Abstract:
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).
