Natalka, Matrosov, Omchak, Pavlik

Siberia, Russia

Main commodities: Au
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The Natalka gold deposit, also known as the Matrosov mine, with the nearby Omchak and Pavlik lode gold deposits, comprise the Omchak mining district. The deposit is located some 390 km NNW of Magadan in the Magadan Province, north of the Sea of Okhotsk in eastern Siberia (#Location: 61° 35' 5"N, 147° 3' 21"E).

The Omchak mining district lies within the 1000 km long, northwest-trending Yana-Kolyma metallogenic belt which is defined by a numerous mesothermal gold-quartz vein deposits and prospects. This belt is located in a complex collision zone between the North Asian craton margin to the west, and an assemblage of passive-continental-margin, island arc and accretionary wedge terranes of the Kolyma-Omolon super-terrane, overlapped by post-accretionary sedimentary and volcanic assemblages. The metallogenic belt is largely restricted to the turbidite sequence of the Kular-Nera terrane, comprising a thick sequence of Late Permian to Early Jurassic argillite, siltstone and minor sandstones, and the Middle to Late Jurassic turbidites of the Verkhoyansk Complex which is part of the overlapping Inyali-Debin foredeep.

The structural framework of the Yana-Kolyma metallogenic belt comprises zones of linear (both longitudinal anfd diagonal) and domal folding, a network of generally NW-trending faults, belts of granitoid plutons and dyke swarms, and low grade greenschist metamorphism. The distribution of gold deposits within the belt is largely controlled by major NW-trending Mesozoic faults. These faults postdate the Jurassic folding and straddle emplacement of the batholiths. Dating of pre- and post-ore batholiths suggest an age range of mineralisation, which occurred as a number of different episodes between 135 and 100 Ma.

The sedimentary rocks in the Omchak district belong to the Late Jurassic Verkhoyansk Complex and comprise four conformable units, from the base:
Pioneer Suite - a 2300 to 2600 m thick suite of mainly un- to poorly-bedded and sometimes weakly spotted black to dark-grey silty argillite, with common intercalated 10 to 50 cm thick beds of grey, calcareous quartz-feldspar sandstone and sandy siltstone. The upper parts of the unit comprises thin, evenly and horizontal bedded dark-grey silty argillite.
Atkhan Suite - composed of 350 to 500 m of tuffaceous shales, pebble argillites, siltstones, sandstones and diamictites. The diamictites consist of around 10% sand and gravel sized clasts of highly altered felsic, intermediate and mafic volcanics, sandstones, granitoids and quartz, all set within an argillite matrix. The other beds of sediment are discontinuous along strike, while the upper parrt of the suite is characterised by calcareous concretions of up to 20 cm in diameter.
Omchak Suite - which totals 1600 m in thickness of argillites and siltstones, with rare sandstones, gravelstones, conglomerates and diamictites, which are predominantly horizontally bedded, with cross and graded stratification. The upper part of the sequence contains rare calcarous sandstone.
Staratel Suite - comprising 900 to 1300 m of poorly bedded dark-grey sandstones, siltstones and silty argillites, with local interbeds of light-grey, fine-grained sandstone. The middle part of the sequence is characterised by abundant thin argillites and siltstones with rare sandstones in packets of up to several metres in thickness.

The sequence has been subjected to both regional greenschist facies (chlorite-sericite and stilpnomelane with disseminated cubes and framboids of pyrite after sedimentary sulphides) and contact metamorphism. Contact metamorphism is represented by zones of hornsfels that occur along the NW-trending faults and range from biotite grade rocks with sphene and ilmenite, to cordierite-andalusite hornfels and garnet-pyroxene skarn. The main sulphide associated with the inner hornfels core is pyrrhotite, with pyrite in the outer zones. Gold-bearing quartz veins are found within all of the contact metamorphic zones.

The igneous rocks in the district are small to medium bodies and dykes of Late Jurassic to Late Cretaceous granitoids of various compositions which were emplaced subsequent to the accretion of the Kolyma-Omolon super-terrane to the North Asian craton. More than five magmatic complexes are distinguished by age and composition, and each exhibiting several mafic to felsic phases. At Natalka only dykes of mafic, intermediate and felsic composition have been found, ranging up to 15 m in thickness, with lengths of tens to hundreds of metres. The mafic dykes are more persistent, thicker and older than felsic dykes, dated at around 150 Ma. Ore formation post-dates both the mafic and felsic dykes.

The Natalka deposit is confined to the major, NW-trending Tenka fault, which controls the margin of a horst block of folded basement. The apical section of the horst is covered by 4 km of sediment, while the same sequence on either side is 5 to 6 km thick. The horst block undulates along strike corresponding to smaller individual segments, while major domal folds which make up the Tenka anticline, correspond to basement blocks along the upper parts of the Tenka fault. The Omchak mining district is in the south-western part of the 90 km long, NNW trending Omchak domal fold on the Tenka fault zone. Swarms of dykes occur along the core of the anticline, while small to intermediate sized igneous bodies ranging from gabbro and diorite to leuco-granite are found on its periphery. Two mineralised systems of en echelon faults are related to horst-anticlines. The Natalka deposit occurs in the northern fault system, while Pavlik is 10 km to the south.

