GEOLOGY AND DISTRIBUTION OF PLATINUM-GROUP ELEMENTS, BUCKO LAKE INTRUSION, THOMPSON BELT, MANITOBA

The Bucko Lake Ni-Cu sulfide deposit is hosted by the Bucko Lake intru sion (BLI), one of several ultramafic bodies of komatiitic affinity of Early Proterozoic age in the Thompson belt, Manitoba. The BLI cuts pr esumably Archean gneissic country-rock. The intrusion is composed pred ominantly of harzburgite, dunite, and olivine orthopyroxenite. Metasom atic alteration of the intrusion occurred in at least two stages. The first stage involved the pervasive serpentinization of the intrusion, with good preservation of relict textures. During the second stage of alteration, next to granitic pegmatite dikes, original textures were d estroyed, and a new growth-oriented fabric was generated. Primary diss eminated Ni sulfide mineralization at Bucko Lake is typical of that in komatiitic dunite-associated deposits. Mobilized sulfides occur in am phibolite xenoliths, sheared granitic pegmatite dikes and sheared peri dotite, and are subdivided, on the basis of their relative timing of e mplacement, into two types: early xenolith-hosted sulfides and later s tringer sulfides. The abundance of platinum-group elements, Au, Cu and Ni in primary sulfides was apparently unaffected by serpentinization and amphibolite-grade metamorphism, but Cu and Au were lost during the metasomatic alteration adjacent to granitic pegmatite dikes. The comp osition of xenolith-hosted mobilized sulfides is similar to that of th e primary sulfides, but stringer sulfides are relatively enriched in C u and depleted in Ni and Ir. The composition of sulfides in the southe rn part of the Thompson belt is similar to that of Archean deposits, b ut is more primitive than that of komatiite-hosted deposits in the Cap e Smith belt. This is significant, since the Thompson belt and Cape Sm ith belt are segments of the Early Proterozoic Circum-Superior belt.