Abstract

Recent work on the platinum-group element (PGE) content in Ni sulfide ores has shown that those associated with komatiite magmas generally have low (Pt + Pd)/(Ru + Ir + Os) ratios (<2.5), whereas those associated with basaltic magmas have higher ratios (>13). The Levack West and Little Stobie 1 and 2 deposits conform with this generalization, having ratios of 18.6, 19.2, and 13.2, respectively (Hoffman et al., 1979). Pt and Pd are present in roughly equal proportions in these deposits, ranging between 1,000 and 3,000 ppm in massive sulfides.New data from the Strathcona deposit indicate values of close to 400 ppm for both Pt and Pd, approximately one-third of the levels at the nearby Levack West deposit. Both Strathcona and Levack West are strongly zoned with respect to platinum-group elements, Au, and base metals; Cu, Ni, Zn, Pt, Pd, and Au increase away from the Irruptive hanging wall into the footwall and Co, Rh, Ru, Ir, and Os increase in the reverse direction. At Strathcona, this zoning results in a decrease of the (Pt + Pd)/(Ru + Ir + Os) ratio from 132 in the footwall to 2.2 in the hanging-wall ore.Relevant phase equilibria data indicate that Cu, Ni, Zn, Pt, and Pd are likely to become enriched in the residual liquid of a fractionating sulfide melt whereas Co, Rh, and Ru will concentrate in the early crystallizing monosulfide solid solution. The correspondence between the experimental data and natural observations is strong evidence that the zoning is the consequence of the fractionation of a sulfide melt, with the residual liquid being expelled progressively into the footwall.Average data obtained for the Falconbridge depost resemble those of the hanging wall at Strathcona, both in the low (Pt + Pd)/(Ru + Ir + Os) ratio of 2.3 and in a low Cu/Ni ratio, suggesting that the ore sampled represents an early-crystallizing fraction and that a late-crystallizing, Pt-rich fraction equivalent to the footwall ore at Strathcona and amounting to perhaps twice the present volume of ore was not included in the sampling and may well not be within the present orebody. The deposit is located largely within a vertical fault zone. It is possible that the sulfide melt was introduced up the fault, crystallizing as it moved upward, and that the Pt-rich portion crystallized farthest up the fault and has been removed by erosion. However, sampling over a 1,600-m vertical interval of the ore zone has failed to reveal any progressive variation in the (Pt + PD)/(Ru + Ir + Os) ratio within the fault zone. An alternative and preferred hypothesis is that the deposit was originally zoned as at Strathcona, with the Pt-rich portion forming away from the Irruptive within the footwall. This portion may have been faulted off, leaving the early crystallizing portion behind to become remobilized into the fault zone. If true, the present location of the Pt-rich portion is intriguing.In addition to having lower concentrations of platinum-group elements than Levack West, the Strathcona deposit also has lower concentrations of Ni and Cu. These compositional differences can be modeled in terms of a larger amount of sulfide equilibrating with a given amount of the host silicate magma at the former deposit than at the latter. Sequences of similar magmatic events, coupled with a proposed fractionation of olivine, can account for many of the compositional variations between different Sudbury deposits.

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