Overview of Results from the Greenstone ... - Geology Ontario

Table 5. Simplified classification of primary mineralization types in komatiite-associated magmatic Ni-Cu-PGE deposits
(modified from Lesher and Keays 2002) in the Abitibi greenstone belt.
Origin
Type
I
II
Type
Stratiform Basal
Stratabound Internal
Sulphide distribution At or near the bases of komatiitic Within komatiitic
peridotite or komatiitic dunite units peridotite or dunite units
Sulphide textures Massive, net-textured, disseminated Disseminated, interstitial Disseminated
Timing and
paragenesis
Examples in the
Abitibi greenstone
belt
Segregated prior to or during
emplacement
- Shaw Dome (Langmuir #1,
Langmuir #2, McWatters, Hart,
Redstone, Galata)
- Bartlett Dome (Texmont)
- Halliday Dome (Sothman)
- Bannockburn area (Thalweg,
Rahn Lake, C-Zone)
- Dundonald area (Alexo, Kelex,
Dundeal, Dundonald South)
- Munro area (Mickel)
- Lamotte area (Marbridge Zone 1-4,
Cubric–Bilson, Ataman, Quebec
Moly)
Segregated during
crystallization of cumulate
host rock
- Bannockburn area
(Bannockburn)
- Lasarre–Amos area
(Dumont Nickel)
III
Stratiform “Reef-style”
At/near contact lower ultramafic cumulate
zones and upper mafic zones
Segregated in final crystallization stages
of host rock
Common, but uneconomic
- Round Lake Dome (Boston Creek sill)
- Munro area (Fred’s flow)
Komatiite-Associated Deposits
Nickel-copper-(platinum group element) (PGE) sulphide mineralization associated with komatiitic rocks
(komatiites and komatiitic basalts) in Archean and Proterozoic greenstone belts are an important part of
the broader category of magmatic Ni-Cu-PGE mineralization. Although komatiitic basalts and komatiites
occur in several greenstone belts worldwide, the majority of known Ni-Cu-(PGE) deposits occur in only a
few of these belts, including the Abitibi and Cape Smith belts of Canada, the Norseman–Wiluna and
Forrestania belts of Western Australia, the Bindura–Shamva and Shangani–Filabusi belts of Zimbabwe,
and the Crixas and Minas Gerais belts in Brazil.
Komatiite-hosted Ni-Cu-(PGE) deposits contain a variety of mineralization types (Lesher and Keays
2002), which may be subdivided into 3 broad genetic categories: magmatic, mobilized metamorphically
hydrothermally, and mobilized tectonically. Each of these categories may be subdivided into several types
and subtypes (Table 5; Lesher and Keays 2002). The most favourable Ni-Cu-(PGE) primary mineralization
types in the Shaw Dome area are, in order of economic potential, stratiform basal (Type I: Kambalda-type),
stratabound internal (Type II: Mt. Keith type), and stratiform internal (Type III: “reef-type”).
In the past decades, researchers have recognized that the physical volcanology of komatiites is the
single most important parameter in assessing the mineral potential of komatiites (e.g., Lesher, Arndt and
Groves 1984; Lesher 1989; Hill et al. 1990, 1995; Barnes et al. 1999). Furthermore, a recent study on
chalcophile elements geochemistry reveals that all komatiitic rocks are a favourable magma source for
Ni-Cu-(PGE) mineralization in the Abitibi greenstone belt, regardless of age. These results corroborate
that the physical volcanology is more important than magma composition in determining the prospectivity
of komatiitic rocks (Sproule et al., in press). Thus, several factors are considered to be critical to the
genesis of economically significant magmatic sulphide deposits (e.g., Lesher and Keays 2002, Naldrett
2004), including:
1. Source of Metal: The magma must be initially undersaturated in sulphide so that it contains
sufficient concentrations of Ni, Cu, and PGE;
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