Sibay, Sibai


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Sibay or Sibai comprises five clusters of volcanic hosted massive sulphide orebodies, distributed over an area of 7 km in a north-south on the eastern margin of the Ural Mountains, 80 km SSW of Magnitogorsk in Russia. Novyi Sibai is the largest of the five clusters (#Location; 52° 41' 43"N, 58° 38' 18"E).

For details of the regional setting, see the Regional Setting section of the 50 Let Oktyabrya record.

The deposit is situated in the core of the Sibay antiform developed on the western limb of the regional Magnitogorsk Synclinorium, in the Tagil-Magnitogorsk Megaterrane of the Uralian Orogenic Belt. The Sibay antiform is approximately 12 km long and 2 to 3 km wide, elongated near north-south. It closes to the south and is truncated by a north-side-down diagonal fault to the north. The fold is asymmetric, with gentle dips of 10 to 30°W on the western limb, and steeper 60 to 70°E dips on the eastern limb. Two parallel north-south faults also bracket the antiform on its eastern and western limbs. The Eastern fault represents a down throw of at least 300 m to the east, and is reflected by an up to 70 m thick crush zone (Smirnov, 1977).

The original (Old) Sibai deposit was discovered in 1913 under gossanous outcrop and has been exhausted. Novyi Sibai was discovered in 1932 and exploited from a 500 m deep open pit until 2000, when mining went underground. Novyi Sibai contains at least 15 stacked, mineralogically zoned orebodies in two groups at four stratigraphic levels: the upper group is exposed in the Novyi Sibai open pit, the lower group occurs at depth below the pit.

Size in 1999 was quoted at: 115 Mt @ 1.0% Cu, 0.04% Pb, 1.56% Zn, 0.6 g/t Au, 16 g/t Ag
Annual production in 1990 totalled 1.63 mt @ 0.9% Cu, 2.5% Zn.

The total pre-mined resource of the field is estimated to have contained 5.6 Mt Cu, 6 Mt Zn, 7000 t Ag and 70 t Au (Laznika, 2006).

Novyi Sibai produced 110 Mt @ 2% Cu, 2.6% Zn, ~50 g/t Ag, ~0.5 g/t Au from the upper level of the open pit, with potentially a further contained 3.51 Mt Cu and 2.6 Mt Zn in both upper and lower levels (Laznika, 2006, quoting N.I. Tatarko, pers com, 2006).

The sequence within the Sibay antiform, from the oldest rocks, which occupy the core, is as follows (Smirnov, 1977):

Middle Devonian,
Karamalytash Group, subdivided into:
• Lower Basaltic Member, >400 m thick - mainly lavas;
• Dacitic Member, 200 to 1000 m thick - lavas with minor seams of tuffs. Maximum thicknesses may represent vent positions;
• Middle Basaltic Member, 0 to 200 m thick - massive and spheroidal lavas with bands of radiolarian jaspers, scoriaceous tuffs and tuffaceous sandstones;
• Rhyolitic Member, 0 to 500 m thick - approximately equal amounts of lavas and pyroclastics. In the upper section there is a 30 m thick horizon of agglomeratic tuffs of mixed composition.
• Upper Basaltic Member, 0 to 700 m thick - coarse pyroclastics with minor lava bands;
The complete sequence is only preserved in the northern part of the antiform, with the upper members thinning to the south. Rhyolites and dacites make up about 40% of the volume of the pile.
Bugulygyr Horizon - red jasper.
Ulutau Group - a terriginous volcano-sedimentary sequence.
Upper Devonian

Two dyke series cut all of the orebodies in the district. The first is composed of dolerites and pyroxene-plagioclase porphyries which are related to the end of the Upper Basaltic Member of the Karamalytash Group; and younger, post upper Devonian gabbro-dolerite dykes (Smirnov, 1977).

The Sibay deposit is located on the northern part of the eastern limb of the Sibay antiform, bounded to the east by the Eastern fault described above, and to the west by the parallel Central fault. Within this fault bounded block the Rhyolite Member attains its maximum thickness. All of the orebodies at Sibay are within the rhyolite member, and are located mainly to the east of the Central fault (Smirnov, 1977).

