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Norilsk - Talnakh, Polar Division - Oktyabrsky, Komsomolsky, Taimyrsky, Kharayelakh, Skalisty, Severnijy, Mayak, Mokulaevskoe, Bear Creek Division - Norilsk-1, Maslovskoye, Chernogorskoye
Siberia, Russia
Main commodities: Ni Cu PGE PGM


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The Permo-Triassic Noril'sk and Talnakh clusters of deposits of Siberia, northern Russia are 35 km apart and are developed near the north-western margin of the Siberian Platform. The deposits are found within and adjacent to gabbro-dolerite sills that represent part of the feeder zone of the up to 3500 m thick, >2.5 million km2 Permo-Triassic Siberian Trap basalts.

The Talnakh cluster of interconnected deposits, Oktyabrsky, Komsomolsky, Taimyrsky, Kharayelakh, Skalisty, Severnijy, Mayak and Mokulaevskoe are operated as the Polar Division, while those at Noril'sk, which include Norilsk-1, Maslovskoye and Chernogorskoye belong to the Bear Creek Division of Nornickel.

The basement to the Siberian Platform is composed of crystalline Proterozoic rocks overlain by late Neoproterozoic sediments, lower Palaeozoic marine dolostones, argillites and sandstones, and by Devonian marls and evaporites. The late Neoproterozoic to Palaeozoic sequence is 7 to 10 km thick, with >1 km of evaporite-bearing sequences. These are followed by up to 1 km thickness of Carboniferous to Permian shallow water limestones, continental sediments and coal measures of the Tunguska Group.

Significant mineralisation, including the Noril'sk and Talnakh orebodies and the sub-economic deposits at Talminsky (100 km NE of Talnakh) and South Noril'sk (100 km SW of Noril'sk), all lie along the deep seated Noril'sk-Kharayelakh Fault zone, which has been shown to extend to the Mohorovicic Discontinuity. The sub-economic mineralisation at Imangdinsky (60 km east of Noril'sk) falls on a parallel deep structure to the east, the Imangdinsky-Letninsky Fault Zone. All are localised along these deep faults near where they are intersected by NNW-SSE cross structures. In the case of the Noril'sk and Talnakh orebodies, these fall on the margins of the Kayerkansky-Pyasinsky Uplift. The South Noril'sk occurrence is on the margin of the Dudinka Uplift, while Talminsky lies at the intersection with both a NNW-SSE fault zone, the NE-SW North Kharayelakh Fault and the Noril'sk-Kharayelakh Fault zone. These cross fault related uplifts represent zones of thinning and exposure of the sub-Trap Permo-Carboniferous coal measures and carbonate-anhydrite bearing lower to Middle Palaeozoic sediments immediately adjacent to the mineralised zones and Noril'sk-Kharayelakh Fault zone.

Mafic sills and dykes, which disrupt the Palaeozoic sequence below the Trap basalts have divided into 6 groups in the Noril'sk District, as follows: Group 1 - alkaline and sub-alkaline affinity; Group 2 - high Ti dolerite dykes; Group 3 - dolerite sills; Group 4 - differentiated intrusions NOT related to ore; Group 5 - differentiated intrusions found in the vicinity of ore - subdivided into a Cr rich (>400 ppm) Group 5A and Cr poor (<400ppm) Group 5B; Group 6 - other intrusions.

The Group 5A, Cr Rich Noril'sk Group - are all predominantly present as sills, none are alkaline, while all have low TiO
2, high Cr (>400 ppm Cr) and high MgO (often in the range 7 to 13%). These sills cut all of the early Upper Permian and the lower sections of the Lower Triassic Stage 2 Trap basalts. They include the well differentiated Talnakh and Chernogorsky (at Noril'sk) Type intrusions which are layered with diffuse gradational boundaries, ranging from picrite and plagioclase-peridotite to anorthositic or leuco-gabbro in composition. The former is associated with both disseminated and massive sulphides while the latter only has disseminated mineralisation. Other Group 5A intrusions include the Kruglagorsky Type, which are leuco-gabbro sills that often flank the Talnakh and Chernogorsky Type intrusions, usually with no associated significant sulphides and are only weakly differentiated. The Zubovsky Type, which, while containing 10 to 20% ultramafic layers, consists predominantly of intermediate composition rocks lacking leuco-gabbro, is weakly differentiated and carries weak disseminations only. The relatively weakly differentiated Dvuborginsky Type has a low proportion of olivine rich layers, and has no significant sulphides. The Makusovsky Type is not differentiated, and also has no associated significant sulphides.

Group 5B, Cr Poor Lower Talnakh Group - are believed to be slightly older than the Noril'sk Group of sills, but closely associated. They also occur as sills, are not alkaline, have low Cr (<400 ppm Cr) and low Ti, are strongly differentiated and have weak associated disseminated sulphides in places, but not of ore grade. They generally occupy a position several tens of metres below the Noril'sk Group sills, though sometimes laterally removed. At Talnakh however, while this is the case on the southern side of the field, on the northern side the Lower Talnakh sill occurs above the mineralised intrusive. The Lower Talnakh Sill has a very similar appearance and zoning to the main Group 5A sills and is difficult to distinguish, other than by 'stratigraphic position'.

The host sills to ore are developed within and adjacent to, being found within 7 km on either side, of the continental scale, NNE trending, deep seated Noril'sk-Kharayelakh fault. These ore bearing sill like intrusive bodies have known dimensions of as much as 15 x 0.5 to 2 km and are from 50 to 300 m in thickness, although they only represent 1 to 2% of the total mafic intrusives in the district. A typical ore related sill is said to consist of a lower olivine bearing, melanocratic gabbro-dolerite with a picritic composition, passing upwards into leucocratic gabbro-dolerites near the top. This has been variously interpreted to result from differentiation or from multiple intrusive pulses. On both the upper and lower margins there are zones of taxite containing increased sulphides, platinoids and chromite. A taxite is a mafic igneous rock with a very variable texture (from fine to pegmatitic) - and composition (leucocratic to melanocratic) - with discrete ghost-like remnants of inclusions of other gabbroic and country rocks. The upper zone is more weakly taxitic. A strong hornfels halo extends for 100 m below the sills and up to 250 m above.

Of the six different groups of mafic intrusives, only the Group 5 sills, which are those associated with mineralisation, have appreciable contained sulphides. The sulphur content of the barren groups of sills is very low (0.03 to 0.08% S) and light sulphur isotopes predominate. The average sulphur content of the Group 5 sills is higher, generally between 0.2 and 2.2% S and is significantly enriched in heavier S isotopes. The sulphur content of the Group 5A ore associated sills is the highest (0.95 to 2.2% S), exceeding that of Group 5B (0.2 to 0.3% S) which only contain sub-economic mineralisation. Both have enriched heavy S isotope levels, with Group 5B having the maximum (Grinenko 1985). The heavy S is characteristic of a sediment hosted sulphate source, which has been demonstrated to be the case for the gypsum and anhydrite of the country rock sediments which are rich in heavy S isotopes (Grinenko 1985). Heavy sulphur at any point within the mineralisation associated mafic intrusives closely coincides with the sulphate content of the immediate sediments they cut. In addition there is a direct correlation between the value of heavy S and the total S content of these sills. These observations ar taken to indicate that a percentage of the sulphur of the sills has been derived in situ from the adjacent sulphate bearing sediments (Grinenko 1985).

As well as cutting the Palaeozoic evaporite-rich sequences within both the Talnakh and Noril'sk deposits, the Noril'sk-Kharayelakh Fault zone, the host mafic sills/dykes, and overlying Trap basalts have locally strongly affected the Carboniferous to Permian coal measures of the Tunguska Group. This has involved upgrading the coal to anthracite, and in places transforming it into large scale deposits of graphite. Such transformation is envisaged to have potentially released a reductant in the form of hydrocarbons, which will include sour, H
2S bearing gas, derived sulphidic coal measures.

Experimental studies by Iacono-Marziano et al. (2017) support the hypothesis that the high temperature (~1200°C) chemical assimilation of abundant sulphate-bearing evaporitic rocks supplied additional S to the mantle derived magmas at Noril'sk-Talnakh. This assimilation was shown to suppress sulphide saturation and reduce olivine crystallisation, although extreme assimilation led to sulphate saturation in the magma. They also showed that conversely, coal assimilation promotes sulphide segregation and magma crystallisation, while decreasing the dissolved H
2O content of the melt and increasing the amount of coexisting fluid phase. Therefore, the magma, which had assimilated evaporites was enriched in S, which was then transported during magma ascent in the form of dissolved, oxidised S. Subsequently, interaction with coal bearing carbonaceous sediments resulted in a substantial reduction of the oxidised magma, which then induced sulphide segregation. This mechanism, they suggest, can potentially produce massive sulphide deposits by important sulphate assimilation and minor organic matter assimilation. Lesser assimilation of sulphate-bearing evaporites and/or coal would lead to the development of disseminated to sub economic or no sulphide.

