The Post-Magmatic Genesis of Sperrylite from some Gold Placers in South Part of Krasnoyarsk Region (Russia)

G.I.Shvedov1, V.V. Nekos1 and A.M. Uimanov2 1Krasnoyarsk Mining-Geological Company. St.K.Marksa 62, Krasnoyarsk, 660049, Russia 2Mining Company "Tamsiz". St.Artelsky 6, Severo-Yeniseysk, 663280, Russia e-mail: [email protected]

Krasnoyarsk Region has been one of the hand, the placers sperrylites from rivers – Karagan main gold-mining provinces of Russia from the last (Eastern Sayan), Rudnaya, Bolshoy Khaylyk, century and up to nowadays. While placer deposits Zolotaya (Western Sayan) are characterized by the being mining they have marked constantly Ir, Ru, Rh unpurities presence. It witnesses for platinum-group minerals with gold in common. In absolutely another native sources. For the the main, they were "platinum" (Fe-Pt compounds) comparison, they show the composition of the and "osmiridium" (Os-Ir-Ru alloys). Receiving and sperrylites from original rocks in some ultramafic- studying heavy concentrates from placer deposits, mafic massifs of Eastern Sayan. we have discovered, that practically all of them The majority of the investigated sperrylitic contained sperrylites somehow, independently of grains from placers contains itself inclusions of the region, or another PGM presence or absence. different ore- and rock-forming minerals. One can One has discovered the mineral in placers of the and should use the mineral compositions for following rivers and creeks: Coloromo, Krasavitza, solving the problem on supposed native sources for Garevka, Lombancha, Danilovsky, Bolshaya PGM. Quartz, pyroxenes, micas, common potash Kuzeyeva (Yenisey Ridge), Karagan, Levaya feldspars, plagioclases, garnets, talc and some Zhaima, Mana, Bolshaya Sinachaga (Eastern others were identified by the composition among Sayan), Rudnaya, Zolotaya, Bolchoy Khaylyk the analyzed mineral inclusions. Besides that, one (Western Sayan), Beika (Kuznetsky Alatau) and can find pyrrhotite, ilmenite, rutile and some others. The picked out sperrylite is presented, chalcopyrites among the inclusions. Quartz, micas, mainly by diversely rounded nodule fragments and garnets (spessartite in particular), ilmenite and more rarely, by separate crystals and its aggregates. rutile are the most common inclusions for the first As a rule, the biggest crystals (0,1-0,5mm) are less group sperriylites, which are distinguished by the rolled than smaller ones (less than 0,1mm). There higher compositions purity. The inclusions of are smoothed forms among them mainly. The orthopyroxenes, talc, olivine and some others are studied sperrylite crystals have different characteristic for the second group of sperrylites. crystallographic forms. More often we can find While studying the minerals in reflected light, one hexahedrons among them (Karagan River), has found out that some of their nodules were sometimes we find octahedrons, cube-and- substituted by platinum from the edges to the center tetrahedron combinations, cube-and-octahedron (placers of rivers: Coloromo, Danilovsky, Karagan, ones (Coloromo River) and some others. One Bolshaya Kuzeyeva), up to the formation of perfect marks cube-and-trigon-trioctahedron and cube-and- platinum pseudo-morphs by the sperrulite (Figure). didodecahedron combinations in isolated cases. A chemical composition of the platinum, The chemical compositions of sperrylites originating by the sperrylite, inherits admixtures of were determined by the X-ray microanalyser the elements, which has been present in the "Camebax-Micro" at UIGGM (Novosibirsk) substituted sperrylites in higher quantities. For (Table). example, the higher As and Sb contents are marked The Table analysis indicates the presence in the platinum composition from the placer of of two groups of the minerals. Their compositions Coloromo River and they are characteristic for the differ from each other. So the placer sperrylites sperrylite out of the same placer. And on the whole, from rivers – Mana, Bolshaya Sinachaga, Levaya the native platinum by sperrylite, compared with Zhaima (Eastern Sayan), Coloromo, Garevka, the magmatic platinum, is characterized by a higher Krasavitza, Danilovsky, Bolshaya Kuzeyeva assay (a composition purity), by the iron admixture (Yeniseisky Ridge) – have antimony and sulphur absence and, evidently, it should be considered as admixtures, but they have no practically PGE independent mineral species. unpurities, except small Rh contents. On the other

Figure 1. Stages of replacement of sperrylite (white) by native platinum (gray). 400x.

