The composition of nickel‐copper sulphide deposits and their host rocks from the Cape Smith Fold Belt, Northern Quebec

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Abstract
Ni‐Cu sulphide deposits containing 2–4% Ni, 1% Cu and 2–3 ppm Pt + Pd occur within sills and flows formed from basaltic komatiite in the Cape Smith Fold Belt of Northern Quebec. The sulphide compositions in these deposits range from metal‐rich (10–15% Ni, 3–4% Cu and 10–20 ppm Pt+Pd) to metal‐poor (5–7% Ni, 1–2% Cu and 3–7 ppm Pt + Pd). Assuming that all of these sulphides formed from silicate magma of the same composition the difference in metal content of these deposits may be due to a larger weight fraction of sulphide having segregated to form the metal‐poor sulphides (0.3 vs 0.1 wt%). All of the sulphides have higher Pd/ Ir, Rh/Ir and Pd/Pt ratios and lower Ni/Cu ratios than the chilled margins of the flows and sills or of the pillowed basalts that overlie the deposits and contain 20% MgO. It is difficult to model the composition of the sulphides if the metal contents of the 20% MgO liquid are used. However, the composition of the sulphides may be modelled using the metal contents of the spinifex‐textured basalts that contain only 15% MgO. Using the composition of the chills and pillowed basalt may lead to an overestimate of the MgO content of the initial liquid due to the presence of 5–10% olivine phenocrysts and minor chromite in these rocks. The presence of the olivine and chromite may also lead to an overestimate of the Ni, Os and Ir content of the initial liquids.

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