Platinum-Group Elements in the Upper Central Layered Zone of the Fox River Sill, Northeastern Manitoba*

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%), pentlanditc, chalcopyrite (<5%), and minor mackinawitc 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, abou