The Monchegorsk nickel-copper district is situated north of the Arctic Circle in the Kola Peninsula of the Russian Federation, 135 km south of Murmansk. Monchegorsk was the first major industrial source of Soviet nickel. It was discovered and developed before the annexation of Pechenga in 1940 and the development of Noril'sk. The ore field contains a number of nickel and also chromium, PGE deposits of various styles, all genetically associated with the Palaeoproterozoic Monchegorsk mafic-ultramafic layered intrusion. The pre-mining resource resource is estimated at some 1,750 t Ni, 450 Mt Pt+Pd, ? kt Cu, ? kt Co and 2.5 mt Cr2O3. The first orebody was discovered in 1930, with nickel mining continuing until the mid-1970s. The large Severonikel' metallurgical complex still operates using concentrate shipped from Pechenga and Noril'sk. Currently (2006), a small chromite deposit (Sopcheozero) is mined by open pit and a PGE prospect is under exploration.
Geologically, this area is positioned in the north-eastern extremity of the Precambrian Baltic Shield, which continues south and west into Scandinavia. The north-west trending Palaeoproterozoic Imandra-Varzuga Zone, which has been interpreted as a palaeorift, is located close to the juncture of the Archaean Beloe More and Central Kola Blocks. The zone preserves a suite of greenschist- to amphibolite-metamorphic facies volcanic and sedimentary rocks, and also several remnants of mafic-ultramafic layered intrusions that represent the magmatic chambers of Proterozoic volcanism. These intrusions fall into two age groups: (1) an older group, 2.51-2.47 Ga, and (2) a younger group, 2.46-2.44 Ga, that correlates with the better known layered intrusions of central Finland. All rocks were deformed and metamorphosed during the Svecofennian Orogeny around 2.0-1.9 Ga.
The Monchegorsk layered intrusion (Monchepluton) occupies an area of 65 square km and has been dated at 2.506-2.487 Ga. It was emplaced into the Imandra-Varzuga supracrustal series, is horseshoe-shaped, trends north-easterly, and is convex to the south-west. Monchegorsk is floored in part by Archaean basement. The intrusion comprises erosional remnants of two magma chambers, (1) a NNE-oriented NKT Chamber, and (2) an ENE-oriented SNP chamber. In both instances igneous layers dip gently in symmetrical synclines, and both host nickel orebodies. The igneous stratigraphy from the base upwards comprises, (1) basal quartz norite and gabbronorite, (2) harzburgite and alternating harzburgite and orthopyroxenite, (3) orthopyroxenite, and (4) gabbronorite with leucogabbro and anorthosite. Numerous dyke rocks are present. In the east, Monchepluton intrusives are overlain by Palaeoproterozoic meta-volcanic rocks. A palaeo-regolith horizon is present beneath the metavolcanic cover, characterised by bleaching, sericitisation and quartz veining. Holocene glaciation deeply scoured the Complex and filled valleys and depression with glacial drift or alluvial material. No oxidation or secondary enrichment zones are developed or preserved.
At least six styles of metallic mineralisation have been identified:
YOUNGEST
· M1: at the top of the Monchepluton: erratically disseminated PGE carrying less than 1% of sulphides, in sericitised and quartz-veined leucogabbro (Plate 4090, Vurechuaivench)
· M2: narrow, late magmatic to hydrothermal massive Fe, Ni, Cu, Co sulphide veins grading to magnetite and ore-silicate pegmatoids in late fractures which intersect the magmatic chambers (Plates 4086 Sopcha, and 4088 Nittis)
· M3: irregular, patchy, disseminated and veinlet Fe, Ni, Cu sulphides in melanocratic norite of the "critical zone" (contaminated) melanorite (Plate 4087 Nyud II)
· M4: layers and lenses of Fe, Ni and Cu sulphide-bearing olivine harzburgite in orthopyroxenite, conformable with igneous layering in Unit 5. Difficult to recover, as a portion of the Ni is contained within the olivine lattice (Plate 4086, Ore layer 330, Sopcha)
· M5: disseminated, stringer, to small masses of Fe, Ni and Cu sulphides in peridotite to gabbronorite, 5-10 m above the basal contact of the Monchepluton (Plate 4089 Travyannaya)
· M6: chromitite layer in a Dunite Block in the orthopyroxenite, dunite of Unit 6 (Plate 4091, Sopcheozero).
OLDEST
The Vurechuaivench PGE prospect is located near the eastern end of the eastern chamber of the Monchegorsk Pluton, 6 km SSE of Monchegorsk. It contains a new, enigmatic, style of mineralisation first discovered in the late 1990's and described in 2000. Presently, the outcropping prospect is exposed in a series of blast trenches and is being drilled.
Mineralisation occurs in uppermost Unit 4 of the Monchegorsk Pluton, which comprises gabbronorite and leucogabbro. Hummocky rock outcrops discovered in taiga forest, lie above the nonconformity, formerly buried under Palaeoproterozoic Kuksha Formation conglomerate and the Strel'nya Formation of andesitic meta-basalt. Lithologies present in the Monchegorsk Pluton are partly bleached and sericite-altered. Several (up to 20) zones bearing trace PGE’s are peneconcordant with the magmatic layering. The Main Reef under exploration is 1-3 m wide and traceable intermittently for 6-8 km. The host gabbronorite has first been retroactively amphibolitised and subsequently sericite-altered and quartz veined. PGE mineralisation is invisible to the eye (potentially economic intervals grading 2-7 g/t PGE) and associated with sparse scattered sulphides (mostly pyrite). Grades are highest in fractured intervals near faults, which are marked by a greater frequency of quartz veinlets.
(Source: Peter Laznicka, 2006)
