intrusive
Geological Setting (12.2.a), (12.2.b) Layered intrusions generally occur in a cratonic
setting, in some cases associated with intracontinental rifts and flood
basalts; some occur in Archean greenstone belts. The Sudbury
deposits probably originated from a structure of meteoritic impact.
Host Rock Various mafic phases of intrusive complexes includes norite, gabbro,
troctolite, feldspathic pyroxenite, amphibolite, gabbro-diabase, picrite.
The North Range deposits at Sudbury occur in pyroxenite,
amphibolite, gabbro-diabase, and picrite. The North Range deposits
at Sudbury occur in brecciated leucocratic footwall rocks. The
Merensky Reef is coarse grained feldspathic pyroxenite associated
with thin chromitite layers.
Associated Rocks A variety of phases of the mafic intrusive complexes includes diorite,
peridotite, pyroxenite, anorthosite, gabbro, norite; and wall rocks of the
intrusive complexes.
Form of Deposit (12.2.a) Conformable layers or lenses, commonly located in a local
depression or embayment at or near the base of the host layered
intrusion. Mineralized material consists of massive sulphides,
sulphide-matrix breccia, interstitial sulphide network, and disseminated
sulphide. In well preserved deposits, the rich mineralized material lie
nearest the base, and are overlain by leaner disseminated sulphide.
Sulphide veins and disseminations commonly penetrate the footwall
rocks.
Principal Minerals Pentlandite, chalcopyrite, cubanite, millerite; various PGE minerals
including sulphides, tellurides, arsenides and alloys.
Associated Minerals Pyrrhotite, pyrite, sphalerite, millerite, marcasite; plagioclase,
hypersthene, augite, olivine, hornblende, biotite, quartz, and a variety
of alteration minerals.
Host Rock Age Various ages; most are Precambrian (Sudbury, 1.85 Ga; Great Lakes
Nickel and Duluth Complex, 1.1 Ga; Bushveld Complex, 2.1 Ga;
Stillwater Complex, 2.7 Ga; Lynn Lake ) 1.8 Ga) but Noril'sk-Talnakh
intrusions are Permo-Triassic, and other Paleozoic and Mesozoic
examples are known.
Genetic Model Mafic magma (probably mantle-derived in most cases) was generally
emplaced quiescently as multiple pulses in upper levels of the crust, in
some cases apparently in a tensional environment associated with
rifting. Early sulphur saturation of the magma produced flow- and
gravity-segregations of nickel-copper bearing sulphides at the base of
the intrusion. Contamination of the magma probably made a
significant contribution to the sulphide saturation in many deposits,
either through the addition of sulphur, or assimilation of siliceous
material. Most of the sulphur in several large districts (Noril'sk-
Talnakh, Duluth Complex) was probably derived from the underlying
sedimentary rocks.
Mineralized Material
Control
The basal contacts (particularly embayments in the basal contacts)
and immediate overlying zones (up to about 200 m) in layered
intrusions, are the most common sites of nickel-copper sulphide
mineralized material
Prophecy Platinum Corp. 8-2 1193460500-REP-R0001-02
Wellgreen Project Preliminary Economic Assessment,
Yukon, Canada