Platinum-Group Minerals in the Placer of the Kitoy River, East Sayan, Russia

Abstract

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. The article shows for the first time the results of a study of placer platinum mineralization found in the upper reaches of the Kitoy River (the southeastern part of the Eastern Sayan (SEPES)). Using modern methods of analysis (scanning electron microscopy), the authors studied the microtextural features of platinum-group minerals (PGM), their composition, texture, morphology and composition of microinclusions, rims, and other types of changes. The PGM are Os‑Ir‑Ru alloys with a pronounced ruthenium trend. Many of the Os‑Ir‑Ru grains have porous, fractured, or altered rims that contain secondary PGE sulfides, arsenides, sulfarsenides, Ir-Ni-Fe alloys, and rarer selenides, arsenoselenides, and tellurides of the PGE. The data obtained made it possible to identify the root sources of PGM in the placer and to make assumptions about the stages of transformation of primary igneous Os-Ir-Ru alloys from bedrock to placer. We assume that there are several stages of alteration of high-temperature Os-Ir-Ru alloys. The late magmatic stage is associated with the effect of fluid-saturated residual melt enriched with S, As. The post-magmatic hydrothermal stage (under conditions of changing reducing conditions to oxidative ones) is associated with the formation of telluro-selenides and oxide phases of PGE. The preservation of poorly rounded and unrounded PGM grains in the placer suggests a short transport from their primary source. The source of the platinum-group minerals from the Kitoy River placer is the rocks of the Southern ophiolite branch of SEPES and, in particular, the southern plate of the Ospa-Kitoy ophiolite complex, and primarily chromitites.

Platinum–group minerals in the placer deposit in northwestern Hokkaido, Japan: description of a new mineral, tomamaeite

“…This trend is also well known in the IPGE-based alloys derived from ophiolites and is considered to be the result of alloy crystallization when the immiscibility gap is vertically deformed under high pressure (Bird and Bassett, 1980). It has also been suggested that the Ru content may be related to the timing of crystallization, and alloys formed at a later stage in magmatic processes tend to be richer in Ru and PPGE (e.g., Tolstykh et al, 2015;Airiyants et al, 2022). After Os-Ir alloy crystallizes, the residual system is rich in Ru and PPGE and may occasionally form a eutectic crystal consisting of ruthenium and isoferroplatinum, such as shown in Figure 5a.…”