Siberia - Chukotka, Russia
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The Kekura intrusion related gold deposit is located in northeastern Siberia, in the Bilibino administration district of the Chukotka Region. It is ~195 km SE of the administrative centre of Bilibino and and 550 km west from the regional capital of Anadyr, in the Chukotka Autonomous Okrug of the Far East of the Russian Federation (#Location 67° 2' 31"N, 166° 31' 49"E).
The Western Chukotka region of Russia hosts a number of gold, silver, copper and molybdenum deposits and prospects, including the Kupol and Dvoinoe high sulphidation epithermal gold-silver deposits; the Karalveem intrusion-related gold deposit (0.645 Mt @ 14.1 g/t for 9.1 t of gold; Akimova et al., 2016), the Klen intermediate sulphidation epithermal gold deposit (3.72 Mt @ 5 g/t Au for 18.6 t of gold); and the Peschanka, Nakhodka (~910 Mt @ 0.34% Cu, 54 ppm Mo, 0.30 g/t Au, 1.2 g/t Ag) and smaller Mangazeika porphyry style Cu-Au(-Mo) deposits. Kupol and Peschanka are ~120 km ESE and 125 km SW of Kekura respectively (Nagornaya et al., 2020).
Kekura is an intrusion related gold deposit located within the Lower Cretaceous, composite granitoid Gvardeisky magmatic complex intruded into an island arc setting in central Chukotka.
Western Chukotka has been subdivided into four tectonic terranes, which are, from SW to NE (after Nagornaya et al., 2020):
i). a marginal fringe of the Omolon cratonic terrane which comprises a slightly deformed Palaeozoic and Mesozoic cover sequence of clastic and volcanic strata overlying Precambrian crystalline basement;
ii). the Oloy Zone, which comprises a tectonic collage of Palaeozoic and Mesozoic active margin complexes, but is dominated by Jurassic and Early Cretaceous continental arc magmatic sequences. It hosts the large Peschanka and Nakhodka, and small Mangazeik porphyry Cu-Au-Mo deposits, as well as the Klen intermediate sulphidation epithermal gold deposit;
iii). the South Anyui Suture Zone, which was formed in the Early Cretaceous, following closure of the oceanic basin separating the Chukotka Block and the Siberian Craton, and their subsequent collision. It hosts the Kekura deposit and has a length of ~ 600 km and width of 15 to 40 km. It is is characterised by a series of NW-SE trending tectonic slices that comprise intensely folded clastic rocks that are occasionally intercalated with basalts and cherts. The age of the volcanic and sedimentary rocks of the South Anyui Zone ranges from Late Triassic to Early Cretaceous (Sokolov et al., 2009). The youngest detrital zircons extracted from the clastic rocks of the zone have ages of ~125 Ma (U-Pb; Amato et al., 2015), suggesting that the final orogenic collision occurred during the early Aptian; and
iv). the Anyui Zone, the former passive margin of the Chukotka block (Parfenov 1991; Nokleberg et al., 2001), which hosts the large Karalveem intrusion-related gold deposit.
All four of these zones have been overprinted by a post-collisional, 121 to 112 Ma magmatic event (Tikhomirov et al., 2017; Kara et al., 2019) and by later 106 to 74 Ma subduction-related volcanism of the Okhotsk-Chukotka volcanic belt (Akinin and Miller 2011; Tikhomirov et al., 2012).
A Late Triassic, highly deformed flysch-like sequence, composed of folded and fractured intercalated mudstone, siltstone and sandstone, is the principal sequence in the in the Kekura deposit area. This succession is intruded by the composite three phase Kekura Granitic Pluton which is considered to be part of the Early Cretaceous Gvardeisky igneous complex which has been dated at 124 to 94 Ma (Bulk rock K-Ar; Furman 1999). The outcrop of the mineralised Kekura stock covers an area of ~13 km2 and comprises three phases of magmatism, from oldest to youngest: i). medium-grained diorite, ii). medium- to coarse-grained quartz monzodiorite and syenite, and iii). medium- to coarse-grained granodiorite. Each of these phases contains xenoliths of gabbro. The pluton has not been influenced by any substantial compressional event, although it is cut by numerous and multi-directional dykes of pre-mineral granodiorite and granite porphyries, and by a post-mineral diorite porphyry (Nagornaya et al., 2020).
