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Bereznyakovskoe, Bereznyaky
Chelyabinsk Oblast, Russia
Main commodities: Au Ag

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The Bereznyakovskoe (or Bereznyaky) ore field, which includes the main Central Bereznyakovskoe and the Northern, Southeastern, Southwestern, Northeastern, Western and Northwestern Bereznyakovskoe deposits, that are located in the Yemanzhelinsk district of Chelyabinsk oblast, ~20 km SE of the city of Chelyabinsk in the Southern Urals of Russia.

The ore field is located in the broader Birgil'da-Tomino ore district of the East Uralian Volcanic Megaterrane (see the Mikheevskoe record for the regional setting). The Bereznyakovskoe ore field comprises a tract of metasomatically altered rocks that is ~7 x 1 to 2 km, containing several centres of hydrothermal Au-Ag mineralisation. The total possible resource has been estimated as 50 to 100 t of contained gold at unspecified grades (Lehmann et al., 1999). Mineralisation appears to be related to minor intrusions of porphyritic diorite, which were taken to indicate shallow-seated porphyry gold (Grabezhev and Moloshag, 1993; Grabezhev et al., 1998; Moloshag et al., 1993, 1994; Sazonov et al., 2001; Puzhakov and Kuznetsov, 2008), although ore mineralisation and metasomatic alteration were interpreted by Lehmann et al. (1999) and Lehmann and Grabezhev (2000) to classify the deposits as possibly low-sulphidation (adularia-sericite) epithermal type.

The surrounding 20 x 30 to 40 km district is bounded by Late Carboniferous granite and Early to Middle Carboniferous granodiorite of the composite Chelyabinsk Pluton in the north, and by the NNE trending major Troitsk Fault in the east. Ordovician basaltic rocks occur in the north, overlain by Middle Ordovician to Upper Devonian clastic and carbonate sequences. In the central part of the district, Upper Devonian to Lower Carboniferous volcano-sedimentary rocks are unconformably overlapped by Lower Carboniferous andesitic tuff. Intrusive rocks include the Middle Ordovician Voznesenka gabbro-diorite-granodiorite complex, granite and granodiorite of the Chelyabinsk pluton, and porphyritic diorite of the possibly Late Devonian to Early Carboniferous Birgil'da-Tomino Complex. The Tomino and Kalinovskoe porphyry deposits (together have resources' of 331 Mt @ 0.46 wt.% Cu and 0.1 g/t Au; Volchkov et al., 2015), as well as the Birgil'da and Yaguzak porphyry copper occurrences; the Biksizak silver-base metal occurrence; and the Michurino and Bereznyakovskoe gold-silver ore field are all related to the latter complex (Grabezhev et al., 1998, 2000).

The country rocks in the ore field comprise volcano-sedimentary sequences of the Upper Devonian to Lower Carboniferous Bereznyakovskoe Formation, underlain by the Pervomaisky Formation which outcrops to the SE and is composed of Silurian limestone with occasional carbonaceous sections (Grabezhev et al., 1998). The Bereznyakovskoe Formation is up to 1 km thick and is composed of lithic and crystal tuffs and tuffaceous breccia, lesser tuffstone, and very rare dacitic-andesitic lavas, which are cut by comagmatic subvolcanic intrusions. Limestone beds and lenses that vary from a few cm to a few metres in thickness are often intercalated with the tuffaceous breccia and tuffstone. At depth, but not at the surface, the Bereznyakovskoe Formation is intruded by porphyritic diorite of the Birgil'da-Tomino Complex (Grabezhev et al., 1998, 2000). This complex has been dated as 428 ±3 Ma and 427 ±6 Ma (Grabezhev et al., 2013). The subvolcanic andesitic bodies have been recorded grading into porphyritic diorite at depth (Puzhakov and Kuznetsov, 2008). Rhyodacite and dolerite dykes intrude the volcanosedimentary rocks in the central and southeastern sections of the ore field. A 20 to 30 m thick mantle of weathering covers the entire ore field, oxidising the hypogene ores.