There are three main ore types developed at Noril'sk and Talnakh. These are:
i). Disseminated sulphides within the differentiated gabbro-dolerite sills, principally on the lower margins of the mineralised sills in the taxitic and to a lesser degree the picritic zones. They occur as droplets, schleiren and fine sulphide veinlets, forming sheet like conformable bodies up to 40 m thick and comprise combinations of chalcopyrite, cubanite and pyrrhotite with troilite and pentlandite. Grades average 0.5 to 0.6% Ni, 0.6 to 0.7% Cu, and 5 to 6 g/t PGE;
ii). Massive sulphides found principally on the lower contact of the mineralised sills, both within the enclosing rocks and to a lesser degree the sill, and are often separated from the sill by several metres of barren sediment or cupriferous mineralisation. Sometimes they are also found on the upper margin of the sill. In other locations, the massive sulphides cut across the sill to its upper margins. The massive sulphides are divided into either pyrrhotite, cubanite or chalcopyrite types, depending on the dominant sulphide, with associated pentlandite moihoekite and talnakhite. Individual massive sulphide bodies may be up to 60 m thick as at Oktyabr'sky where it covers an area of 3.5 km
2. Grades vary drastically with the sulphide assemblage, but are of the order of 2.8% Ni, 5.6% Cu and 15 g/t PGE. There is evidence that the massive sulphides post date the disseminated sulphides; and
iii). Vein disseminations of 'cupriferous' ore which are found below the massive sulphide, between the massive sulphide and the lower margins of the sill, and within sediments on the upper margin of the sill, often associated with zones of 'skarn' altered brecciated dolerite and marl/argillite. They may be 10 to 20 m thick. The main metallic minerals are pyrrhotite, pentlandite, chalcopyrite, cubanite, millerite, pyrite, magnetite, bornite, chalcocite, etc.

The total production + resource in the Noril'sk-Talnakh district are quoted by Naldrett (2004) at:
     1.309 Gt @ 1.77% Ni, 3.57% Cu, 0.061% Co, 9.5 g/t PGE (including 1.84 g/t Pt, 7.31 g/t Pd).

Ore reserve and mineral resources at 31 December, 2011 (Noril'sk Nickel webpage, 2013) were:
  Talnakh proved ore reserves
      Massive sulphide ore - 46.867 Mt @ 2.71% Ni, 3.36% Cu, 5.81 g/t Pd, 1.33 g/t Pt, 0.15 g/t Au, 7.54 g/t 6PGM
      Cuprous ore - 34.468 Mt @ 1.04% Ni, 4.19% Cu, 9.95 g/t Pd, 2.37 g/t Pt, 0.70 g/t Au, 12.44 g/t 6PGM
      Disseminated ore - 40.966 Mt @ 0.45% Ni, 0.79% Cu, 3.49 g/t Pd, 1.29 g/t Pt, 0.19 g/t Au, 4.97 g/t 6PGM
    TOTAL Talnakh proved ore - 122.301 Mt @ 1.48% Ni, 2.73% Cu, 6.20 g/t Pd, 1.61 g/t Pt, 0.32 g/t Au, 8.06 g/t 6PGM
  Talnakh probable ore reserves
      Massive sulphide ore - 79.074 Mt @ 2.55% Ni, 2.69% Cu, 4.84 g/t Pd, 0.89 g/t Pt, 0.14 g/t Au, 6.30 g/t 6PGM
      Cuprous ore - 49.412 Mt @ 0.79% Ni, 3.53% Cu, 7.52 g/t Pd, 1.93 g/t Pt, 0.56 g/t Au, 9.67 g/t 6PGM
      Disseminated ore - 32.996 Mt @ 0.37% Ni, 0.62% Cu, 0.76 g/t Pd, 1.29 g/t Pt, 0.19 g/t Au, 3.47 g/t 6PGM
    TOTAL Talnakh probable ore - 161.482 Mt @ 1.56% Ni, 2.53% Cu, 5.17 g/t Pd, 1.18 g/t Pt, 0.28 g/t Au, 6.75 g/t 6PGM
  Talnakh measured + indicated mineral resources
      Massive sulphide ore - 20.470 Mt @ 4.23% Ni, 5.83% Cu, 12.95 g/t Pd, 2.54 g/t Pt, 0.51 g/t Au, 15.91 g/t 6PGM
      Cuprous ore - 1.505 Mt @ 0.80% Ni, 2.26% Cu, 6.64 g/t Pd, 1.91 g/t Pt, 0.41 g/t Au, 8.74 g/t 6PGM
      Disseminated ore - 1332.722 Mt @ 0.52% Ni, 1.05% Cu, 2.89 g/t Pd, 0.83 g/t Pt, 0.19 g/t Au, 3.89 g/t 6PGM
    TOTAL Talnakh resources - 1354.697 Mt @ 0.57% Ni, 1.12% Cu, 3.04 g/t Pd, 0.86 g/t Pt, 0.19 g/t Au, 4.08 g/t 6PGM
  Noril'sk disseminated ore
      Proved reserves - 32.897 Mt @ 0.34% Ni, 0.48% Cu, 4.03 g/t Pd, 1.64 g/t Pt, 0.17 g/t Au, 5.98 g/t 6PGM
      Probable reserves - 22.306 Mt @ 0.28% Ni, 0.36% Cu, 4.28 g/t Pd, 1.75 g/t Pt, 0.20 g/t Au, 6.37 g/t 6PGM
      Measured + indicated resources - 25.802 Mt @ 0.33% Ni, 0.45% Cu, 4.24 g/t Pd, 1.67 g/t Pt, 0.15 g/t Au, 6.30 g/t 6PGM
  Combined Talnakh and Noril'sk ores
      Proved + probable reserves - 338.986 Mt @ 1.33% Ni, 2.26% Cu, 5.38 g/t Pd, 1.42 g/t Pt, 0.28 g/t Au, 7.12 g/t 6PGM
      Probable reserves - 22.306 Mt @ 0.28% Ni, 0.36% Cu, 4.28 g/t Pd, 1.75 g/t Pt, 0.20 g/t Au, 6.37 g/t 6PGM
      Measured + indicated resources - 1380.499 Mt @ 0.57% Ni, 1.11% Cu, 3.07 g/t Pd, 0.87 g/t Pt, 0.19 g/t Au, 4.12 g/t 6PGM
      Inferred resources - 462.709 Mt @ 0.89% Ni, 1.85% Cu, 4.40 g/t Pd, 1.12 g/t Pt, 0.26 g/t Au, 5.75 g/t 6PGM.



The following is a more detailed description of the   History,   Tectonic Setting,   Regional Stratigraphy,   Mafic Rocks   and   Contact Alteration   at, and surrounding, the Noril'sk and Talnakh deposits.

History

  Cu-Ni mineralisation in the Noril'sk region was first indicated in around 1860 (von Gruenewaldt 1991) and further established in 1920 in the Noril'sk I deposit by N Urvanstev (Smirnov 1977). Detailed exploration and small scale exploitation commenced in 1935, while mining at Noril'sk-I began in 1943 using forced labour. The first mines were relatively small, one of which produced native PGE's alone. Coal was also mined for power from the Permian coal measures within a few kilometres of Noril'sk I. In 1945 the larger underground Zapolyarny Mine was commissioned at Noril'sk, followed by three open pits, the North, South and Coal Creek (or Medvezhy Ruchey).
  By the early 1960's some 3 Mt of ore had been extracted from the underground mine, while 9.5 Mt of ore and 60 Mt of waste had been removed from the open pits. At that stage it was decided at the Soviet Government level that the operation should be closed. However in August 1960 drilling in the Talnakh area intersected the mineralised host intrusive and following more intensive exploration, including underground development, high grade ore was delineated in appreciable quantities. As a consequence the Noril'sk operation was allowed to continue without significant direction from Moscow. The first ore was mined at Talnakh in 1965 although the large Komsomolsky Mine commenced operation in 1971 with 800 m shafts, followed by the Oktyabr'sky Mine in 1974 whose shafts are 1200 m deep. The other operating mines were the Taymyr, construction of which was completed with 1600 m deep shafts in the early 1980's, and the Mayak Mine with 1200 m shafts. The Skalisty mine shaft was commissioned in 1997 at a depth of approximately 1000 m, while the deep Glubokijy shaft at 2500 m, was under construction. Each of these shafts services a separate mine. At Noril'sk the underground Zapolyarny and open cut Medvezhy Ruchey (Noril'sk-I) were operated.
  In 1993, by decree of the President of the Russian Federation, the Noril'sk Nickel Kombinate was transformed into Russian Joint Stock Company (RJSC) Norilsk Nickel, which in 2001, was restructured to become MMC Norilsk Nickel (Nornickel), the shares of which were listed on the RTS (Russian Trading System) and MICEX (Moscow Interbank Currency Exchange) stock exchanges.



