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Mineralogy of Platinum-Group Elements and Gold in the Ophiolite-Related Placer of the River Bolshoy Khailyk, Western Sayans, Russia

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Mineralogy of Platinum-Group Elements and Gold in the Ophiolite-Related Placer of the River Bolshoy Khailyk, Western Sayans, Russia minerals Article Mineralogy of Platinum-Group Elements and Gold in the Ophiolite-Related Placer of the River Bolshoy Khailyk, Western Sayans, Russia Andrei Y. Barkov 1,*, Gennadiy I. Shvedov 2, Sergey A. Silyanov 2 and Robert F. Martin 3 1 Research Laboratory of Industrial and Ore Mineralogy, Cherepovets State University, 5 Lunacharsky Avenue, 162600 Cherepovets, Russia 2 Institute of Mining, Geology and Geotechnology, Siberian Federal University, 95 Avenue Prospekt im. gazety “Krasnoyarskiy Rabochiy”, 660025 Krasnoyarsk, Russia; [email protected] (G.I.S.); [email protected] (S.A.S.) 3 Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, QC H3A 0E8, Canada; [email protected] * Correspondence: [email protected]; Tel.: +7-8202-51-78-27 Received: 21 May 2018; Accepted: 5 June 2018; Published: 12 June 2018 Abstract: We describe assemblages of platinum-group minerals (PGM) and associated PGE–Au phases found in alluvium along the River Bolshoy Khailyk, in the western Sayans, Russia. The river drains the Aktovrakskiy ophiolitic complex, part of the Kurtushibinskiy belt, as does the Zolotaya River ~15 km away, the site of other placer deposits. Three groups of alloy minerals are described: (1) Os–Ir–Ru compositions, which predominate, (2) Pt–Fe compositions of a Pt3Fe stoichiometry, and (3) Pt–Au–Cu alloys, which likely crystallized in the sequence from Au–(Cu)-bearing platinum, Pt(Au,Cu), Pt(Cu,Au), and PtAuCu2, to PtAu4Cu5. The general trends of crystallization of PGM appear to be: [Os–Ir–Ru alloys] ! Pt3Fe-type alloy (with inclusions of Ru-dominant alloy formed by exsolution or via replacement of the host Pt–Fe phase) ! Pt–Au–Cu alloys. We infer that Rh and Co mutually substitute for Fe, not Ni, and are incorporated into the pentlandite structure via a coupled mechanism of substitution: [Rh3+ + Co3+ + ! 3Fe2+]. Many of the Os–Ir–Ru and Pt–Fe grains have porous, fractured or altered rims that contain secondary PGE sulfide, arsenide, sulfarsenide, sulfoantimonide, gold, Pt–Ir–Ni-rich alloys, and rarer phases like Cu-rich bowieite and a Se-rich sulfarsenide of Pt. The accompanying pyroxene, chromian spinel and serpentine are highly magnesian, consistent with a primitive ultramafic source-rock. Whereas the alloy phases indicate a highly reducing environment, late assemblages indicate an oxygenated local environment leading to Fe-bearing Ru–Os oxide (zoned) and seleniferous accessory phases. Keywords: platinum-group elements; gold; platinum-group minerals; placer deposits; ophiolite complexes; western Sayans; Russia 1. Introduction It was L.A. Yachevskiy who first found grains of platinum-group minerals (PGM) identified as “osmian iridium” and “platinum” in a placer in 1910 in the Usinskiy area of the western Sayans [1]. These grains, enriched in platinum-group elements (PGE), were recognized in a heavy-mineral concentrate provided by Chirkov, a gold miner. Later, B.M. Porvatov, M.K. Korovin, and N.K. Vysotskiy [1–3] described the geology and occurrences of Au–PGM-bearing placers in the Usinskiy area and other regions of the western Sayans. Krivenko et al. [4] conducted large-scale investigations of associations of PGM in Au–PGE-bearing zones of placers of the Altai-Sayan folded region. Tolstykh et al. [5] reported the occurrence of a highly unusual association of PGM in a placer deposit of the Minerals 2018, 8, 247; doi:10.3390/min8060247 www.mdpi.com/journal/minerals Minerals 2018, 8, 247 2 of 24 Minerals 2018, 8, x FOR PEER REVIEW 2 of 25 ofZolotaya the Zolotaya River (“GoldenRiver (“Golden River”) River”) watershed, watershed, located located ca. 15 ca. km 15 from km thefrom Bolshoy the Bolshoy Khailyk Khailyk placer. There,placer. arsenotelluridesThere, arsenotellurides and telluroarsenides and telluroarsenides of Ir–Os–Ru of Ir–Os–Ru and an and arsenoselenosulfide an arsenoselenosulfide of Ir occurof Ir occur with witha Pt–Au–Cu a Pt–Au–Cu alloy alloy phase phase in association in association with with grains grains of of Os–Ir–Ru Os–Ir–Ru and and Pt–Fe Pt–Fe alloys. alloys. TheseThese placer occurrences were were attributed attributed to to lode lode serpentinite serpentinite rocks rocks of ofthe the Kurtushibinskiy Kurtushibinskiy ophiolite ophiolite belt belt [5]. [5In]. addition,In addition, G.I. G.I. Shvedov Shvedov and and V.V. V.V. Nekos Nekos [6] [6 found] found grains grains of of placer placer PGM PGM at at the the River River Bolshoy Bolshoy Khailyk. Khailyk. In the Aktovrakskiy complex [7], [7], which forms part of the Kurtushibinskiy belt in Krasnoyarskiy Krasnoyarskiy kray (Figure 11),), serpentititeserpentitite bodiesbodies areare fairlyfairly abundantabundant atat BolshoyBolshoy Khailyk,Khailyk, thethe sitesite ofof ourour investigation.investigation. The area largely consists of volcanogenic and sedimentary rocks of the Chinginskaya suite of Lower Cambrian age,age, discordantlydiscordantly overlainoverlain by by Ordovician Ordovician "terrigenous" "terrigenous" rocks. rocks. Dikes Dikes and and intrusive intrusive bodies bodies of ofDevonian Devonian rocks rocks (granite, (granite, diorite, diorite, and metagabbroicand metagabbr rocks)oic rocks) are considered are considered responsible responsible for placer for goldplacer in goldthe area. in the area. Figure 1. Regional geologygeologyof of the the Bolshoy Bolshoy Khailyk Khailyk area area (a )(a based) based on on [6], [6], with with minor minor modifications, modifications, and anda map a map (b) showing (b) showing the location the location of the of placer the placer area in area the in Russian the Russian Federation. Federation. The red The square red square (a) shows (a) showsthe sampling the sampling area (this area study). (this study). Our aims in the present article are to document the occurrences and mineralogical characteristics Our aims in the present article are to document the occurrences and mineralogical characteristics of assemblages of PGM and PGE–Au-rich phases in the alluvial placer associated with the River of assemblages of PGM and PGE–Au-rich phases in the alluvial placer associated with the River Bolshoy Khailyk (Figure 1), a tributary of the Urbun (Urgun) River, which flows into the River Bolshoy Khailyk (Figure1), a tributary of the Urbun (Urgun) River, which flows into the River Yenisei. Yenisei. Our results and observations provide genetic implications and insights into the mineral-forming Our results and observations provide genetic implications and insights into the mineral-forming environments and trends of crystallization of the ophiolite-related PGE–Au mineralization at environments and trends of crystallization of the ophiolite-related PGE–Au mineralization at Bolshoy Khailyk. Bolshoy Khailyk. 2. Materials and Methods We have examinedexamined numerous PGM grains and micrometricmicrometric inclusions found in heavy-mineral concentrates collectedcollected at at thirteen thirteen prospecting prospecting dug du pitsg pits at a smoothat a smooth bend alongbend thealong riverbed the riverbed of Bolshoy of BolshoyKhailyk, Khailyk, within the within mining the concession mining concession outlined (Figure outlined1). The(Figure concentrates 1). The concentrates are invariably are enriched invariably in enriched in grains of chromian spinel (~15–75 vol %), magnetite (up to ~30%), clinopyroxene and amphiboles (up to ~20% each), with subordinate amounts (<5%) of epidote, zoisite, and grossular-rich garnet. Minerals 2018, 8, 247 3 of 24 grains of chromian spinel (~15–75 vol %), magnetite (up to ~30%), clinopyroxene and amphiboles (up to ~20%Minerals each), 2018, with8, x FOR subordinate PEER REVIEW amounts (<5%) of epidote, zoisite, and grossular-rich garnet.3 of 25 Among the detrital grains of PGM, the Os–Ir–Ru alloy minerals strongly predominate (~80 vol %). Among the detrital grains of PGM, the Os–Ir–Ru alloy minerals strongly predominate (~80 vol %). Grains ofMinerals these 2018 alloys, 8, x FOR vary PEER in REVIEW size from <0.5 to ~2 mm across. Commonly, they display a well-preserved3 of 25 Grains of these alloys vary in size from <0.5 to ~2 mm across. Commonly, they display a well-preserved hexagonal outline (Figure2) and are composed of intergrowths of crystals of varying compositions hexagonalAmong outline the detrital(Figure grains 2) and of arePGM, composed the Os–Ir–Ru of intergrowths alloy minerals of strongly crystals predominate of varying (~80compositions vol %). (e.g., Figure3b,f). Variously rounded grains also are present, however (Figure3e). (e.g.,Grains Figure of these 3b,f). alloys Variously vary in rounded size from grains <0.5 to also ~2 mm are across.present, Comm howeveronly, they(Figure display 3e). a well-preserved hexagonal outline (Figure 2) and are composed of intergrowths of crystals of varying compositions (e.g., Figure 3b,f). Variously rounded grains also are present, however (Figure 3e). Figure 2. Photo of a concentrate of platinum-group minerals collected in the Bolshoy Khailyk placer Figure 2. Photo of a concentrate of platinum-group minerals collected in the Bolshoy Khailyk placer deposit.Figure The 2. Photo brownish of a concentrate film present of onplatinum-group the surface of minerals some grains collected represents in the Bolshoy iron hydroxides. Khailyk placer deposit. The brownish film present on the surface of some grains represents iron hydroxides. deposit. The brownish film present on the surface of some grains represents iron hydroxides. FigureFigure 3. 3.Back-scattered Back-scattered electron electron (BSE) (BSE) imagesimages showingshowing chcharacteristicaracteristic textures textures of of placer placer grains grains of of Figure 3.platinum-groupBack-scattered minerals electron from the (BSE) Bolshoy images Khailyk showing placer. ( characteristica) A partly rounded textures grain of Ir–Os placer alloy grains of platinum-group minerals from the Bolshoy Khailyk placer. (a) A partly rounded grain of Ir–Os alloy platinum-groupcontains inclusions minerals of Pt–Fe from alloy. the Bolshoy(b,f) Intergrowths Khailyk of placer. subhedral (a cr)ystals A partly of Os–Ir–Ru rounded alloys. grain (c) A of Ir–Os contains inclusions of Pt–Fe alloy. (b,f) Intergrowths of subhedral crystals of Os–Ir–Ru alloys.
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