Platinum-Group Minerals of Pt-Placer Deposits Associated with the Svetloborsky Ural-Alaskan Type Massif, Middle Urals, Russia

Abstract

The alteration of platinum group minerals (PGM) of eluval, proximal, and distal placers associated with the Ural-Alaskan type clinopyroxenite-dunite massifs were studied. The Isovsko-Turinskaya placer system is unique regarding its size, and was chosen as research object as it is PGM-bearing for more than 70 km from its lode source, the Ural-Alaskan type Svetloborsky massif, Middle Urals. Lode chromite-platinum ore zones located in the Southern part of the dunite "core" of the Svetloborsky massif are considered as the PGM lode source. For the studies, PGM concentrates were prepared from the heavy concentrates which were sampled at different distances from the lode source. Eluvial placers are situated directly above the ore zones, and the PGM transport distance does not exceed 10 m. Travyanistyi proximal placer is considered as an example of alluvial ravine placer with the PGM transport distance from 0.5 to 2.5 km. The Glubokinskoe distal placer located in the vicinity of the Is settlement are chosen as the object with the longest PGM transport distance (30-35 km from the lode source). Pt-Fe alloys, and in particular, isoferroplatinum prevail in the lode ores and placers with different PGM transport distance. In some cases, isoferroplatinum is substituted by tetraferroplatinum and tulameenite in the grain marginal parts. Os-Ir-(Ru) alloys, erlichmanite, laurite, kashinite, bowieite, and Ir-Rh thiospinels are found as inclusions in Pt-Fe minerals. As a result of the study, it was found that the greatest contribution to the formation of the placer objects is made by the erosion of chromite-platinum mineralized zones in dunites. At a distance of more than 10 km, the degree of PGM mechanical attrition becomes significant, and the morphological features, characteristic of lode platinum, are practically not preserved. One of the signs of the significant PGM transport distance in the placers is the absence of rims composed of the tetraferroplatinum group minerals around primary Pt-Fez alloys. The sie of the nuggets decreases with the increasing transport distance. The composition of isoferroplatinum from the placers and lode chromite-platinum ore zones are geochemically similar.

Platinum-group minerals from the Malaya Kamenushka River placer, Middle Urals, Russia

“…In addition, the coincidence of PGM associations from the chromites of the Kamenushensky massif and the Malaya Kamenushka shows that there is no radical vertical zoning, at least for the size of the erosional area of the massif. Similar results were obtained for the alluvial system associated with the weathering of the Svetloborsky massif rocks (Stepanov et al , 2019a).…”

“…Based on our observations and previous studies (Oberthür, 2018), such a difference in the sizes of PGM aggregates is associated not with the growth of platinum grains in water, but with the nature of the release of metal grains from bedrock intergrowths. Large segregations are accumulated next to the bedrock during the transition to placers, where the PGM grain sizes decreases with an increase in the distance of transport, leading to an uneven distribution of grains (Stepanov et al , 2019a).…”

“…The features of the PGM assemblages of the placers associated with these massifs are not well characterised (Stepanov et al , 2019a). The Nyasminskaya placer system, including the Malaya Kamenushka River placer, is related to the eroded rocks of the Kamenushensky massif and is the third largest in the Urals by volume of extracted PGM.…”

Kufahrite, PtPb, a new mineral from Ledyanoy Creek placer, Galmoenan ultramafic complex, Koryak Highlands, Russia

“…The main evidence for this conclusion are: (1) the occurrence of both tetraferroplatinum and tulameenite as rims over isoferroplatinum grains (Fig. 2; Nixon et al , 1990; Tolstykh et al , 2004, 2015; Stepanov et al , 2019, 2020); and (2) the presence of these minerals as separate phases in serpentine veinlets (Fig. 2e,f); and (3) experimental studies which proved the possibility of formation of Pt–Fe alloys under hydrothermal conditions (Evstigneeva and Tarkian, 1996).…”

Editorial for the Special Issue “Platinum-Group Minerals: New Results and Advances in PGE Mineralogy in Various Ni-Cu-Cr-PGE Ore Systems”

“…The study by Stepanov et al [4] demonstrated that the degree of alteration of PGM in placers associated with the Ural-Alaskan type Svetloborsky massif is linked to the transport distance. In particular, these authors had shown that at a distance of more than 10 km, the degree of PGM mechanical attrition becomes significant, and the primary morphological features, characteristic of lode platinum, are rarely preserved.…”