Tectonic Setting

  The Noril'sk district lies on the far north-western margin of the Siberian Craton, near the Arctic Ocean coast of central Asia. Some 100 km to the north of Noril'sk the craton is bounded by the Khatangsky Rift Zone (or Yenesejsko-Khatangsky Trough) which separates it from the Taimyr Block that is to the north. Approximately 150 to 200 km to the west the craton is fringed by the West Siberian Lowland, a broad zone of Cenozoic sediments separating it from the Uralian Orogenic Belt and the East European (or Baltic) Craton. The West Siberian Lowland is underlain by two elements, the Central Asian Rift Zone and the northern segment of the Kazakhstan Block. The Siberian Craton was cratonised during the Proterozoic, while the Taimyr Block was not consolidated until the end of the Palaeozoic. The Taimyr Block apparently separated from the Canadian Craton during the Early Cretaceous as the Arctic Ocean opened up.
  Extension and associated crustal thinning in the Central Asian Rift Zone apparently commenced in the Triassic following compression during the Carboniferous and Permian reflected by tectonism in the Uralian Orogenic Belt (Zonnenshain, et al., 1990 and Naldrett, et al., 1992). The timing of development of the Khatangsky Rift Zone is less well defined, although Jurassic sediments have been mapped. Subsidence within both rift zones was accompanied by sedimentation from the Late Triassic to Mid Tertiary which reaches thicknesses of up to 10 km (Naldrett, et al., 1992).
  Both the Central Asian and Khatangsky Rift Zones correspond closely to thinning/attenuation of the crust (or depth to the Mohorovicic Discontinuity) which is less than 30 km in the cores of the two rift zones. This thickness increases rapidly to >40 km at the margin of the Siberian Craton and 35 to 40 km over the northern Kazakhstan Block.
  The most outstanding feature on the Siberian Craton is the development of the extensive Siberian Traps, the thick mafic volcanics and related intrusives of Permo-Triassic age which occupy much of the western section of the Craton. This volcanism commenced in the Late Permian and continued until the Middle Triassic, coincident with the early stages of extension in the Central Asian Rift Zone. Trap basalts are also found on the Taimyr Block and below Jurassic sediments in the Khatangsky Rift Zone (Naldrett 1992). On the Siberian Craton these mafic rocks are up to 4000 m thick and cover an area of 340 000 km
2, although it is estimated that they originally had an areal extent of 4 million km2 (Wooden, et al., 1992). These volcanics are considered to be related to the Mesozoic breakup of Pangea as are many of the other great continental basalt accumulations, specifically the Jurassic Karoo basalts of southern Africa the Jurassic to Cretaceous Parana Basin basalts of Brazil and probably the Jurassic dolerites of Tasmania and Antarctica. According to Wooden, et al., (1992) the Siberian trap basalts differ from other continental flood basalt provinces in that they are characterised by initial and continuing explosive volcanism.
  All of the significant Ni-Cu-PGE mineralisation in the Noril'sk district lies, straddles and falls within 7 km on either side of the major Noril'sk-Kharayelakh Fault zone. This major NNE-SSW trending structure appears to be intimately related to the mineralisation and to the intrusive phases of the mafic extrusives in the area. It is clearly evident on regional magnetic data images, being traceable below and through the Central Asian Rift Zone sediments for more than 1000 km.
  Duzhikov, et al., (1988) note that deep seismic and electrical traverses across the Noril'sk-Kharayelakh Fault zone show a sharp change in depth of the Mohorovicic Discontinuity from 45 to 50 km from west to east, suggesting that it is a deep structure extending to the mantle. However Naldrett, et al., (1992) note that where intersected in mine openings it is observed as a crush zone 10 m wide with vertical displacements of less than a few hundred metres. During a visit in 1993 we were told that the movement on the fault was ~200 m vertically, with an unspecified dextral lateral displacement. At the surface it is reflected by a broad U shaped valley around 1 km wide.
  The Noril'sk-Kharayelakh Fault falls within a broader 10 to 35 km wide belt of fault bounded troughs and blocks controlled by a dense network of parallel to sub-parallel dislocations. Faulting within this zone was active from prior to the Proterozoic cratonisation and has been re-activated periodically from the Precambrian to the Tertiary (Naldrett, et al., 1992). Earlier depositional troughs within the same region include the NNE-SSW trending Taimyr-Tungussky Trough of 'Karelide' age (around 1800 Ma) which also occupied the north-western corner of the current Siberian Craton and cuts the central section of the Taimyr Block to the north, and the Middle to Upper Proterozoic Pre-Yenejsky Trough (Duzhikov and Strunin 1988). The major deep faults within this zone are parallel to the Uralian Orogenic Belt to the west. Duzhikov and Strunin (1988) state that "... in the Noril'sk region, an area of fault activity extending intermittently over a significant interval of geological time, there is a high density of trans-crustal faults as compared to adjacent terrains".
  Continued activity on the Noril'sk-Kharayelakh Fault zone occurred during the deposition of the Lower to late Middle Palaeozoic, where it appears to represent a major basement dislocation manifested in the Palaeozoic sediments as a ductile flexure (more than a brittle fault) forming the western margin of the Khantajsko-Rybninsky uplift. Movement on this structure, while pronounced until the end of the Devonian, was less marked during the Permo-Carboniferous. It appears to be the main axis of the lower part of the Permo-Triassic Trap volcanism, as well as being reactivated as a transform to the later Khatangsky Rift Zone (Duzhikov and Strunin 1988). Within the numerous zones of faulting in the Noril'sk district there are mylonites, blasto-mylonites and cataclastics after sedimentary and igneous rocks and the sulphide ores (Smirnov, 1977).
  The triangular north western corner of the Siberian Craton, that defines the Noril'sk district, is separated from the rest of the craton by another series of major NNE-SSW trending deep faults which are parallel to, and to the east of, the main Noril'sk-Kharayelakh Fault zone (Naldrett, et al., 1992). One of the more important of these is the Keta-Urbinski fault which, while obvious on geophysical data, has only a weak trace at the surface. This fault zone is a major terrain boundary between blocks of differing structural and geological character during the Proterozoic and Palaeozoic. Within this triangular shaped area, most faults trend either in a NNE direction, sub parallel to the Keta-Urbinski fault or NE which is parallel in part to the Khatanga Rift Zone. A few trend NNW.
  It has been proposed that the Noril'sk district represents the remnant of an earlier Proterozoic Mobile Belt parallel to the Uralian Orogenic Belt to the west.
  Within the Noril'sk-Talnakh region, there are a number of basin like structures as defined by the base of the Trap basalts. These basins are formed by the interaction of a series of positive and negative warps, which are partly pre- and partly post- Trap volcanism. The main negative structures are predominantly post-Trap in age and include the Kharayelakh, and the interconnected Vologochansky and Noril'sk structural basins to the NE and SW respectively, separated by the NNW-SSE trending Kayerkansky-Pysasinsky Anticline/Uplift. A 10 to 12 km wide, NE-SW zone of more intense activity, across which the thickness and facies of the Palaeozoic sediments change, separates the Vologochansky and Noril'sk structural basins which are respectively to the NW and SE. The Dudinka Uplift forms the western margin of both the Vologochansky and Noril'sk basins, while the Kayerkansky-Pysasinsky uplift separates both from the Kharayelakh basin to the NE. The Kharayelakh, and Noril'sk basins are both depressions centred on the Noril'sk-Kharayelakh Fault zone. The major north-south elongated Tunguska Syncline occurs on the eastern margin of the region. It is separated from the other basins by the NNE-SSW Khantajsko-Rybninsky Anticline which parallels, and is to the east of the Noril'sk-Kharayelakh Fault, and is characterised by a zone of over-thrusting. The Noril'sk and Talnakh deposits both lies on the Noril'sk-Kharayelakh Fault zone, located on the SW margin of the Kharayelakh Basin and the NE margin of the Noril'sk Basin respectively. While the basins are largely post-Trap, warping is believed to in part be coincident with the development of the basalts, as reflected in thickness variations in the tuffs and lavas.



Regional Stratigraphy

  Rocks ranging in age from Mesoproterozoic to Cainozoic are represented in the Noril'sk area. The regional sequence is as follows, from the base:

MIDDLE TO UPPER PROTEROZOIC - Proterozoic (Riphean - i.e., 1600 to 650 Ma) rocks in the Noril'sk area are restricted to the western margin of the Siberian Craton, while a core of Archaean crystalline basement is rimmed by Proterozoic sediments to the east. According to Duzhikov and Strunin (1988) the Proterozoic sediments comprise,
Igarsky Suite, 1500 to 1600 m thick - spilitic lavas, tuff breccias, lava breccias and porphyries with both trachy-basaltic and tholeiitic compositions and interbedded jasper to jasperoid beds up to 4 m thick, which may be traced over a strike length of 300 km and width of 50 to 60 km. This suite is confined to the western margin of the Siberian Craton.
Ludovsky Suite, up to 2000 m thick - black, green-black and rarely mottled phyllitic schists with rhythmic interbeds of quartz sandstone.
Gubinsky and Staromostovsky Suite, 1500 m thick - quartz and rarely quartz-feldspar sandstone, conglomerates (sometimes glauconitic) with interlayers of clay and argillite comprise the Gubinsky Suite, while a 150 m thick unit in the upper part of the section, which also contains interlayers of assorted tuffs, tuffites and lavas of olivine basalt composition, is the Staromostovsky Suite. Conglomerates carry clasts of sandstone, siltstone and felsites. Interpreted correlates of the Gubinsky Suite form the basal section of the Proterozoic platform sediments, unconformably over Archaean crystalline basement in the Anabarsky uplift in the core of the craton to the east.
Chernorechensky Suite, 700 to 1300 m thick - dark grey to black argillaceous limestones and lime dolostones with interlayers of black claystones, and sometimes calcareous graphitic schist, marl and rarely quartz sandstone. The unit thins to the west.
Izluchinsky Suite, up to 1130 m thick - rhythmically layered red coloured variegated sandstones, siltstones, argillites and conglomerates with rare dolostones.
• Yenesejsky Mountain Range Metamorphism and Granites, dated at 850-900 Ma - resulted in the overthrusting of the Igarsky and succeeding Ludovsky suite onto the younger sediments of the Gubinsky, Staromostovsky, Chernorechensky and Izluchinsky suites, together with intrusions of gabbro, alkaline syenites and lamprophyres. The large thrusts dip predominantly to the west, and were reactivated periodically in the Phanerozoic.

