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Nezhdaninskoye, Sentachan, Sarylakh

Siberia - Sakha-Yakutia, Russia

Main commodities: Au Sb
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The deposits of the ~1000 km long, Mesozoic to Tertiary Verkhoyansk-Kolyma Fold Belt on the eastern margin of the Siberian Shield, includes the Nezhdaninskoe or Nezhdaninskoye Au mining complex, and the Sentachan and Sarylakh Au-Sb deposits. The Nezhdaninskoye deposit is located in the southern part of the Yana-Kolyma orogenic belt, 450 km east of the city of Yakutsk in Yakutia, in east-central Siberia. Sentachan is several hundred km north of Nezhdaninskoye, and Sarylakh is a further few hundred km to the NW (#Location: Nezhdaninskoye 62° 31' 12"N, 139° 09' 00"E; Sentachan 66° 28' 48"N, 137° 03' 0''E; Sarylakh 64° 16' 48"N, 142° 46' 12"E).

Nezhdaninskoye

The Nezhdaninskoye group of deposits are located in a flexure like anticline at the junction of the north-south trending Nezhdaninsk Shear zone with the NE trending Suntarsk Fault zone.   They are hosted by greenschist facies lower to upper Permian to Triassic siltstones and sandstones, intruded by the small Cretaceous Kurum Massif (an amphibole-biotite granodiorite), several small diorite stocks, dykes of gabbro-diorite, amphibole and biotite lamprophyre. In main the deposit area, the intrusions comprise at least two larger granite stocks (121 to 92 Ma), each covering approximately 5 to 7 km2 and various dykes (153 to 137 Ma lamprophyre and gabbro-diorite; 140 to 110 Ma quartz diorite, granodiorite and plagiogranite-porphyry; and 81 to 79 Ma rhyolite. The main group of deposits occur in the closure of a regional (10 x 60 km) anticline, and are related to a series of generally steep-dipping to subvertical and flat-lying cataclastic- and shear-zones. This setting has allowed the development of continuous zones containing mineralisered structures over intervals that persist for at least 15 km along strike and 2000 m down-dip. Within the vicinity of the main group of deposits there are more than 100 individual mineralised structures. More than 50 orebodies have been delineated over a 10 km strike length and to a depth of 1500 m in the core of the anticline, over widths ranging from 2 to 40 m, located within high angle, north-south trending sinistral faults.
    The gold-orebodies are of several types, including i). quartz-sulphide veins, typically with grades of >5 g/t Au, ii). mineralised tectonic zones carrying 2 to 4 g/t Au, and iii). mineralised stockworks averaging 2 to 5 g/t Au, and up to 40 m in thickness. The mineralised intervals are accompanied by hydrothermal alteration which includes silicification and quartz-Fe carbonate-sericite±albite alteration. The mineralisation is characterised by a low sulphide content, typically not exceeding 5 vol. %. The dominant sulphides are arsenopyrite and pyrite, both of which are enriched in gold (30 to 500 g/t in arsenopyrite and 10 to 150 g/t in pyrite). Other sulphides include sphalerite, chalcopyrite, galena, tennantite-tetrahedrite, bournonite, boulangerite and stibnite.
    Three major mineralising phases have been recognised, namely: i). metamorphism, with the formation of low-Au (<2 g/t) quartz veins, ii). a main stage with gold-bearing pyrite-arsenopyrite, scheelite-arsenopyrite-pyrite, sphalerite-chalcopyrite-galena and stibnite-fahlore phases and related quartz-Fe-carbonate-sericite alteration, and iii). late hydrothermal activity, accompanied by re-distribution of earlier minerals and the formation of a freibergite-pyrargyrite-electrum assemblage. Gold varies in fineness from 560 to 900. In places, gold-bearing intervals are enriched in graphite.   The mineralisation of most economic significance comprises low grade disseminated pyrite and arsenopyrite in altered rocks.
    The Nezhdaninskoe group of deposits contain a resource of more than 950 tonnes of contained gold, with an average grade of ~4 g/t Au, i.e., in ~235 Mt of ore.

