Native osmium from the Guli Massif, Northern Siberia, Russia

Summary: Native osmium from two different placer occurrences (i. e., Ingaringda and Burlakovsky) within the Guli Massif, Maimecha-Kotui Province, was evaluated for mineral compositions that show systematic differences between the localities. Grains of native osmium show increases in iridium towards the rim, and nuggets consisting of aggregates define trends of preferential substitution of osmium by iridium. A statistically reliable difference in Os/Ir ratios between the two studied placers can be demonstrated. From textural and compositional characteristics it is concluded that the native osmium of the Burlakovsky placer formed at temperatures higher than can be assumed to be reasonable for crustal conditions. The difference in the Os/Ir ratios between the two placers can be attributed to fractional crystallization of native osmium either in the mantle source, which was tapped in different events to form the Guli Massif, or to decreasing Os/Ir values during crystallization under crustal conditions after intrusion of the silicate melt(s).