Typomorphic features of placer gold of Vagran cluster (the Northern Urals) and search indicators for primary bedrock gold deposits |
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| Institution: | 1. Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry of Russian Academy of Science (IGEM RAS), 119017 Moscow, Staromonetny 35, Russia;2. Institute for Natural Science of Perm State University, 614990 Perm, Genkelya St., 4, Russia;3. Perm State University, 614990 Perm, Bukireva St., 15, Russia;4. Geoplacer Exploration Ltd., 13 Tigereye Crescent, Whitehorse, Yukon Y1A 6G6, Canada;5. Terra Solutions, P.O. Box 369, Rolla, MO 65402, USA;1. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, 3, Koptyug ave., Novosibirsk 630090, Russia;2. Novosibirsk State University, 2, Pirogova str., Novosibirsk 630090, Russia;3. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, 3, Koptyug ave., Novosibirsk 630090, Russia;4. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, 1А Favorsky str., Irkutsk 664033, Russia;1. Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Moscow, Russia;2. Institute of Volcanology and Seismology, Far East Division, Russian Academy of Sciences, Petropavlovsk-Kamchatsky, 683006, Russia;1. Departamento de Geología y CEACTierra, Universidad de Jaén, Campus Científico-Tecnológico de Linares, 23700 Linares, Spain;2. Departamento de Ciencias de la Tierra, Universidad de Cádiz, CASEM, 11510 Puerto Real, Spain;3. Departamento de Estratigrafía y Paleontología, Universidad de Granada, 18071 Granada, Spain;1. Departamento de Paleontología, Facultad de Ciencias Geológicas, calle José Antonio Novais, 2, Universidad Complutense de Madrid, 28040 Madrid, Spain;2. Departamento de Ciencias Geológicas, Facultad de Ingeniería y Ciencias Geológicas, Avenida Angamos, 0610, Universidad Católica del Norte, Antofagasta, Chile;1. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia;2. Irkutsk Research Technical University, ul. Lermontova 83, Irkutsk, 664074, Russia;3. Department of Earth Sciences, Carleton University, Ottawa, ON K1S 5B6, Canada;4. Tomsk State University, ul. Lenina 36, Tomsk, 634050, Russia;5. Lund University, 12 Sulvegatan, Lund, 223 62, Sweden;6. J. Sutterlay Geochronology Laboratory, Toronto University, Toronto, ON N5S 3B1, Canada;7. V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia;8. Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia |
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| Abstract: | The Vagran placer cluster is located on the eastern slope of Northern Urals. During > 100 years of gold mining history approximately 40 tons of gold have been extracted from the placer deposits.Bedrocks of the region consist of high metamorphic Upper Proterozoic and Paleozoic terrigeneous, terrigeneous-volcanogenic and igneous rocks. Gold placer deposits are mostly alluvial genesis deposits and of Quaternary to Oligocene (?) age. The alluvial deposits consist of gravel with pebbles, boulders, and sandy clay covered by sandy silt and a soil layer. The thickness of the alluvial sequence is usually 5–10 m and reaches 18 m in the main watercourses of the third order. Nearly all of the alluvial sediments are gold bearing but concentrations of economic importance prevail in the bottom part of the sequence above the bedrock.There are four different types of gold particles: (I) rounded and well-rounded particles of high fineness and homogeneous inner structure, (II) rounded to sub-rounded high fineness particles with a pure gold rim developed over a core, (III) crystallomorphic (idiomorphic) high fineness with a homogeneous inner structure, and (IV) irregular angular and subangular particles of medium fineness with a significant content of Ag (10–40 wt.%) and elevated Hg (up to 1.15 wt.%).The first type is prevalent and comprises up to 65% of the total gold particles; it is uniformly distributed throughout the territory. There are features with initially complicated dendritic and laminar shaped particles which were rounded during transportation. The second and third types have a propensity for zones of the inherited erosion–tectonic depressions. Apparently, types I, II and III are related with orogenic mesothermal gold-sulfide-quartz mineralization; the differences of these types depend on the primary zonation of ore bodies and supergenic transformation of the alloys. They were connected with middle-depth ore bodies of an orogenic gold-sulfide-quartz formation. The fourth type is evident of nearby transportation from primary sources and a short duration of supergenic influence. It is controlled by a zone of NW-SE orientation, diagonal to the main structures of Ural Fold Belt.The plot of Au content vs coefficient of heterogeneity (ratio of the Au content in the core and in the rim of the grains) is the distinguishing factor between the four types of gold grains both by primary hypogenetic characteristics and supergenetic features.No corresponding lode occurrence of gold-sulfide-quartz mineralization has been identified to date in this region. Placer gold concentrations are related to the intermediate hosts of the Mesozoic-Cenozoic surfaces of the Ural peneplain uplift in the Oligocene and eroded in Miocene-Quaternary time. This factor determines the widespread distribution of placer gold in the territory of the Vagran cluster.The large, Carlin-type Vorontsovsk gold deposit is located 60 km south-east from the Vagran area. It has a shallow erosional level, small size of native gold, and its distal location from the placer deposits makes it an unlikely primary source for the Vagran placers. However, mineralization of this type of deposit is noted within the cluster.Gold of the fourth type nearly resembles the gold of the Vorontsovsk deposit and, apparently, the source is related to the same hydrothermal mineralization event. ICP MS analyses of the quartz-sulfide lodes in the floor of gold-bearing valleys revealed a gold content of 2.0–6.9 g/t in the zone of type IV distribution. Therefore, gold of the fourth type can be used as an indicator for the exploration of primary bedrock mineralization. The geological setting and typomorphic features of this placer gold shows that the primary gold mineralization is similar to the Vorontsovsk deposit and within the zone of distribution of the placer gold of the fourth type. |
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