Mineral parageneses and the origin of isoferroplatinum nuggets from the Ignali placer deposit (Siberian Platform)

Results of a study of platinum group minerals (PGM) and associated mineral paragenesis from the Inagli placer deposit (the Aldan Shield, Siberian Plat form) are given. The Ir trend is typical for isoferroplatinum, which is a p rincipal PGM in the Inagli massif. It is continuous in Ir content, reaching 8 wt % and more. Some decrease in the Ir content along the placer is cause d by a vertical zoning of the primary sources of Pt. The isoferroplatinum nuggets from the placer were examined for the first ti me. They are classified as small nuggets. Their sizes and weights are from 5 to 15 mm and 0.5 to 4.5 g. The largest nugget, 45 x 30 x 22 mm in size, w eighs 100.22 g and consists of the isoferroplatinum aggregate with abundant chrome-spinelide impregnation. Chrome-spinelides containing 44 to 62 wt % Cr2O3 Cr-bcaring (up to 6 wt % Cr2O3) diopside, olivine (Fa(2-5)), serpenti ne, phlogopite, Ni-bearing (up to 5 wt % NiO) vermiculite, K-feldspar, pect olite, apatite, and unidentified Ti-bearing K- and Ca-silicates in PGM were distinguished as intergrowths and inclusions in PGM. Chrome-spinelides int ergrown with isoferroplatinum were identified as magnesium ferrichromites o l more rarely as magnochromites. They are identical to chrome-spinelides fr om schlierens and differ from accessory chrome-spinelides due to their high er ME content. This is evidence that principal primary sources of PGE in th e Inagli placer are chromite segregations in dunites. The formation of poly phase multicomponent sc,lid solutions displaying wide compositional variati ons and containing numerous various inclusions of parent crystals and exsol ved isolated lamella is considered on the base of the ternary Ir-Os-Pt diag ram.