ASSOCIATION OF PLATINUM-GROUP ELEMENTS WITH CHROMITITE IN LAYERED INTRUSIONS AND OPHIOLITE COMPLEXES

Many chromitites from layered complexes contain much higher proportions of Pt and Pd relative to Ru, Ir, and Os than do those from ophiolite complexes; this is attributed in part to the fact that chromitites from layered intrusions originally contained more sulfide than those from ophiolites. Within layered intrusions, the chromitites richest in platinum-group elements are those occurring close to or above the level at which plagioclase first appears as a cumulate phase. Magma mixing would appear to be the best mechanism to explain the formation of massive chromitite layers in layered intrusions. Observations and hypotheses lead to the conclusion that chromitites within or close to the level at which norites, gabbros, and anorthosites first appear in an intrusion are better prospects for platinum-group elements enrichment than those deep within ultramafic cumulates. Additional aspects of the subject are discussed.