Abstract

Within the Belleterre-Angliers greenstone belt of the Pontiac subprovince there are a number of Ni-Cu sulfide occurrences associated with synvolcanic gabbros. The stratigraphy and geochemistry of the belt suggest that it is a rifted island arc and that the gabbros which contain the Ni-Cu sulfides were derived from the mantle plume that rifted the island arc.The conditions under which the sulfides formed may be modeled either by recalculating the sulfide compositions to 100 percent sulfide or by using the Cu/Pd ratios. Both approaches suggest that most of the sulfide occurrences formed from an enriched MORB magma with R factors of 200 to 1,000. One of the occurrences (La Force) is depleted in platinum-group elements (PGE) relative to Cu and this depletion may be the result of the removal of sulfides from the magma prior to emplacement at this stratigraphic level.The advantage of using the Cu/Pd ratio rather than recalculating to 100 percent sulfide is that it is not necessary to have S analyses to model the conditions of formation of the sulfides. Further, the Cu/Pd ratio may be used as a prospecting tool. Rocks with ratios greater than mantle values formed from magmas depleted in Pd, possibly due to removal of sulfide from the magma. This implies the potential for a PGE-bearing sulfide deposit at stratigraphically lower levels. Such an approach has been applied to PGE targets in the eastern Abitibi subprovince. Samples from four Ni-Cu occurrences with Cu/Pd ratios close to mantle ratios have the potential to produce PGE as a by-product. Samples from seven Ni-Cu sulfide occurrences and the layered intrusions Lac Dore, Bell River, and Cummings Complex have Cu/Pd ratios much greater than mantle ratios, indicating that these intrusions and sulfides are not good targets for PGE at this stratigraphic level, although there may be PGE-rich sulfides present at depth.

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