MINERAL PARAGENESES AND THE ORIGIN OF ISOFERROPLATINUM NUGGETS FROM THE IGNALI PLACER DEPOSIT (SIBERIAN PLATFORM)

Abstract

Results of a study of platinum group minerals (PGM) and associated mineral paragenesis from the Inagli placer deposit (the Aldan Shield, Siberian Platform) are given. The Ir trend is typical for isoferroplatinum, which is a principal 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 caused by a vertical zoning of the primary sources of Pt. The isoferroplatinum nuggets from the placer were examined for the first time. 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, weighs 100.22 g and consists of the isoferroplatinum aggregate with abundant chrome-spinelide impregnation. Chrome-spinelides containing 44 to 62 wt % Cr2O3, Cr-bearing (up to 6 wt % Cr2O3) diopside, olivine (Fa2-5), serpentine, phlogopite, Ni-bearing (up to 5 wt % NiO) vermiculite, K-feldspar, pectolite, apatite, and unidentified Ti-bearing K- and Ca-silicates in PGM were distinguished as intergrowths and inclusions in PGM. Chrome-spinelides intergrown with isoferroplatinum were identified as magnesium ferrichromites or more rarely as magnochromites. They are identical to chrome-spinelides from schlierens and differ from accessory chrome-spinelides due to their higher Mg content. This is evidence that principal primary sources of PGE in the Inagli placer are chromite segregations in dunites. The formation of polyphase multicomponent solid solutions displaying wide compositional variations and containing numerous various inclusions of parent crystals and exsolved isolated lamella is considered on the base of the ternary Ir-Os-Pt diagram.