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Gaya, Gai, Gaiskoye |
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Russia |
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Cu Zn
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Super Porphyry Cu and Au
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Gaya, Gaiskoye or Gai, in Russia, is located to the east of the Ural Mountains, 30 km north of Orsk, 245 km south of Magnitogorsk in southern Russia, 15 km north of the Kazakhstan border (#Location; 51° 28' 48"N, 58° 29' 19"E).
For details of the regional setting, see the Regional Setting section of the 50 Let Oktyabrya record.
The deposit is situated within the Uralian Orogenic Belt, on the southern section of the western limb of the regional Magnitogorsk Synclinorium, within the Tuba-Gaya structural zone, and is confined to the axial portion of the Gaya volcanic dome. The Gaya volcanic dome is approximately 8 km long and 3 to 4 km in width, elongated north-south. The dome is also asymmetric, with dips on the western limb not exceeding 20°W, while the eastern limb dips at 40 to 60°E. The core of the dome is occupied by lower to middle Devonian lavas, pyroclastics and sub-volcanic intrusives, overlain on the limbs by terriginous volcano-sedimentary rocks of the middle Devonian Ulutau Group. On the western limb the core volcanics may have been overthrust onto the Ulutau Group. Within the dome there is a widespread development of syn-volcanic faults, although these have been largely obscured in the core sections (Smirnov, 1977).
Size quoted in 1999 was >300 Mt @ 1.6% Cu, 0.06% Pb, 0.8% Zn.
Annual production at Gaya amounted to 1.18 Mt @ 2.05% Cu, 1.8% Zn, in 1990.
The main deposit was discovered in 1950 when it was estimated to have reserves of 470 Mt @ 1.4% Cu, 0.7% Zn, 1.175 g/t Au (Laznicka 2010).
The stratigraphy of the Gaya volcanic dome may be summarised as follows:
Middle Devonian,
Lower andesite-dacite sequence, >800 m thick - which hosts all of the ore mineralisation. It is composed of various pyroclastics from fine clastic ashy tuffs to blocky agglomerates with subordinate bands of lava. They are distinguished by a colour variability, from light green to greenish-grey to lilac and red, often with a streaky appearance. The composition of the pyroclastics varies, upwards, from andesite, to dacite and then rhyolite. The thickness of individual pyroclastics is impersistent, associated with complex mutual transitions (Smirnov, 1977).
The formation of these tuffs and lavas was accompanied by intrusion of vent and sub-volcanic facies in the axial portion of the pile to form a complex neck of ignimbrites, vent volcanics, lavas and sub-volcanic rocks, mainly rhyodacitic. Sub-volcanic rhyolite porphyry and rhyodacite occupies a considerable portion of the pyroclastic sequence. The neck has a complex mushroom shape and is elongated north-south in the axial section of the Gaya volcanic dome (Smirnov, 1977).
Upper andesite-basalt sequence, >1000 m thick - which developed in the axial zone and on the eastern limb of the Gaya volcanic dome, unconformably resting on the lower sequence. It comprises block and coarsely clastic scoriaceous tuff-agglomerate with subordinate tuffs, and plagioclase and pyroxene-plagioclase porphyry lavas, all of which have a basalt-andesite composition. They are generally dark in colour. The upper pyroclastics are cut by numerous dykes and sills of plagioclase porphyry and gabbro-dolerite (Smirnov, 1977).
Unconformity
Ulutau Group - a terriginous volcano-sedimentary sequence.
The rocks of the volcanogenic complex have undergone regional 'greenstone' alteration to prehnite-pumpellyite and chlorite-epidote associations. In the central portion of the 'vent zone' the rocks are largely obscured by quartz and quartz-sericite alteration, grading outwards into chlorite, then carbonate, albite and finally epidote. The intense alteration of the vent zone extends along the whole length of the Gaya volcanic dome, varying from a few tens of metres to 500 m in width. Alteration is confined to the lower sequence and associated intrusives, being only weak in the upper sequence, except where it was depressed into the lower zone. The structure of the inner zone is complex, with intense shearing composed of quartz-sericite schists and zones of boudinaged, sheared, altered, schists (Smirnov, 1977).