The Natalka deposit occurs within a 13 km long 'Z-shaped' strike-slip fault zone, with mineralised structures trending at 320 to 340° and dipping at 60°N in the south-eastern part of the deposit, becoming near vertical in the centre, and south dipping in the north-west. Most of the Natalka deposit is found within a network of faults that form a horsetail structure of the sub-parallel, east dipping Main fault, which in turn merges with the major Omchak fault to the south. The Main and mineralised faults reflect a complex pattern of strain ranging from compression in the north, strike-slip in the centre and extension/dip-slip to the south. The mineralisation is found at the transition from compressive to extensional domains. Pre-ore dykes and and ore-bearing veins were emplaced during compressive and sinistral movement, while subsequent horizontal compression corresponded to post-ore folding and dextral movement. Local post-ore dykes were emplaced during a renewal of dextral movement, while late granitoid bodies were emplaced during a dextral phase.

The ore zone at Natalka comprises around 100, including more than 8 major, mineralised structures that in plan view lie within a 5 km long area that is 100 to 200 m wide in the NW, 350 to 400 m wide in the centre and 600 m in the SE. The mineralised structures consist of quartz veins, stringers and altered wall-rock. Sub-parallel stringers occupy longitudinal tension fractures which lie at a moderate angle to schistosity and contain the bulk of the resource. In general veins are in the core of individual structures, enclosed by stringer zones and altered hosts and peripheral alteration and sulphidisation.

Quartz veins occur in the core of the structures and exhibit brecciated, brecciform, banded or massive forms, with sulphides mostly confined to host rock clasts within brecciated and banded veins, and to a lesser degree in the quartz matrix. Individual veins are up to several metres thick and extend for a much as hundreds of metres along srike.

The sub-parallel quartz stringers are the most widespread of the ore types. They range from less than 1 mm to several cm in thickness and are found at a density of 5 to 50 per metre in the ore structures and generally constitute up to 10% of the wall-rock, locally up to 40%. Thin, variably oriented, quartz, quartz-sulphide and sulphide stringers are also present. The stringers are accompanied by silica, sulphide and carbonate hydrothermal alteration of sediments and intrusive rocks. Steep, sub-parallel quartz stringers are common to the NW, while thin, variably oriented quartz, quartz-sulphide and sulphide stringers in altered host rocks are common in the SE. Hydrothemal alteration covers a broad zone extending up to several hundred metres outwards from the ore zone, but is more intense within the ore zone. Post ore displacement is widespread, although offsets are usually less than 0.5 to 1.5 m, but as much as 10 to 15 m on gentle steep faults.

The distribution of gold is irregular. High grades correspond to areas of quartz veining and intense sulphidisation. The major gangue mineral is quartz, with subordinate carbonates (calcite, magnesite, dolomite and ankerite), feldspars (albite and K feldpar), chlorite, sericite, kaolinite, montmorillonite, barite and apatite. The main sulphides are arsenopyrite lesser pyrite and minor pyrrhotite, Co-Ni sulpharsenides, sphalerite, chalcopyrite, galena, native gold, ilmenite and rutile, and local scheelite, millerite, tetrahedrite, bournonite, boulangerite and stibnite. The total sulphide content in the ore structures is 1 to 3%, locally up to 5%.

The deposit is believed to have resources of approximately 1280 t Au at an average grade of 1.13 g/t Au, corresponding to 1125 Mt of ore (Seltman et al., 2010).

In January 2007 the State Reserves Committee (GKZ Rosnedra) of the Ministry of Natural Resources of the Russian Federation estimated a resource of 1500 t of gold in Russian B+C1+C2 categories within the pit outline at an average grade of 1.7 g/t Au, including 1283 t of B+C1. In addition to this, there are 335.3 t of Au as off-balance reserves outside the pit outline (Polyus Gold, 2007).

Remaining JORC compliant Ore Reserves and Mineral Resources at 31 December 2017 were (Polyus Ore Reserves and Mineral Resources Update 20 March, 2018):
  Proved + Probable Ore Reserves - 293 Mt @ 1.7 g/t Au for 498 t of contained gold;
  Measured Mineral Resources - 150 Mt @ 1.7 g/t Au;
  Indicateded Mineral Resources - 261 Mt @ 1.8 g/t Au;
  Inferred Mineral Resources - 148 Mt @ 2.1 g/t Au;
  TOTAL Mineral Resources - 558 Mt @ 1.9 g/t Au for 1060 t of contained gold;

The most recent source geological information used to prepare this summary 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.

  References & Additional Information
   Selected References:
Eremin R A, Voroshin, S V, Sidorov V A, Shakhtyrov V G, Pristavko V A and Gashtold V V,  1994 - Geology and Genesis of the Natalka Gold Deposit, Northeast Russia: in    International Geology Review   v36 pp 1113-1138
Seltmann, R., Soloviev, R., Shatov, V., Pirajno, F., Naumov, E. and Cherkasov, S.,  2010 - Metallogeny of Siberia: tectonic, geologic and metallogenic settings of selected significant deposits: in    Australian J. of Earth Sciences   v.57, pp. 655-706.

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

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