Sperrylite presence in the most parts of allowing to distinguish it from the primary- gold-bearing placers in the south of Krasnoyarsk magmatic analogue associated genetically with the Region parallel to the absence of any basic and crystallization, of a stratiform basic-ultrabasic ultrabasic rocks in the nourishment areas of some magma or formed at platinum minerals substituting placers make us suppose, that there are another of alpinotypic ultramafites. In the first turn, we original sources for this mineral some how. In our should attribute its chemical composition to such opinion, gold ore deposits and hydrothermal type characteristic sings of a hydrothermal sperrylite. manifestations can be these untraditional sources. The chemical composition differs from the others This supposition is based on the next two moments. by the absence of another PGE admixtures, iridium, Firstly, in the last years literature there appeared to osmium and ruthenium especially; they are typical be the data on direct finds of sperrylites in gold ore representatives of a hightemperatured paragenesis. deposits (Laverov et.al., 1997). Secondly, they have As a rule, one marks higher contents the elements experimental works concerning sperrylitic synthesis only with chalcophile properties (as Sb, for under hydrothermal conditions (Evstigneeva et. al., example) in the composition of hydrothermal 1996; Tihomirova et. al., 2000). It witnesses about sperrylite. the possibility of the mineral origin under nature Considerably greater variety of conditions, jointly or separately from gold while crystallomorphologic forms, unlike the mineral, gold ore deposits forming. The sperrylite, born in a accompanying different platinum-metallic hydrothermal process, has some specific features parageneses of traditional industrialgenetic types, is the other sign of the sperrylite, originated under hydrothermal genesis: cubes, octahedrons, cube- postmagmatic conditions. Accordingly to the and-tetrahedron combinations, cube-and- published data, cubic habit sperrylites are octahedron combinations, cube-and-diplaid characteristic for nickel-copper deposits of the combinations and some others. Norilsk type; octahedronic forms are prevailing After all, rock-forming minerals inclusions among the sperrylites from circular alkaline- of a typical hydrothermal parageneses with average ultrabasic massifs of Konder and Inagly types, and low temperatures for the crystallization are among the sperrylites from alpinotypic ultrabasites, characteristic sings for the sperrylites from among the sperrylites from nickel-copper deposits hydrothermal deposits, unlike the inclusions of of Cambalda type (Australia), associated with a chrome-spinellids, ortho-puroxenes, olivines and basalt-komatiitic formation. Unlike this one marks the others, inherent to PGM from mafit-ultramafitic different forms among the sperrylites of a associations.

Table. Chemical compositions of sperrylite from some gold placers and intrusive massifes of the south of Krasnoyarsk region. Amounts Limits of contents, (wt.%) of Placer deposits analysis Pt Ir As Ru Rh Sb S Yenisey Ridge 56,29- 43,13- 0,07- 0,02- 3 0,00 0,00 0,00 Krasavitza 56,76 43,37 0,38 0,03 55,79- 42,84- 0,00- 0,01- 15 0,00 0,00 0,00 Koloromo 56,58 43,51 0,30 0,06 56,09- 43,14- 0,00- 0,00- 10 0,00 0,00 0,00 Garevka 56,78 44,12 0,16 0,09 56,23- 42,39- 0,06- 0,00- 0,00- 8 0,00 0,00 Danilovsky 57,24 43,84 0,16 0,19 0,05 55,49- 42,78- 0,00- 0,00- Bolshaya 12 0,00 0,00 0,00 56,84 43,61 0,18 0,44 Kuzeyeva Eastern Sayan 56,21- 42,70- 0,00- 0,01- 14 0,00 0,00 0,00 Mana 57,29 43,57 0,16 0,05 56,57- 43,28- 0,06- 0,00- Bolshaya 3 0,00 0,00 0,00 56,93 44,43 0,13 0,05 Sinachaga 56,20- 43,44- 0,00- 0,04- 12 0,00 0,00 0,00 Kuvai 57,28 44,22 0,09 0,19 56,41- 42,91- 0,00- 0,08- Levaya 4 0,00 0,00 0,00 56,97 43,30 0,25 0,27 Zaima 55,50- 0,00- 42,18- 0,00- 0,00- 0,02- 38 0,00 Karagan 57,04 1,11 46,14 0,05 0,27 0,46 Western Sayans 55,49- 0,00- 41,52- 0,00 0,00- 0,00- 0,01- 39 Rudnaya 57,01 0,90 43,68 0,07 0,15 0,11 53,25- 0,89- 43,43- 0,03- 1,19- 0,96- 2 0,00 Bolshoy Khaylyk 54,57 1,66 44,01 0,11 1,59 1,36 Intrusive massifes of Eastern Sayan 54,09- 0,00- 42,37- 0,00- 0,00- 0,07- Massif 18 0,00 56,61 2,16 43,73 0,05 0,28 0,39 Kingash 53,06- 0,00- 40,64- 0,00- 0,00- 0,00- 0,02- Massif 29 56,76 2,62 43,78 0,06 0,09 1,08 0,46 Kuskanak 55,18- 0,29- 43,93- 0,23- 0,58- Romanovsky 2 0,00 - 55,77 0,91 43,95 0,42 0,80 massif Clim 0,146 0,132 0,048 0,038 0,036 0,038 0,012

Acknowledgements Laverov N.P., Distler V.V., Mitrofanov G.L. et.al., The authors thanking A.P.Krivenko and 1997, Platinum and other native metals in N.D.Tolstykh (UIGGM, Novosibirsk) for granting ores of a deposit a Sukhoi Log - Doklady is exemplars of sperrylites from placer deposits Akad. Nauk, 355, 5, p.664-668 (in Garevka and Kuvai. Russian). Tihomirova V.I. & Chichagov A.V., 2000, References Hydrothermal synthesis and Tatiana Evstigneeva & Mahmud Tarkian, 1996, crystallography aurostibite (AuSb2), Synthesis of platinum-group minerals sperrylite (PtAs2) and geversite (PtSb2). – under hydrothermal conditions. Doklady Akad. Nauk, 372, 3, p.382-384 Eur.J.Mineral., 8, p.549-564. (in Russian).