Micrometric Inclusions in Platinum-Group Minerals from Gornaya Shoria, Southern Siberia, Russia: Problems and Genetic Significance

Micrometric inclusions in platinum-group minerals (PGMs) from alluvial placers carry considerable information about types of primary rocks and ores, as well as conditions of their formation and alteration. In the present contribution, we attempt to show, with concrete examples, the significance of the data on the composition and morphology of micrometric inclusions to genetic interpretations. The PGM grains from alluvial placers of the Gornaya Shoria region (Siberia, Russia) were used as the subject of our investigation. In order to determine the chemical composition of such ultrafine inclusions, high-resolution analytical methods are needed. We compare the results acquired by wavelength-dispersive spectrometry (WDS; electron microprobe) and energy-dispersive spectrometry (EDS) and scanning electron microscopy (SEM) methods. The results obtained have good convergence. The EDS method is multi-elemental and more effective for mineral diagnostics in comparison with WDS, which is its certain advantage. The possible conditions for the formation of inclusions and layers of gold, sulfoarsenides and arsenides in Pt3Fe grains, which have an original sub-graphic and layered texture pattern, are discussed. They are the result of solid solution and eutectic decompositions and are associated with the magmatic stages of grain transformation, including the result of the interaction of Pt3Fe with a sulfide melt enriched with Te and As

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

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

Native Gold and Unique Gold–Brannerite Nuggets from the Placer of the Kamenny Stream, Ozerninsky Ore Cluster (Western Transbakalia, Russia) and Possible Sources

We carried out a comprehensive study of native gold (morphology, composition, intergrowths, and microinclusions) from alluvial deposits of the Kamenny stream (Ozerninsky ore cluster, Western Transbaikalia, Russia). The study showed that there were four types of native gold, which differed significantly in their characteristics and probably had different primary sources from which placers were formed: gold–quartz, oxidized gold–sulfide, gold–silver, and zones of listvenites with copper–gold and gold–brannerite (Elkon-type). Particular attention was paid to the study of unique, both in size and in composition, gold–brannerite nuggets of the Kamenny stream. It was established that the gold in the gold–brannerite nuggets (GBNs) had wide variations in chemical composition and mineral features. According to them, there were five different fineness types of native gold: 750–800‰; 850–880‰; 880–920‰; 930–960‰; and 980–1000‰. The data obtained indicated a multistage, possibly polygenic, and probably polychronous formation of GBN gold–uranium mineralization. The first stage was the formation of early quartz–nasturanium–gold–W–rutile–magnetite association (Middle–Late Paleozoic age). The second was the crystallization of brannerite and the replacement of an earlier pitchblende with brannerite (Late Triassic (T3)–Early Jurassic (J1) age). The third was the formation of the hematite–barite–rutile–gold association as a result of deformation–hydrothermal processes, which was associated with the appearance of zones of alteration in brannerite in contact with native gold with 8–15 wt.% Ag. The fourth was hypergene or the low-temperature hydrothermal alteration of minerals of early stages with the development of iron hydroxides (goethite) with impurities of manganese, tellurium, arsenic, phosphorus, and other elements. The carbon isotopic composition of an organic substance indicates the involvement of a biogenic carbon source. In the OOC area, there were signs that the composition of the GBNs and the quartz–chlorite–K–feldspar-containing rocks corresponded to Elkon-type deposits