UPPER PROTEROZOIC (Vendian - 650 to 570 Ma) TO LOWER CAMBRIAN - sediments of this age represent the onset of platformal deposition on the newly consolidated Siberian Craton and overlie the Middle to Upper Proterozoic (Riphean) sequences with angular unconformity. The succession includes the following (from Duzhikov and Strunin 1988),
Sukharinsky, Gremyakinsky and Polbansky Suites, 420 to 540 m thick - commencing with layers of quartz sandstone and conglomerate, succeeded by rhythmically layered limestone and dolostone with intervening layers of marl and sandstone. The sequence thins from west to east. Further to the south-east a comparable sequence, the 340 to 385 m thick Platonovsky Suite comprising lacustrine carbonates and bedded sulphate horizons, may be an equivalent.
  Unconformity

CAMBRIAN TO ORDOVICIAN - comprising (from Duzhikov and Strunin 1988)
Krasnoporozhsky Suite, 60 to 220 m thick - decreasing in thickness from the south and west towards the north. Comprises assorted carbonates, mainly limestones, dolostones and marls. Dolostones are replaced by argillaceous limestones to the east.
Shumninsky Suite, 0 to 290 m thick - a grey coloured limestone unit with dolostones to the west which are also replaced by argillaceous limestone to the east. This unit thins to the north and is almost absent near Noril'sk.
Ust'brussky Suite, 150 to 450 m thick - mainly 'lacustrine carbonates'.
Lbazny Suite, 200 to 630 m thick - mainly 'flyschoid lacustrine carbonates', which thin towards the Noril'sk area.
Oraktinsky and Kulyumbinsky Suites to the south of Noril'sk, and the Chopkinsky and Tukoladinsky Suites at Noril'sk, 1100 to 1300 m thick - varied 'sulphate-lacustrine-carbonate' units.
Early Middle Ordovician Suites, 850 to 1000 m thick - comprising multi-coloured sulphate-lacustrine-carbonates of the Ujgursky and Ust'mundujsky suites; dolostone-limestone and clay-dolostone-calcareous sediments of the Il'tyksky suite; and sulphate-carbonate-lacustrine formations containing zones of quartz sandstone of the Guragirsky suite. The quartz sandstones are followed by a regional disconformity.
Middle Ordovician Suites, tens of metres thick - comprising varied lithologies, ranging from marls, argillites and limestone with organic detritus, sandy limestones and phosphorite bearing quartz sandstones, with terriginous rocks increasing towards the south.
Upper Ordovician Suites, are generally absent in the Noril'sk region as a result of a regression at the base of the Silurian and consequent erosion of these sections of the Ordovician.
  Unconformity

SILURIAN - comprising (from Duzhikov and Strunin 1988),
Chambinsky Suite, 100 to 120 m thick - a graptolite bearing limestone-clay, or marl formation containing graptolitic argillites with interlayers of organogenic limestones containing pyrite concretions.
Ugiyuksky and Tanimensky Suites to the south, and Talikitsky and Omnutakhsky Suites in the northern and central areas, 200 to 330 m thick - mainly argillaceous limestones, thickening towards the south.
Muktensky, Khyuktinsky and Uragdansky Suites, 65 to 130 m thick - containing massive coral-stromatoporoid limestone, usually with silica concretions, while the upper sections comprise clay-concretion like limestones with interbeds of organogenic rocks. The thickest section is to the south.
Tukal'sky, Kongdinsky and Makussky Suites, 80 to 140 m thick - typically a complex limestone with elements of a reef. In the northern section of the area, around Noril'sk where the sequence is thinnest, there is a slightly higher dolostone and sulphate content, particularly in the upper section. The remainder of the formation is mainly lagoon to marine limestone-dolostones. These units are largely of Upper Silurian age.
Nizhnepankogirsky Sub-suite and Postnichny Suite, 75 to 220 m thick - predominantly a sulphate-argillaceous-dolostone formation. The lower section comprises the Nizhnepankogirsky Sub-suite south of Noril'sk and the Postnichny Suite in the Noril'sk area, the latter of which comprises argillaceous limestone-dolostone, limestone and dolostone with interlayered marl. The thinner sections correspond to the structural highs such as the Dudinka High. The upper part of the section grades into the Devonian and comprises dolostone with interlayered marl, anhydrite and gypsum.

DEVONIAN TO LOWER CARBONIFEROUS - the following is based on a detailed stratigraphic column provided by Noril'sk Nickel, details in Duzhikov and Strunin (1988) and discussions over drill core at Noril'sk.
Lower Devonian - comprising,
Yamnakhtinnia Suite, which is lower Gedinnian in age and 40 to 93 m thick - in the Noril'sk district this unit comprises massive coarse dolostone with multiple interlayers of anhydrite and gypsum. The carbonates sometimes have lenses and clusters of celestite (strontium sulphate). The sulphate content of the sequence increases towards the north of the region, particularly in the vicinity of the pre-existing troughs (see the Tectonics segment above).
Khrebtovsky Suite, of middle Gedinnian age, and 43 to 87 m thick - marl, argillite, dolostone and anhydrite in thin rhythmic layers;
Zubovsky Suite, of upper Gedinnian age, and 100 to 150 m thick - regionally, generally comprising grey coloured dolostone-marls, interlayered with clayey dolostone, argillite and dolostone. The thickness is generally fairly constant throughout the region. Core viewed at Noril'sk had around 25% anhydrite/gypsum within grey marls as thin laminae and beds up to several cm's thick. In the Noril'sk district, where the Zubovsky Suite ranges from 97 to 160 m in thickness, it is subdivided into,
 - Lower Sub-suite, 35 to 90 m thick - variegated marl, dolostone and anhydrite. A layer of anhydrite generally found at the base is up to 20 m thick.
 - Upper Sub-suite, 45 to 90 m thick - grey to greenish-grey marl with bands of anhydrite, gypsum and dolostone.
Kureisk Suite, of Siegenian age, and 67 to 85 m thick - which regionally, generally comprises multi-coloured/variegated dolostone, calcareous argillite and marls with rare inter-layers of siltstones and limestones. The sequence is thickest to the south west of Noril'sk. In the Noril'sk district there are two facies, the first which, tends to occur along the margins of the uplifts, comprises grey to greenish-grey, variegated and evenly banded marl, siltstone and argillite. The second facies is concentrated in the deeper basins with thickest sections and is composed of greenish-grey marl, clay-siltstone, dolostone and limestone and is 20 to 70 m thick. Where seen in core at Noril'sk it was mainly red and green 'splotchy' argillites with black interbands of calcarenite within the argillite.
Razvedochninsky Suite, of Emsian age, and 0 to 250 m thick - which regionally comprises transgressive micro-cycles of multi-coloured siltstones, sandstones and gravels/conglomerates. Terriginous rocks become more abundant to the south where these rocks contain acid pyroclastic material, grading into volcanomict sandstone, argillite and tuffite. These sections commonly contain elevated manganese in the form of mangano-siderite. This suite is everywhere characterised by considerable sulphate. It never exceeds 110 to 150 m in thickness along the uplifts, but in the deeper sections of the basins, as near Noril'sk, it approaches 150 to 235 m in thickness where the sulphate and carbonate content increases.
  In the Noril'sk district the unit is described as comprising phosphorous bearing, variegated, argillite, with lenses and bands of dolostone and limestone in the upper sections and lenses of phosphorous bearing gravels/conglomerates and sandstones in the lower sections. Examples seen in core at Noril'sk were mainly argillite and siltstone with lenses and lumps of phosphorite.
Middle Devonian - comprising,
Manturovsky Suite, 180 to 700 m thick - which has been subdivided into,
 - Lower Manturovsky Sub-suite, 120 to 150 m thick - which is, in turn, split into five units in the immediate Noril'sk district, as follows,
 i). 25 to 130 m of variegated marl with anhydrite clots and lenses, and bands of black argillite. A ferrous siltstone with sandstone lenses is found at the base of the unit;
 ii). 15 to 40 m of variegated marl with anhydrite clots;
 iii). 10 to 80 m of grey dolostone, anhydrite and marl;
 iv). 30 to 150 m of variegated marl with anhydrite clots;
 v). 10 to 60 m of anhydrite, dolostone and marl.
 - Upper Manturovsky of Eifelian to Givetian age, and 45 to 180 m thick - which has two different facies types. The first of these facies comprises brecciated grey carbonate and marl with greenish-grey dolostone that is mainly localised over the uplifts such as the Dudinka and Khantajsko-Rybninsky. In the Noril'sk area, these sedimentary rocks are up to 75 m thick. In these areas there is a short lived regional disconformity at the base of the sequence. The second facies comprises predominantly dolostone, marl and anhydrite, with a red marl in the middle and a sub-suite at the base composed of brecciated salt bearing carbonate and layers of halite. This units is better represented in the basinal structures, and is best developed to the east of the Imangdinsky-Letninsky Fault in the Tunguska Syncline where they are 400 to 536 m thick, although they are usually only 120 to 210 m thick and decrease to the south. According to Duzhikov and Strunin (1988) this second facies, the evaporite unit, is localised within erosional features above the Razvedochninsky Suite. There is a lateral transitional facies between the two on the margins of the uplifts. Core from the Manturovsky Suite in the Noril'sk area comprised marls with appreciable anhydrite as blocks/clasts within a black 'shale' matrix and as irregular bands interlayered with dolostone and marl. This unit is.
Yuktinsky Suite late Givetian age, and 10 to 130 m thick - comprises lacustrine-carbonate sediments 12 to 40 m thick which prevail on the uplifts (see the Tectonics' segment above), while sulphate bearing sequences up to 55 m thick predominate in the troughs (Duzhikov and Strunin 1988). The thickness of the sulphate bearing facies increases sharply to the NE from 55 to 130 m. In the Noril'sk area this unit is generally 10 to 70 m thick and the two facies comprise dolostone with carbonate breccia, and dolostone with carbonate breccia and anhydrite.
Upper Devonian - comprising,
Nakakhozsky Suite, of lower Frasnian age, which is 2 to 130 m thick - in general comprising multi-coloured clay-carbonate rocks which ranges in thickness from 2 to 60 m over the uplifts and 80 to 130 m in the troughs. In the NE they approach 160 m in thickness as sulphate-carbonate sequences replacing the lacustrine-carbonates (Duzhikov and Strunin 1988). Separate sulphate layers up to 15 to 20 m thick or more, sometimes with salt, are found in the sequence. In places, over the crests of the uplifts, the middle Devonian below may be eroded. In the Noril'sk district the suite is 2 to 110 m thick and comprises two facies, the first of which is found over the uplifts and is composed of grey to greenish-grey marl and siltstone, while the second is made up of variegated marl and anhydrite and occurs in the main basins.
Kalargonian Suite, 120 to 270 m thick - this unit, which is of Frasnian age, is interpreted as representing the peak of a marine transgression and comprises thick grey sulphate-limestone-dolostone accumulations. Lacustrine carbonates from 120 to 180 m in thickness, with secondary sulphates, accumulated over the uplifts, while dolostones, dolomitic marl, limestone and anhydrite from 170 to 270 m thick were deposited in the basins. The uplifts were still marked with erosion of the underlying Nakakhozsky and sometimes the Yuktinsky Suite. The sequence again thins to the south. In the Noril'sk district, the suite ranges from 130 to 180 m in thickness, and has been subdivided as follows:
 - Lower Kalargonian, 40 to 90 m thick - comprising the two facies detailed above, the first of which is represented by dolostone, carbonate breccia and marl, while the second is present as dolostone, anhydrite, brecciated carbonate and marl.
 - Middle Kalargonian, 50 to 90 m thick - composed again of two facies, the first being dolostone and carbonate breccia with pockets of limestone, while the second comprises anhydrite, dolostone and marl.
 - Upper Kalargonian, 20 to 40 m thick - composed exclusively of dolostone and carbonate breccia.
Where sighted in core at Noril'sk the Kalargonian Suite comprised bituminous limestone (60%), with lesser dolostone (20%) and marls (20%).
Fokinsky Suite, 220 to 500 m thick - this suite is of Fammemian age, and is interpreted to represent a regressive phase. It emphasises the uplifts and basins/depressions the most strongly. It is an evaporitic sulphate-lacustrine-carbonate unit which includes anhydrite, dolomitic-anhydrite and dolomitic marls with interlayers of limestone and lenses of halite and clay-carbonate breccias. The thickness is generally 220 to 420 m, reaching 500 m in the western Volgochansky trough. The unit is absent in the immediate Noril'sk-Talnakh mine areas.
Lower Carboniferous, comprising
• Turneyski Suite, 35 to 85 m thick - which is believed to reflect a new stage of transgression during the Tournaisian. It comprises a marine limestone formation of limestone and fragmentary-organogenic limestones, sometimes with silex (compact limestone) lenses. It rarely contains dolostones and carbonate breccias. This suite is only preserved in the pre-existing basins where it unconformably overlies late Devonian sediments. Its thickness is quite constant and no more than 65 to 85 m, but decrease to the south to 35m. It is not represented in the Noril'sk-Talnakh area.
Tundrinski Suite, 60 to 150 m thick - representing interpreted renewal of regression during the Visean, with the development of multi-coloured marls with interlayered gypsum and anhydrite. The amount of terriginous material increases to the north where the suite is about 150 m thick, while to the west of the Noril'sk and Vologochansky troughs they are 70 to 114 m thick. To the south it decrease to only 60 m, but contains more carbonates. It is not represented in the Noril'sk-Talnakh area.
Brussky Suite, up to 98 m thick - comprising multi-coloured sulphate bearing lacustrine-carbonate lithologies. This unit is latest Visean age and is only preserved from erosion to the south of Noril'sk and is not represented in the Noril'sk-Talnakh area.
  Unconformity reflecting considerable tectonic activity, with up to 1200 m of erosion, which in places exposed units down to the Ordovician. The main activity included continued uplift along the Khantajsky-Rybninsky uplift, and subsidence in the area to the east of the Imangdinsky-Letninsky Fault (forming a rift trough along the western section of the Tunguska Syncline) and in the western part of the Noril'sk, Kharayelakh and Volgochansky troughs.