Sentachan

Sentachan is localised within the Sentachan Fault, approximately 300 m NE of the Adycha Taryn Fault zone, a 40 km wide, 900 km long, NW trending belt of intensely and multiply deformed clastic sediments.   Gold-quartz and gold-antimony deposits are localised within dextral faults that are subsidiary to the Adycha Taryn Fault zone and are hosted by late Triassic shales, siltstones and sandstones. The ore deposits occur in shales, siltstones and sandstones of Norian and Rhaetian (Late Triassic) age, localised within a monocline. The three main orebodies are 0.2 to 6 m thick and 80 m in length, the central parts of which are massive quartz-stibnite veins, accompanied by numerous quartz-ankerite veinlets and halos of disseminated pyrite, and transformed into separate quartz lenses on the flank of the mineralised zone. Veins are accompanied by numerous quartz-ankerite veinlets and halos of disseminated pyrite in the hanging wall, and by carbonatised sandstone, with finely disseminated pyrite in the footwall, while brecciated quartz veins fill steeply dipping fissures, with quartz clasts cemented by fine-grained quartz-stibnite aggregates. Stibnite dominates in the gently dipping structural zones. Milky quartz boundins are cross-cut by fine chlorite-ankerite-quartz veinlets with rare stibnite.
    The paragenesis includes earliest barren quartz which is cataclased and brecciated. This stage is overprinted by arsenopyrite I and pyrite I, disseminated in fault-controlled quartz-sericite halos surrounding the quartz-stibnite veins. The quartz, arsenopyrite I and pyrite I are fractured with the cracks filled by stibnite. The next phase comprises galena-sphalerite-sulphosalts filling subsidiary fissures to major quartz filled fractures, replacing ankerite, arsenopyrite I and pyrite I. Rare and fine-grained arsenopyrite II is associated with galena, sphalerite and sulphosalts. Native gold occurs preferentially at the corroded contacts of relict quartz and stibnite. Arsenopyrite III is commonly found as rare inclusions in monominerallic massive stibnite veins, and is replaced by native gold, enriched in zones where stibnite overprints earliest arsenopyrite and pyrite.
    Ore grades at the Sentachan deposit average 25% Sb, 37 g/t Au, and 13.4g /t Ag.

Sarylakh

The Sarylakh Sb-Au deposit is also located in the Yana-Kolyma orogen and hosted within a Triassic sandstone-shale sequence. In the deposit area, these rocks have been folded into a broad 12 km wide syncline, with the mineralisation being localised in a district- to regional-scale fault zone intersecting the syncline. A 120 Ma quartz-diorite porphyry stock, and 133 Ma plagiogranite porphyry sill, are exposed ~2 km from the deposit. The plagiogranite is cut by quartz veinlets carrying pyrite, arsenopyrite and stibnite.
    The deposit comprises a series of subvertical quartz-sulphide veins and vein zones, the largest of which is ~10 to 20 m thick and persists over a strike length of several hundred metres, accompanied by quartz-sulphide stockwork developments. There are several generations of stibnite, with the early giant-crystalline (up to 15 cm crystals) phase deformed by mylonitisation. Stibnite and quartz predominate, with other minerals (pyrite, arsenopyrite, bertierite, ankerite and sericite) together accounting for 1 to 10 vol.% of the veins. Trace minerals include native Au, Ag, Sb, galena, chalcopyrite, tetrahedrite, zinkenite and jamsonite. The mineral paragenesis comprises: i). quartz-carbonate-pyrite; ii). weakly auriferous quartz-pyrite-arsenopyrite; and iii). quartz-stibnite-bertierite-polysulphide. The final stage also carries high-fineness native Au (Berger, 1978).
    The Sarylakh deposit has a resource of 2.17 Mt @ 6 wt.% Sb and 6 g/t Au for 0.130 Mt of contained Sb.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2010.     Record last updated: 10/5/2013
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
 References to this deposit in the PGC Literature Collection:
Genkin A D, Bortnikov N S, Cabri L J, Wagner F E, Stanley C J, Safonov Y G, McMahon G, Friedl J, Kerzin A L, Gamyanin G N  1998 - A multidisciplinary study of invisible Gold in arsenopyrite from four mesothermal Gold deposits in Siberia, Russian Federation: in    Econ. Geol.   v93 pp 463-487
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.


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