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

2. Source of Sulphur: The magma must have access to an external sulphur source to achieve early
sulphide saturation and to segregate significant abundances of immiscible sulphides at a high
(crustal) level;
3. Dynamic System: The ores must form in a dynamic system where the magmas can interact with
country rocks (to extract S) and where the sulphides can equilibrate with a sufficient amount of
magma to generate high chalcophile element contents in the sulphides (i.e., high R factor);
4. Physical Trap: The sulphides must be concentrated in some type of physical trap (embayment,
inflection).
Abitibi Greenstone Belt
NICKEL SULPHIDE DEPOSITS
Nickel-copper-(platinum group element)(PGE) discoveries in the Abitibi greenstone belt occurred
sporadically, including several periods of intense exploration and discovery alternating with other periods
with only minimal mineral exploration (Figure 25): Period I (1907–1915) included the discovery of the
Alexo Mine; Period II (1950–1951) included the discovery of the Texmont Mine; Period III (1960–1977)
included the discovery of Langmuir Mine (#1 and #2); and Period IV (1989–1998) included the discovery
of the Dundeal Horizon, and Period V (2003 to current) includes the discovery of C-Zone in the
Bannockburn area. Thus, Ni-Cu-(PGE) mineralization continues to be discovered whenever there is
sufficient exploration activity.
Komatiite-associated Ni-Cu-(PGE) deposits, prospects and showings in the Abitibi greenstone belt
occur exclusively in the 2719 to 2710 Ma Kidd–Munro assemblage (e.g., Alexo, Dundonald South,
Dumont, Marbridge) and the 2710 to 2703 Ma Tisdale assemblage (e.g., Langmuir, McWatters, Redstone,
Texmont, Sothman, Bannockburn). No deposits have been discovered in the 2750 to 2735 Ma Pacaud
assemblage and 2723 to 2720 Ma Stoughton–Roquemaure assemblage. The likely reasons for this are
discussed below.
The deposits almost always occur in clusters. For example, there are 6 known deposits in the Shaw
Dome, 4 in the Bannockburn area, 5 in the Dundonald area, and at least 4 in the Lamotte area. This is the
normal pattern for komatiite-associated Ni-Cu-(PGE) deposits worldwide, exemplified by the deposits in
the Kambalda–St. Ives–Tramways–Widgiemooltha–Carnilya Hill district in Western Australia (Barnes
2004). Clearly, areas where only one or two deposits have been discovered, such as the Bartlett and
Halliday domes, have significant potential for discovery of additional deposits.
KOMATIITE PHYSICAL VOLCANOLOGY
Interpreting volcanic environments is difficult as it may be complicated by primary factors (e.g., more
than one eruptive centre in each assemblage, differences in eruption rate and style, differences in mode
and environment of emplacement) and secondary factors (e.g., metamorphism, alteration, structural
complexity). However, there are clear lithological, geochemical, textural, and volcanological differences
between the komatiitic rocks in the different assemblages of the Abitibi greenstone belt (Table 6).
The Pacaud assemblage is dominated by sheet-like flows with lesser pillowed flows. These sheetlike
flows include abundant differentiated flows (ortho- to mesocumulate lower zones and poorly
developed spinifex-textured upper zones) and less abundant massive flows (olivine-phyric or ortho- to
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