Alluvial platinum-group minerals as indicators of primary PGE mineralization (placers of southern Siberia)

The platinum-group minerals (PGM) in placer deposits provide important information on the types of their primary source rocks and ores and formation and alteration conditions. Different characteristics of minerals can be determined by a set of conventional and modern in situ analytical techniques (scanning electron microscopy (SEM) and electron probe microanalysis (EPMA)). A study of PGM from placers of southern Siberia (Kuznetsk Alatau, Gornaya Shoria, and Salair Ridge) shows that the morphology and composition of PGM grains, the texture, morphology, and composition of silicate, oxide, and intermetallic microinclusions, and the type of mineral alteration can serve as efficient indicators of the primary sources of PGM. The widespread rock associations in the Kuznetsk Alatau, Gornaya Shoria, and Salair Ridge, the compositions of PGM and microinclusions in them, and the dominant mineral assemblages testify to several possible primary sources of PGE mineralization: (1) Uralian-Alaskan-type intrusions; (2) ophiolite associations, including those formed in a subduction zone; (3) ultramafic alkaline massifs; and, probably, (4) rocks of the picrite-basalt association. The preservation of poorly rounded and unrounded PGM grains in many of the studied placers of the Altai-Sayan Folded Area (ASFA) suggests a short transport from their primary source.