The mineralisation, like the Sibay antiform, is divided by a diagonal, north-east trending block fault. This structure displaces all members of the Karamalytash Group, as well as the Central Fault. The northern block is known as the Old Sibay, and the southern the New Sibay sector which have markedly different orebody morphologies and attitudes (Smirnov, 1977).

The Old Sibay, or northern sector is characterised by conformable, layered sulphide developments, which are largely composed of banded and lesser uniform sulphides. The orebodies on the western flank are almost horizontal, while those to the east dip at up to 30°E. The thickness of ore is impersistent, and varies from 1 to 20 m. The sulphide bodies are restricted to a horizon of agglomeratic tuffs of mixed composition, often with a welded matrix, that is located in the roof of the Rhyolitic Member. The agglomerate clasts are mainly silicified and sericitised acid volcanics, and numerous lumps of massive sulphide up to 1 m in diameter, but never more (Smirnov, 1977).

The New Sibay, or southern sector, embraces a large accumulation of massive sphalerite-chalcopyrite-pyrite ore with a complicated shape. It comprises two, large, roughly equant, steeply dipping bulges, one 'stratigraphically' above the other. These lensoid bulges are joined by a narrower waist, separating the overall conformable, upper and lower lenses. Each bulge fades out up and down dip into thinner conformable fingers branching off the main mass, and following lithological banding. These fingers contain massive and banded ore, analogous to the layered sulphides at Old Sibay. In polished sections of the New Sibay deposit ores, fossils can be seen to be replaced by sulphides (Smirnov, 1977).

The principal metallic minerals at Sibay are pyrite, sphalerite and chalcopyrite, with minor melnikovite-pyrite, pyrrhotite and magnetite, with impurities of tennantite, galena, arsenopyrite, native gold, hematite, bornite, enargite, greenockite, freibergite, cinnabar and germanite. Quartz and calcite are the most common gangue, with less common siderite, chlorite, sericite, gypsum and barite. The ratio of Cu:Zn varies from 1:1, up to 1:15, averaging 1:1.6. Approximately 90% of the ore is the chalcopyrite-sphalerite-pyrite association. Lesser types are pyritic and sphalerite-pyrite ores (Smirnov, 1977).

Two stages of sulphide formation are implied. The first is represented by the New Sibay bulges and the lower parts of the Old Sibay sector. These are uniform massive sulphides of pyrite, chalcopyrite and sphalerite. They consist of three principal mineral associations, namely: 1) early pyrite; 2) chalcopyrite-pyrite; and 3) sphalerite-pyrite. The early phase pyrite is enriched in Co, but contains none of the late stage trace elements. The various associations are usually spatially combined, and where mixed display patchy, finely veined and brecciated textures. The second stage of ore development is usually banded and of pyrite-chalcopyrite-sphalerite composition. It is typical of the Old Sibay sector and the small conformable layered fingers which branch off the main New Sibay bulges. The surrounding rhyolites, where they are pyroclastics, are characterised by the presence of sulphide clasts (Smirnov, 1977).

Only a weak alteration aureole is evident in the surrounding rhyolite, with quartz-sericite persisting for 10 to 30 m outwards from the sulphides, grading into albite and epidote on the periphery. In outcrop the sulphides have been oxidised to brown ironstones to a depth of 20 m, with secondary sulphides to 40 to 50 m, and to 100 m in the fault zones. The principal minerals of the oxidation zone are goethite, hydrogoethite, kaolinite, covellite and chalcocite (Smirnov, 1977).

The most recent source geological information used to prepare this summary was dated: 1999.    
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.

Novyi Sibai

  References & Additional Information
   Selected References:
Prokin V A and Buslaev F P  1998 - Massive copper-zinc sulphide deposits in the Urals: in    Ore Geology Reviews   v14 pp 1-69
Vikentyev, I.V., Belogub, E.V., Novoselov, K.A. and Moloshag, V.P.,  2017 - Metamorphism of volcanogenic massive sulphide deposits in the Urals. Ore geology: in    Ore Geology Reviews   v.85, pp. 30-63.

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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