No 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.

—We present results of analysis of the morphology and chemical composition of platinum-group minerals from chromite– platinum orebodies of zoned clinopyroxenite–dunite massifs in the Middle Urals (Nizhnii Tagil, Svetlyi Bor, Veresovyi Bor, and Kamenushenskii). Study of more than 500 grains has given an insight into the sequence of formation of platinum-group minerals in chromitites of the studied massifs. Three assemblages of platinum-group minerals have been revealed: magmatic (Os–Ir–Ru intermetallic compounds, isoferroplatinum, ferroplatinum, sulfides of the isomorphous series erlichmanite–laurite and kashinite–bowieite, and thiospinels of the series cuproiridsite–cuprorhodsite–malanite); postmagmatic (with a predominance of tulameenite, tetraferroplatinum, and ferronickelplatinum, resulted from serpentinization of dunites); and latest secondary (minerals with a predominance of PGE sulfides, arsenides, sulfoantimonides, sulfoarsenides, plumbides, and amalgams).

No abstract

No abstract