MIDDLE CARBONIFEROUS TO UPPER PERMIAN
Middle Carboniferous to Mid Upper Permian Tunguska Series, 300 to 1000 m thick - limno-paralic coal-bearing formations are represented by complex rhythmic sequences of sandstone, siltstone, argillites, coal and conglomerate, sometimes with argillaceous limestone, tuffs and tuffaceous sandstone. In the Noril'sk distric, the thickness decreases towards the axes of the uplifts which controlled the main basins (see the 'Tectonics' segment above). Along the axes, the Permian rests unconformably on older Palaeozoic sedimentary rocks. To the west this sequence pinches out against the Dudinka Uplift. Multiple coal beds are found in the sequence, with productive layers of stone coal approaching 20 m in thickness in the Noril'sk region, and 50 m elsewhere. There are 8 coal bearing sections locally over a thickness of 150 to 300 m, with seams of coal up to 12 m thick, averaging 2 to 3 m. In the Talnakh mine area, there is a 2.8 m thick coal seam. The coal is a high energy variety, varying from anthracite to coking coal, although the metamorphic grade of the coal decreases sharply to the east. The grade of the coal is believed to have been influenced by the Permo-Triassic Trap basalts and related sills. In places in the Noril'sk region, the coal has been transformed to graphite, with large scale graphite deposits being outlined. In the Noril'sk District the sequence is as follows:
Adylkanskaya Suite, 0 to 90 m thick - argillite, siltstone and sandstone, with intercalations and lenses of carbonaceous argillite and conglomerate, and concretions of siderite and marcasite.
Talnakhskaya Suite, 30 to 60 m thick - sandstone, siltstone and argillite with limestone lenses.
Daldyskanskaya Suite, 40 to 65 m thick - siltstone, sandstone and argillite with carbonaceous layers.
Shmidtinnian Suite, 20 to 95 m thick - argillaceous sandstone, siltstone, argillite and conglomerate with coal.
Kayerkannian Suite, 55 to 95 m thick - sandstone, conglomerate, siltstone, carbonaceous argillite and coal.
Ambarnunskaya Suite, 0 to 35 m thick - sandstone, conglomerate, argillite, tuffaceous sandstone and tuffaceous argillite. This suite marks the transition from the coal measures to the overlying Permo-Triassic Siberian Trap mafic suite. A characteristic red tuffaceous argillite used for local brick making is found at the transition from the Tunguska Series to the Trap basalts.

UPPER PERMIAN TO MIDDLE TRIASSIC
Late Upper Permian to Middle Triassic Siberian Trap Mafics, up to 4000 m thick - these comprise a complex series of 'flood basalts', tuffs and related sills and dykes cutting the underlying Palaeozoic and Proterozoic sediments, as described below in the 'Mafic Rocks' segment.



Mafic Rocks

  The Siberian Trap Basalts are up to 4000 m thick and cover an area of some 340 000 km
2 centred on the NNW-SSE trending Tunguska Syncline. It is estimated that they originally had an areal extent of 4 million km2 (Wooden, et al., 1992). These volcanics have a gradational lower contact with the underlying Permian Tunguska Series coal measures, with intercalations of tuffs and tuffaceous sediments within the sandstones, conglomerates and argillites of the middle Upper Permian Ambarnunskaya Suite. These are followed by 25 to 120 m of titanium-augite basalts with tuffs and tuff breccias, at the base of the main basalt accumulations.
  The basaltic extrusives are accompanied by a series of sills, dykes and other crosscutting bodies of comparable composition which cut the Palaeozoic sedimentary rocks below, and the lower sections of the Trap basalts themselves.
  In the immediate Noril'sk-Talnakh area, there are two basalt filled 'basins', the Noril'sk and Kharayelakh basins, centred on the Noril'sk-Kharayelakh Fault Zone. Although these basins largely represent folds superimposed on once continuous layers, deep drilling has indicated that the lower suites of the basalts are thickest in the basin centres (Wooden, et al., 1992).
  Basalt development in the Noril'sk area is characterised by initial and continuing explosive activity, with basaltic tuff units up to 300 m thick constituting around 10% of the total thickness of the basalt sequence (Wooden, et al., 1992).

