Abstract

Platinum-group element abundances ranging up to 1,200 ppb total are associated with weak, sporadic concentrations of disseminated sulfides in certain cyclic units of the Fox River sill, northeastern Manitoba. The sill is a 250-km-long, predominantly ultramafic stratiform intrusion of Proterozoic age. The Main Layered series of the intrusion comprises a sequence of cyclic units defined mainly on the basis of the disappearance and abrupt reappearance of cumulus olivine. The lower central layered zone consists largely of olivine-rich adcumulates that contain little sulfide, whereas the overlying upper central layered zone comprises mainly orthocumulates that contain sporadic platinum-group element-bearing sulfides. Pyrrhotite (75-95%), pentlandite, chalcopyrite (<5%), and minor mackinawite form the bulk of the texturally well-preserved interstitial sulfides.Ir, Rh, Pt, Pd, and to a lesser degree Au are strongly intercorrelated in the analyzed samples. This probably reflects their mutually consistent partitioning into the immiscible liquid sulfide phase that separated from the silicate melt, and the lack of significant preferential remobilization of any of these platinum-group elements. Ir appears to show partly compatible behavior, about 1 ppb apparently being associated with cumulus olivine. The lack of correlation of Ru with the other platinum-group elements is not well understood but could result if its principal occurrence were as laurite inclusions in chromite.Although the chondrite-normalized platinum-group element profiles resemble those of other magmatic sulfides, their slopes strongly suggest two populations, one steep (i.e., enriched in Rh, Pt, and Pd) and the other shallow. The latter group is in most cases associated with thick cyclic units that are believed to be derived from large volumes of relatively primitive magma. Little enrichment of Rh, Pt, and Pd in the melt was produced by the time of sulfide separation. However, the first group, enriched in Rh, Pt, and Pd, is generally associated with thin cyclic units that are believed to result from small influxes of magma mixed with more evolved residual magma. The greater degree of fractionation of this magma produced stronger enrichment of Rh, Pt, and Pd in the residual melt, and hence, greater enrichment in the immiscible sulfide phase that subsequently separated. From this, it can be speculated that the best potential for platinum-group element enrichment in the Fox River sill should occur in thin, highly differentiated cyclic units.

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