About 50 bodies of complex, multistage massive and disseminated Fe, Cu, Zn sulphide mineralisation have been identified at Gaya, the great majority of which are sub-economic. They are distributed over a stratigraphic thickness of up to 500 m and mostly comprise sheet-like chalcopyrite and lesser sphalerite that are all located within the inner zone of hydrothermally altered and sheared rocks, and have been recorded throughout its whole length, although the central section has the lowest density of mineralised bodies, with most on the northern end and less to the south. Throughout the entire length of the altered and sheared zone the mineralised bodies are found in its hangingwall. They are conformable with the shearing, strike north-south and dip eastward (Smirnov, 1977).
The largest orebodies have a mushroom shape in cross section, with an undulose, relatively gently sloping upper surface, while the lower surface is very irregular, with a nose and numerous protruding branches. The mineralised bodies branch complexly along strike and down dip over short distances and may wedge out bluntly or as fingers. Marked bulges and pinches have also been recorded. In the deeper part of the orebodies the shape and distribution is controlled by shearing, contacts of vent rocks and by differences in mechanical properties within the hosts (Smirnov, 1977).
The principal commodities are copper, zinc, sulphur, gold and silver. The Cu:Zn ratio is close to 2:1. Cd, Se, Te and Ge are present, with contaminants of As and F. The ore is present as both uniform massive sulphides with >65% sulphides and as segregated ores. The ratio of massive:segregated sulphides is around 2:1. The massive sulphides generally have a clear contact with the country rocks, repeating the undulations and variations in the hangingwall rocks and shear zones. Segregated ores surround the massive pyritic ores downwards and laterally, as well as forming independent downward apophyses. The segregated ores have a well defined contact with the massive sulphides, but are gradational into the pyritic, altered, surrounding country rock. The massive sulphides are composed of chalcopyritic, chalcopyrite-sphalerite and sulpho-pyritic varieties. The ratio of the three in that order are 10:5:1 (Smirnov, 1977).
The principal hypogene minerals are pyrite, chalcopyrite, sphalerite, fahlore and bornite, with minor galena, traces of native gold and silver, tellurides of Pb and Ag, arsenopyrite, pyrrhotite, marcasite, melnikovite-pyrite, etc.. The most common gangue minerals are quartz, calcite, barite, sericite and to a lesser degree chlorite and relict country rock minerals (Smirnov, 1977).
Studies indicate that the pyrite-chalcopyrite mineralisation formed first, with the bulk of the pyrite and a portion of the chalcopyrite of the orebodies being emplaced. At a later stage the pyrite-chalcopyrite-sphalerite phase overprinted the earlier sulphides, emplacing the bulk of the chalcopyrite and sphalerite. Between the two phases the pyrite-chalcopyrite mineralisation was subjected to intense shearing and recrystallisation. Metamorphism and recrystallisation also took place after the pyrite-chalcopyrite-sphalerite phase. The largest amounts of Cu are recorded in the upper parts of the orebodies. A wider development of the pyrite-chalcopyrite-sphalerite phase is also represented in the upper sections, with a tendency for the sulpho-pyritic ores to be better developed at lower levels. In the lower sections of the orebody, segregated ores are only found surrounding massive sulphides in their upper parts. There is no clear mineral zonation however, with the orebodies extending to depths of 1000 to 1200 m without bottoming (Smirnov, 1977). The orebodies have a higher than usual proportion of bornite for a VHMS deposit (Laznicka 2010).
The outcropping orebodies have undergone oxidation to a depth of 10 to 40 m with the formation of brown ironstones, quartz-jarosite, quartz-pyrite sands and secondary sulphides. The supergene zone, which is gold rich, is developed irregularly, to 40 m and locally to maximum depths of 100 to 120 m (Smirnov, 1977; Laznicka 2010).
The most recent source geological information used to prepare this decription was dated: 2010.
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.
Gaya
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Selected References:
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Prokin V A and Buslaev F P 1998 - Massive copper-zinc sulphide deposits in the Urals: in Ore Geology Reviews v14 pp 1-69
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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.
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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, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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