Mafic Extrusives
  The mafic extrusives are distinctly stratified and include basalt layers from a few to 90 to 100 m in thickness, and tuffs ranging from tens of centimetres to 20 to 40 m. The latter may reach 100 to 250 m near vents. The basalts and tuffs occur as relatively homogenous zones with a constant composition and morphology, and may cover areas of up to tens of thousands of square kilometres (Duzhikov and Strunin 1988). They have been divided vertically into three stages, as follows, from the base:

UPPER PERMIAN
First stage, subdivided into:
Ivakinsky Suite, 270 to 330 m thick - represented by four sequences of basalt with lesser tuff and tuffaceous sediments. This suite is made up of four sequences. The first and lowest is composed of alkalic trachy-basalts which are characterised by the lowest silica content, high alkalinity and the highest amount of Fe, Ti and P and is restricted to several limited areas around the limit of the basins. The second consists of sub-alkalic titanium-augite basalt and is only found on the north-eastern side of the Kharayelakh Basin and on the margin of one other basin. The third comprises porphyritic titanium-augite basalt and is distributed over the central Noril'sk and the north-western Tunguska Syncline. The fourth differs greatly from the preceding three, being made up of trachy-andesite basalt, andesitic basalts and andesine basalts and are found throughout the Noril'sk-Kharayelakh basins and the north-western margin of the Tunguska Syncline.
Syverminsky Suite, 20 to 195 m thick - a relatively homogenous sequence of tholeiitic basalts, which are characterised by a lack of tuffs and is made up of 20 layers, each of which is 3 to 5, to 15 to 30 m in thickness. It is restricted to the western side of the Tunguska Syncline where it is 180 to 195 m thick, and the central and eastern sections of the Noril'sk-Kharayelakh depression, where it is 120 to 150 m thick. The suite wedges out on the northern, western and southern borders of the latter depression.
Gudchichinsky Suite, up to 350 m thick - is composed of a lower unit of 2 to 4 sequences of predominantly porphyritic basalts which total up to 160 m in thickness, and an upper unit of picritic (ultramafic/mafic) lava with a thin tuff at the base. The upper unit has a maximum development of 200 m in the northern Kharayelakh basin, but elsewhere varies from 30 to 110 m. The suite is most complete in the Noril'sk-Kharayelakh basins and is locally developed on the western margin of the Tunguska Syncline. The picrites of this suite carry up to 0.15% Ni.

LOWER TRIASSIC
Second stage, the base of which is defined by the thick Kakanchansky Tuff, that separates the predominantly basaltic lavas of the two stages, as follows,
Kakanchansky Tuff, 15 to 250 m thick - marking a break in the volcanic activity. It is composed almost entirely of 'vent' or 'near vent' facies tuffs and occurs in the larger parts of the Noril'sk-Kharayelakh depressions, and along the western margins of the Tunguska Syncline. Minor sub-alkalic basalts are found at some sites.
Tuklonsky Suite, a few tens to 320 m thick - poikilophytic basalts with accessory picritic and tholeiitic basalt. The thickness is variable, with in places the upper sections thinning with a corresponding thickening of the lower parts of the overlying suite. This suite is found in the central and eastern sections of the Noril'sk-Kharayelakh basins and the western sections of the Tunguska Syncline.
Nadezhdinsky Suite, 150 to 570 m thick - divided into three portions, comprising a lower portion made up of 10 to 14 porphyritic basalt flows totalling up to 260 m in thickness, a middle portion of 10 to 13 porphyritic flows which sometimes contain tholeiitic basalts and total up to 285 m, and an upper portion some 30 to 120 m thick, comprising several layers of lava with a tuffaceous band at the base. This suite is extensive in the Noril'sk region and the north-eastern section of the Kharayelakh basin.

MIDDLE TRIASSIC
Third stage, which is more widespread than the first two stages, whose distributions are apparently controlled by the Noril'sk-Kharayelakh and the Imangdinsky-Letninsky Faults (see below for more detail). This stage reflects a marked shift to the NE of the centres of accumulation of basalts, and by the formation of compositionally homogenous rocks, with a constant ~48% SiO
2, 7% MgO, low K and Ba, and elevated Cu (100 to 200 ppm). It is subdivided into the,
Morongovsky Suite, 250 to 700 m thick - distinguished by 30 to 50% tuffaceous rock. It is sub-divided into two by an up to 100 m thick tuff which separates two sequences of basalt.
Mokulaevsky Suite, 400 to 670 m thick - comprising four sequences of basalt, punctuated or characterised by tuff bands or their absence. One such tuff band at the base of the lowest sequence is 40 to 45 m thick.
Kharayelakhsky Suite, 475 to 600 m thick - composed of four sequences, characterised by the presence of lavas, tuffs or agglomero-porphyritic basalts.
Kumginsky Suite, 160 to 210 m thick - comprising 8 to 12 homogenous layers of agglomero-porphyritic basalt. It is only found in the Kharayelakh basin in the Noril'sk district, and is overlapped by the succeeding suite.
Samoyedsky Suite, up to 800 m thick - in places this unit has a gradational boundary with the Kharayelakhsky Suite, but can be sub-divided into three sequences which are similar in their constituent basalts, but vary in the amounts of tuff they contain.

  According to Wooden, et al., (1992), there are two features of the basaltic rocks which are specific to the Noril'sk area. The first of these is that the lower five basalt suites, corresponding to the first two stages, show a broader range of composition (2.7 to 16.8% MgO) than can be found in any other region of the Siberian Traps (where the range is generally 6.0 to 7.6% MgO). This variation corresponds to the presence of variable picritic and trachy-basalts with the more typical tholeiitic basalt in these lower two stages, while the third is a homogenous tholeiitic basalt. Naldrett, et al., (1992) also indicate that the basalt suites of the lower two stages are 'clearly associated with faults and develop in depressions, the long axes of which tend to coincide with the faults'. They further claim that three faults have had an important effect, the Noril'sk-Kharayelakh, the North Kharayelakh and the Imangdinsky-Letninsky, with the former being by far the most significant. Similarly the MgO content of the suites in the lower two stages increases towards the faults. Naldrett (1992) shows that isopachs of the different suites of basalts indicate a close control by the same faults for the first two stages, with the thickness and distribution being greater in the vicinity of the faults, although this is not so for the third stage. Duzhikov and Strunin (1988) state that the third stage basalts are more widespread than the first two, extending beyond the confines of the Noril'sk district and that they represent the essential plateau basalts of the Siberian Craton. They also say that this third stage reflects a marked shift to the north-east of the centres of basalt accumulation and is characterised by the formation of compositionally homogenous rocks. It is not clear whether the first two stages are found outside of the Noril'sk area.
  The second of the two features reporteed by Wooden, et al. (1992) is that the maximum thicknesses of the Siberian Trap flood basalt, wbich exceeds 3500 m, is reached in the Noril'sk area, where all eleven suites of the flood basalt are represented.

Mafic Sills and Dykes
  Mafic sills and dykes disrupt the Palaeozoic sequence below the Trap basalts and now comprise a significant percentage of the section. Naldrett, (1992) and Naldrett, et al., (1992) have divided those intrusives in the Noril'sk District into 6 groups as follows:
Group 1 - Alkaline and Sub-alkaline Affinity - these include the Yergalakhsky Type high Ti trachy-dolerite sills which are believed to be co-magmatic with the lower trachy-basalt rich sequence of the Upper Permian Ivakinsky Suite at the base of the Stage 1 basalts, and the North Kharayelakhsky Type sub-alkalic high Ti sills which contain some picrites. Neither intrusives carries significant sulphides and both show no indications of 'differentiation'.
Group 2 - High Ti Dolerite Dykes - comprising the Avamsky Type quartz bearing leucocratic dolerite dykes which are restricted to the north-east of the area. They are not alkaline, are not differentiated nor do they carry significant sulphides. They have high Ti, Fe and P and appear to be younger than the Group 3 Daldykansky sills.
Group 3 - Dolerite Sills in the Noril'sk Area - incorporate a number of varieties, all of which are present primarily as sills, are not alkaline, are not differentiated nor carry significant sulphides. These varieties include, the Irbinsky Type which cut the Group 1 Yergalakhsky sills and is subdivided into an eastern sub-type related to the Lower Triassic Tuklonsky lava suite and a western sub-type related to the Lower Nadezhdinsky lava suite, both of which are Stage 2 basalts. The second type is the Ambarminsky Type which cuts the Irbinsky Type and comprises sills of two ages, one of which equates with the Morongovsky and the other with the Samoyedsky lava suite both of which are stage 3 basalts. The third is the Daldykansky Type which cuts all of the other intrusives in the area, including the mineralised intrusions, and all of the lavas of the Traps, including the uppermost Samoyedsky lava suite of stage 3.
Group 4 - Differentiated Intrusions NOT Related to Ore - these include the Fokinsky Type, which consists of high Ti, high Cr sills thought to be the intrusive equivalents of the Stage 1 Gudchichinsky picritic lavas; the Kulgakhtakhsky Type, a low Ti, high Cr intrusive complex; and the low Ti, low Cr Ruinny Type intrusive complex which cuts all lava suites up to and including the Mokulaevsky lava suite of the Stage 3 basalts. All three are not alkaline, are strongly differentiated but do not carry significant sulphides.
Group 5 - Differentiated Intrusions Found in the Vicinity of Ore - subdivided into a Cr rich (>400ppm) Group 5A and Cr poor <400ppm) Group 5B, as follows,
 - Group 5A, Cr Rich Noril'sk Group - all are present predominantly as sills, none are alkaline, while all have low TiO
2, high Cr (>400 ppm Cr) and high MgO (often in the range 7 to 13%). These sills cut all of the Stage 1 and 2 basalts to the top of the Tuklonsky and Lower Nadezhdinsky Lava Suites. They include the well differentiated Talnakh and Chernogorsky Type intrusions which have layers with diffuse gradational boundaries ranging from picrite and plagioclase-peridotite to anorthositic or leuco-gabbro in composition. The former is associated with both disseminated and massive sulphides while the latter only has disseminated mineralisation. The Kruglagorsky Type, which are leuco-gabbro sills, often flank the Talnakh and Chernogorsky Type intrusions and usually have no associated significant sulphides and are only weakly differentiated. The Zubovsky Type, which while containing 10 to 20% ultramafic layers, consists predominantly of intermediate composition rocks lacking leuco-gabbro, is weakly differentiated and carries weak disseminations only. The relatively weakly differentiated Dvuborginsky Type has a low proportion of olivine rich layers, and has no significant sulphides. The Makusovsky Type is not differentiated, and also has no associated significant sulphides. Duzhikov, Distler, et al., (1988) suggest that some of the mafic sills, including those of Group 5A, and related picritic basalts of the Siberian Traps, such as the Tuklonsky Suite, are komatiitic in composition. This is based on comparisons with komatiites from elsewhere in the world on element ratio plots. It should be noted that the points from Noril'sk have a broad spread, and while partially overlapping the filed of lower komatiites from elsewhere, also straddle the tholeiite/komatiite boundary. In addition many of the Group 5A sills appear to be the product of alteration of dolerites as indicated below. The comparison with komatiites is therefore dubious.
 - Group 5B, Cr Poor Lower Talnakh Group - these are known as the Lower Talnakh Type and are believed to be slightly older than the Noril'sk Group of sills, but closely associated. They also occur as sills, are not alkaline, have low Cr (<400 ppm Cr) and low Ti, are strongly differentiated and have weak associated disseminated sulphides in places, but not of ore grade. They generally occupy a position several tens of metres below the Noril'sk Group sills, though sometimes laterally removed. At Talnakh however, while this is the case on the southern side of the field, on the northern side, the Lower Talnakh sill occurs above the mineralised intrusive. The Lower Talnakh Sill has a very similar appearance and zoning to the main Group 5A sills and is difficult to distinguish, other than by 'stratigraphic position'.
Group 6 - Unclassified Intrusions.