Most orebodies that form the deposits of the ore field are hosted by the Bereznyakovskoe Formation, with the exception of two that occur within limestone of the Pervomaisky Formation. The individual orebodies are predominantly composed of closely spaced zones of disseminated and stringer-disseminated mineralisation, with lesser, but still frequent, massive, brecciated, cockade and veinlet ores. The principal hypogene ore minerals are pyrite and tennantite-tetrahedrite, with lesser enargite, galena, sphalerite and Au, Ag and Pb tellurides. Gangue minerals are quartz and subordinate carbonate. The ore mineralisation is found in zones of metasomatic silicification, with an intensity extending to the development of secondary quartzites, surrounded by sericite-quartz aureoles, which give way to an outer quartz-sericite zone (Plotinskaya et al., 2009). Grabezhev et al. (2000) observed alteration associated with the porphyritic diorite of the Birgil'da-Tomino Complex at depth to comprise an inner chlorite-mica-quartz-albite assemblage grading locally to mica-quartz or pyrophyllite-quartz rocks, with an outer zone where carbonate and chlorite increase strongly, whilst at the periphery, carbonate-chlorite propylitic rocks are found.

Primary quartz remains in the oxidised mineralisation of the epithermal system, although sulphides and tellurides are converted to limonite and jarosite, with associated native gold. Sericite is replaced by hydromica.

Lehmann et al. (1999) recognised three paragenetic stages:
• an early pyrite stage, marked by metasomatic disseminations and veinlets in host rocks;
• a base-metal stage containing tennantite, sphalerite, chalcopyrite, tellurides of Au, Ag, Pb, Hg and other elements, and sporadic native gold; and
• a supergene stage of jarosite, limonite and native gold as products of breakdown of tellurides, accompanied by residual quartz.
Moloshag et al. (1993), Murzin et al. (1994, 1995), and Grabezev et al. (2000) proposed a similar sequence:
• a gold-base metal stage comprising native gold, tennantite-tetrahedrite and other sulphides;
• a gold-telluride-base metal stage composed of tellurides of Ag, Pb and Bi, with native gold; and
• telluride-base metal with Au tellurides and native tellurium.

A spatio-temporal mineral zonation is evident across the ore field representing the passage from early to late mineral assemblages, from the main Central to the Southeastern Bereznyakovskoe deposits. This zonation progressed from enargite, tennantite, native tellurium, tellurides and selenides → tennantite-tetrahedrite, tellurides-and sulphoselenides (galeno-clausthalite) → tetrahedrite, tellurides, native gold, galena and sphalerite. The quantity of gangue quartz increases and the total sulphide content decreases in the same direction. This progression is interpreted to reflect a decrease in sulphur, tellurium and oxygen fugacities, and an increase in pH from the Central to the Southeastern Bereznyakovskoe deposit, and from the early to late mineral assemblages (Plotinskaya et al., 2009). Ag, Sb, Pb, and Zn contents increase in the same direction, whilst As and Te contents and the Au:Ag ratio decrease, and Se and Sn almost completely disappear. Ore deposition is interpreted to have developed over a temperature range of 380 to180°C during the pyrite stage; from 300 to 170°C in the telluride-base metal stage; and 270 to 220°C during the galena-sphalerite stage. Ore-forming solutions were characterised by a salinity of 4 to 8 wt.% NaCl equiv. and predominantly NaCl composition, and were enriched in bivalent cations at a late stage (Plotinskaya et al., 2009). The same authors conclude the Bereznyakovskoe ore field is an epithermal hydrothermal system with mineralisation that varies from high- to intermediate-sulphidation from the main Central to the peripheral Southeastern Bereznyakovskoe deposits.

Bereznyaky is part of the Uzhuralzoloto Group's Chelyabinsk region operations that also include the Kochkar, Svetloye, South Kurasan and West Kurasan deposits.

The most recent source geological information used to prepare this decription was dated: 2009.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Plotinskaya, O.Yu., Grabezhev, A.I., Groznova, E.O., Seltmann, R. and Lehmann, B.,  2014 - The Late Paleozoic porphyry-epithermal spectrum of the Birgilda-Tomino ore cluster in the South Urals, Russia: in    J. of Asian Earth Sciences   v.79, pp. 910-931.
Plotinskaya, O.Yu., Groznova, E.O., Kovalenker, V.A., Novoselov, K.A. and Seltmann, R.,  2009 - Mineralogy and Formation Conditions of Ores in the Bereznyakovskoe Ore Field, the Southern Urals, Russia: in   Originally published in Russian in Geologiya Rudnykh Mestorozhdenii, 2009, Vol. 51, No. 5, pp. 414-443. Re-published in English in, Geology of Ore Deposits, Pleiades Publishing, Ltd.,   Vol. 51, No. 5, pp. 371-397.

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