  All of the mineralised Group 5A sills of the Noril'sk and Talnakh 'ore junctions' occur close to (within 7 km of) either the Noril'sk-Kharayelakh, North Kharayelakh or Imangdinsky-Letninsky Faults, with all of the economic mineralisation indicated to date being related to the former. The Group 5B Lower Talnakh Type and the Zhubovsky Type sills of Group 5A, both of which are only ever weakly mineralised, are also apparently closely related to the Noril'sk-Kharayelakh Fault zone, but extend much farther from it than do the strongly mineralised Talnakh and Chernogorsky Types.
  The distribution and thickness of the sills varies considerably, with swelling and constrictions which are influenced by the structure of the enclosing rocks, specifically by the deep Noril'sk-Kharayelakh Fault Zone, but also by the widely developed minor faults and fractures which cut the Palaeozoic sediments and Trap basalts; and by folds within the Palaeozoic sediments. The minor faults and fractures within the Palaeozoic are oriented in the four main directions of the district, namely NW, NE, NNE and WNW, and have vertical amplitudes of up to 400 to 500 m. The majority if these faults die out with depth or flatten into shallowly dipping interlayer shears, indicating an extensional regime. The sills also tend to be thickest in synformal structures and thinnest in antiforms. The orientation and location of these folds appear to be related to the Noril'sk-Kharayelakh Fault Zone also.
  In general, the Group 5A mineralised sills are elongate, 'finger like' bodies with dimensions estimated to be of the order of 120:12:1 by Smirnov (1966), as quoted in Wooden, et al., (1992), in contrast to the many more extensive un-mineralised, more equi-dimensional sills in the area. The mineralised Noril'sk I sill for instance has a long dimension of >15 km, with a width of 1.2 to 2.5 km, while the ore bearing Main Sill at Talnakh has a long axis of ~16 km and is generally 1 to 2 km wide. The Kharayelakhsky Sill (see below) at Talnakh, which is also associated with ore, is also elongated but has an approximately 'V' shape. Each has been interpreted as having one or more narrow roots at one extremity (generally not on the 'long' sides) where each dips into the Noril'sk-Kharayelakh Fault Zone.

  The Talnakh Complex comprises two main sections. The Main Sill is mainly on the eastern side of the Noril'sk-Kharayelakh Fault Zone and cuts sediments of the Carboniferous to Permian Tunguska Group coal measures. It crosses to the western side of the Noril'sk-Kharayelakh Fault in the south, remaining within the Tunguska Group, and outcrops on its southern extremity over a limited interval on both sides of the fault, largely concealed by glacial cover. In the area of exposure, the roof of the sill approaches the overlying basalt, while it dips gently towards the north, following the line of the Noril'sk-Kharayelakh Fault, until its base cuts into the Devonian carbonates. The section of this sill on the eastern side of the fault is known as the North-east Branch, while that on the western side is known as the South-west Branch. The Kharayelakhsky Sill however, which is more extensive, is almost entirely found within the Devonian carbonates and evaporites on the western side of the fault. It also dips gently to the north towards the core of the Kharayelakh structural basin. This sill has also been split into two sections, the North-west and Western (or Kharayelakh, or Oktyabr'sky) Branch. The latter is the thicker section of the Kharayelakhsky Sill on its western extremity, while the North-western (or Central) Branch occupies a graben-like structure between the Noril'sk-Kharayelakh Fault and the Western Branch.
  The Talnakh Complex sills range in thickness from zero on the margins and in 'holes', to maximums of >150 m.
  The morphology of the intrusive bodies within the Tunguska Group is relatively simple, while the Kharayelakhsky Sill in the Devonian sediments is more complex with the development of multiple apophyses and breccia like rocks which lie within a thick area of contact metamorphic and metasomatic rocks (Duzhikov, Distler, et al., 1988). This is particularly the case on the western boundary of the Kharayelakhsky Sill.

  The Noril'sk Complex is influenced by the structure of the enclosing sedimentary sequences in the same way as the Talnakh Complex. It is made up of four separate sills. These are Noril'sk I, which is of the Talnakh Type; Noril'sk II and Mt Chernaya (or Chernogorsky) which are of the Chernogorsky Type; and Dvugorbinsky which is of the Dvugorbinsky Type. Each of these types are described above and belong to the Group 5 differentiated intrusions found in the vicinity of ore. Although both the Talnakh and Chernogorsky Types are mineralised, the latter only has associated disseminated and veinlet mineralisation, while the former are accompanied by higher grade massive sulphides. Significant mining has only taken place on mineralisation associated with the Noril'sk I branch. The Noril'sk I sill ranges in thickness from zero to near 350 m at its thickest, but averages 130 m (Smirnov, 1977), with a 'trough-like' form with abrupt sides, sharp sags at its base and thin dyke like branches which occur in volcanic rocks of the Nadezhdinskaya and Morongovskaya suites. It is located close to an intra-formational contact within the Carboniferous section of the Tunguska Group (Duzhikov, Distler, et al., 1988), but transgresses upwards into the Permo-Triassic tuff-lava sequence of the Trap basalts which enclose most of the Noril'sk I sill. Although volcanics of the Traps are the main enclosing lithologies to this sill, the areas of maximum thickness are linked with 'gutters' which plunge into the underlying sediments. These areas of maximum sill thickness correspond closely with the areas of development and maximum thickness of ore grade mineralisation (Duzhikov, Distler, et al., 1988). Within these sags there is usually an increase in the thickness of picrite and gabbro-dolerite (see below).

  The differentiated, mineralised sills at both Talnakh and Noril'sk are generally composed of three series which are in turn divided into a number of suites, as described below (after Naldrett, et al., 1992, with descriptive detail from Duzhikov, Distler, et al., 1988, and von Gruenewaldt, 1991). The boundaries of the individual suites are invariably gradational and difficult to determine. These sub-divisions and their order appear to be idealised and far from universal, with a large number of exceptions and variations. The three series are as follows:

Upper Layered Gabbro Series, 10 to 30 m thick - which generally has a very low olivine content. It is composed of,
Contact Gabbro-dolerites - a zone of contact quenched gabbro-doleritic hybrid rocks whose composition is governed by the enclosing lithologies. At Noril'sk I irruptive breccias occur sporadically on the upper margin. These consist of fragments of the enclosing lithologies and amphibolised dolerites, cemented contact and taxitic gabbro-dolerites. Hybrid metasomatic rocks are found both in the cement of the breccias and at the contact with Tunguska Series rocks where granodiorite, granophyre and meta-diorite occur. This band appears to represent a thin skin of around a metre or so, but possibly may be thicker in places.
Equi-granular and pegmatoidal chromite and anorthite bearing gabbro - these rocks contain segregations of chromite and green chromium-bearing clinopyroxene.
Taxitic* chromite bearing anorthite-gabbro with inclusions of picrite, troctolite and clino-pyroxenite - the association and inter-digitation of melanocratic and leucocratic gabbroic rocks and the wide variation in composition and grain size gives the rocks their taxitic appearance. These rocks are characterised by abundant segregations of chromite, green chromium-bearing clinopyroxene, sulphide and platinum (PGE) mineralisation. The leucocratic gabbros are taken to be an alteration product of the gabbro-dolerites, which occurred at the same time as the alteration to taxites.
Equi-granular and taxitic gabbro.
Main Layered Gabbro Series, generally up to 100 m thick and often more - this series has an increasing olivine content downwards, from zero in the upper diorites, to 5% in the underlying quartz-bearing gabbro-dolerite to 5 to 20% in the gabbro-dolerites and olivine bearing gabbro-dolerites below. The olivine content increases further to more than 20 to 22% in the lower olivine picrites and picritic gabbro-dolerites. The olivine commonly occurs as conspicuous olivine-pyroxene spots which increase in density downwards within this series. The Main Layered Gabbro Series is generally made up of,
Prismatic grained gabbro-dolerite and gabbro-diorite - these have a gradational contact with the Upper Layered Gabbro, and are composed of prismatic grained pegmatoidal rocks which alternate with gabbro-diorites and gabbro-dolerites with a thickness varying from 20 to 100 m. The texture is a coarse grained graphic intergrowth of mafic and leuco minerals with grain sizes of 2 to 3 mm. Rock forming minerals are plagioclase (andesine-labradorite), clinopyroxene and hornblende associated with biotite, quartz and olivine. Duzhikov, Distler, et al., (1988) note that the former presence of olivine is indicated by a spotted texture in the lower half, which increases with depth. The spots which are 1 to 7 mm across are formed by the replacement of olivine by iddingsite, chlorite-serpentine and talc.
Quartz-bearing, Olivine poor diorite with micro-pegmatite - follows the band above across a broad transitional boundary but has a very variable thickness, sometimes reaching 90 m. The component minerals are plagioclase, clinopyroxene, orthopyroxene and varying amounts of quartz or olivine.
Gabbro-dolerite grading downwards into olivine gabbro-dolerite - characterised by a steady downwards increase in olivine, and equally gradational changes in texture and grain size. As the quartz content of the unit above decreases, the olivine percentage increases correspondingly. This band varies from 30 to 70 m in thickness, including the olivine-biotite gabbro-dolerite below, with which it has a gradational contact. The main constituent minerals of the olivine gabbro-dolerite are plagioclase, clinopyroxene, orthopyroxene and olivine. The olivine in this unit and those below is conspicuous as 1 to 3 mm diameter olivine-pyroxene spots which increase in abundance and size downwards.
Olivine-biotite gabbro-dolerite - this band is similar to that above, but is characterised by its olivine and biotite content, and the presence of fine interstitial sulphide. It also has a gradational lower contact over an interval of 10 to 30 cm with the underlying picritic rocks. This band contains elevated chromite.
Picritic gabbro-dolerite, picrite, troctolite, plagio-peridotite - these are enriched in olivine and vary greatly in thickness, ranging from 20 to 40 m in the axial zone of the intrusives, in places up to 50 to 70 m. Duzhikov, Distler, et al., (1988) state that the rocks often display a micro-layering caused by an alternation of mafic olivine rich plagio-olivinites, which may be several cm's to several tens of cm's thick, and plagioclase rich gabbroic layers, usually <1 cm thick. In core at Talnakh the picrite is a dark grey medium grained rock with a variety of coarse grained plagioclase and olivine. The picrite and olivine gabbro above contain rounded and elongate patches and drops of sulphide commonly 1 to 5 mm across, as well as the olivine-pyroxene spots described above. Chromite, PGE's and sulphides are found within this band.
Lower Layered Gabbro Series, 30 to 35 m thick - this series corresponds to a decrease in the olivine content of the sill, generally down to 5%. Both bands of this series contain elevated Cu-Ni sulphides and PGE's. It comprises,
Taxitic olivine bearing gabbro-dolerite and equi-granular gabbro-dolerite - this band is the definitive characteristic of the differentiated Group 5A sills. Such bands are distributed throughout the complex, although they are best developed within both the lateral peripheries and upper and lower flanking zones where they are the major constituent (Duzhikov, Distler, et al., 1988). The taxitic rocks are sometimes locally absent but are always present where Cu is developed in the aureole of the sill.
  The rocks of the horizon have an extremely varied composition and widely developed dissemination of sulphides. Their average thickness is 10 to 15 m, but may reach 40 to 50 m locally. They are characterised by a pronounced taxitic texture, with irregular fragments of remnant picritic gabbro-dolerites and troctolite (plagioclase and olivine), irregularly distributed crystals of coarse plagioclase and olivine appearing almost fibrous in places, and schleiren-like blocks of prismatic grained gabbro-dolerite and gabbro-diorite.
  The taxitic rocks are paler, due to increased plagioclase and have clots of sulphides up to 10 x 5 mm. This band is characterised by the increase in plagioclase and decrease in olivine. This was confirmed in sections observed in core where plagioclase appears to post date the olivine-pyroxene clots, while the sulphides appear interstitial to the plagioclase. In sections where separate sulphide clots were not well developed in core the olivine-pyroxene clots had associated interstitial sulphides. It seems likely that the olivine-pyroxene and sulphide clots are closely related.
  Taxitic gabbro-dolerites are substituted in the lower sections by olivine gabbro-dolerite with indistinct taxitic textures and compositions similar to those in the main Layered Gabbro Series in the centre of the sill. The olivine gabbro-dolerite band varies from a few metres to 20 m in thickness.
  The taxitic gabbro-dolerite appears to be a replacement or alteration product of picrite and picritic gabbro-dolerite rather than a separate intrusive phase of the sill or magmatic segregation.
  • Contact gabbro-dolerite - these are represented by fine and medium grained rocks with porphyry like segregations of plagioclase. Plagioclase prevails in the mineral composition, with subordinate clinopyroxene and rare sporadic olivine. The thickness of the contact varies from 1 to 5 m, and may be up to 10 to 20 m on the lateral margins of the sills.

* NOTE: 'Taxitic' is a term used by Russian geologists to describe a mafic igneous rock with a very variable texture (ranging from fine grained to pegmatitic) and composition (from leucocratic to melanocratic) in which discrete ghost-like remnants of inclusions of other gabbroic and also country rock are present ranging from a few mm's to several tens of cm's in diameter (Naldrett, 1992).



Contact Alteration

  At Noril'sk contact alteration is on the whole insignificant, apart from a thin aureole of hornfelses and calc-skarns in joints. Processes of superimposed biotitisation and chloritisation are widespread and albite-microcline rocks have been recorded associated with ore (Smirnov, 1977).
  At Talnakh the mineralised sills are accompanied by a thick (up to 150 to 200 m) enveloping aureole of altered rocks, the width of which decreases from the frontal to the near root portions. The bulk of the altered rock is located in the roof of the sills where it is generally around 50 m thick, while in their lower exo-contact the thickness is of the order of about 20 to 30 m. Contact hornfelses are regionally insignificant, the most widely distributed rocks being 'metasomatites' of albite-microcline composition, magnesian 'skarnoids', serpentinites and calc-skarns. Pre-ore metasomatites and post-ore prehnite, calcite, quartz and other vein accumulations have been recorded locally (Smirnov, 1977).
  According to Duzhikov, Distler, et al., (1988) there are five styles of contact alteration at Talnakh, as follows,
Hornfels and marble - from carbonate bearing Permo-Carboniferous terriginous rocks, Lower to Middle Devonian argillites and Devonian dolostones and marls,
Alkalic metasomatites - derived from Permo-Carboniferous sandstones, siltstones and argillites, Devonian argillites and contact gabbro-dolerites (making the actual contact less distinct in places);
Magnesian skarns - formed from Devonian dolostones and dolomitic marls;
Calc-skarns - which overlap hornfels, magnesian skarns and the gabbro-dolerites;
Low temperature metasomatites - replacing all contact metamorphic rocks, dolerites and gabbro-dolerites of the contact zone.

  The aureole around the sills comprises an outer layer where the clay cement of the sandstones and silstones is first altered to hydro-micas, sericite, muscovite and chlorite and then to biotite, actinolite and epidote, while andalusite, cordierite and hypersthene is found on the contact. The dolostones and marls are metamorphosed to pure calcite, and marbles, progressively inwards to forsterite, monticellite, clinopyroxene, periclase, brucite, spinel, calcite and anhydrite, followed by garnet-serpentine skarns sometimes on the contact (Duzhikov, Distler, et al., 1988).
  However, while contact alteration is not generally widespread, in proximity of the sills, coal seams have been converted into anthracite to coking coal over significant areas, and in places have been transformed into large scale graphite deposits.

The most recent source geological information used to prepare this decription was dated: 2000.    
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
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Naldrett A J, Asif M, Gorbachev V A, Kunilov V Ye, Stekhin A I Fedorenko V A, Lightfoot P C  1994 - The Composition of the Ni-Cu Ores of the Oktyabrsky Deposits, Norilsk Region: in Naldrett A J, Lightfoot P C, Sheahan P (Eds),  The Sudbury - Norilsk Symposium Ontario Geological Survey   Special Publication 5 pp 357-372
Naldrett A J, Fedorenko V A, Asif M, Shushen L, Kuniloc V E, Stekhin A I, Lightfoot P C, Gorbachev N S  1996 - Controls on the composition of Ni-Cu Sulfide deposits as illustrated by those at Norilsk, Siberia: in    Econ. Geol.   v 91 pp 751-773
Naldrett A J, Lightfoot P C  1999 - Ni-Cu-PGE Deposits of the Norilsk Region, Siberia: Their Formation in Conduits for Flood Basalt Volcanism: in Keays R R, Lesher C M, Lightfoot P C, Farrow C E G, (Eds),  Dynamic Processes in Magmatic Ore Deposits and their Application in Mineral Exploration Geological Association of Canada, Short Course Notes   v13 pp 195-249
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