Gold mineralisation is hosted by the granodiorite intrusion, concentrated around a series of 15 to 40° dipping faults, that control the development of hydrothermally altered zones and quartz veins. These structures appear to be dominantly reverse to thrust faults, with a cumulative displacement in the order of tens of metres across the mineralised package, as indicated by the offset of lamprophyre dykes. The main package of mineralised structures has a strike length of >1 km, a down-dip extent of >400 m, and a thickness of 80 to 120 m. This package, known as the Pologay Zone, outcrops to the SW. The western and northern limits of mineralisation are defined by steeper 50 to 80°E to SE dipping faults, the Vintovoy and Krutaya fault respectively. The latter is also mineralised. Another such structure, the Daykovy Fault, approximately corresponds to the likely boundary between the open pit and underground components of the deposit (Fluor, 2018).
Mineralisation is spatially related to intrusive rocks, and occurred in a number of stages, each accompanied by a distinct hydrothermal alteration assemblage, as follows (after Nagornaya et al., 2020):
• Stage 1, which is characterised by Ni-Co-Fe arsenides and sulphoarsenides (nickeline, safflorite, cobaltite, löllingite) and native bismuth, commonly accompanied by a propylitic assemblage of quartz-oligoclase-actinolite-clinochlore-calcite;
• Stage 2, represented by molybdenite, bornite, chalcopyrite, and pyrite I, with an associated albite-quartz-muscovite-tourmaline alteration assemblage;
• Stage 3, associated with an alteration suite of quartz-dolomite-muscovite and arsenopyrite, and was responsible for the bulk of the economic mineralisation, which occurred in two pulses:
Substage 1, represented by scheelite, arsenopyrite, pyrite II, chalcopyrite II, sphalerite, galena, tennantite-tetrahedrite, maldonite and native gold with a fineness of ~850.
Substage 2, characterised by Bi tellurides and sulphotellurides, native bismuth, bismuthinite and gold with a fineness of 920 to 995 as a decomposition product of maldonite.
• Stage 4, the final mineralisation stage, is characterised by chalcopyrite III, galena II, boulangerite, bournonite, Ag-rich tetrahedrite, Sb-bearing sphalerite, stibnite, a low-fineness Au-Ag alloy and native silver associated with a muscovite-illite-siderite-quartz alteration assemblage (Nagornaya et al., 2020).
Overall, within the deposit, arsenopyrite is the dominant sulphide mineral totalling up to 2 to 3%, whilst molybdenite and chalcopyrite are only minor components. There is a coarse component to the gold distribution, with individual grains commonly ranging from 0.5 to 1 mm, and occasionally as much as 3 to 5 mm. Oxidation is weakly developed to a depth of 20 to 30 m below the surface, reflected by iron hydroxide appearing on fracture surfaces, although it does not have a significant effect on the distribution or gold, nor the processing properties of the mineralised rocks (Fluor, 2018).
Resources and Reserves
JORC compliant Ore Reserves at a cut-off of 1.6 g/t Au in Open Pit and 2.0 g/t Au Underground, at 1 January 2018 (Highland Gold Mining Limited Resource and Reserve statement):
Proved Ore Reserves
Open Pit - 0.65 Mt @ 9.24 g/t Au for 6.0 t of gold;
Probable Ore Reserves
Open Pit - 5.1 Mt @ 7.82 g/t Au for 39.9 t of gold;
Underground - 3.13 Mt @ 5.31 g/t Au for 16.6 t of gold;
TOTAL Reserves - 8.88 Mt @ 7.03 g/t Au for 62.5 t of gold.
Mineral Resources - includes Ore Reserves
Measured Resource - 0.580 Mt @ 11.0 g/t Au for 6.39 t of gold;
Indicated Resource - 8.720 Mt @ 8.0 g/t Au for 69.50 t of gold;
Measured + Indicated Resources - 9.300 Mt @ 8.2 g/t Au for 75.89 t of gold;
Inferred Resource - 0.160 Mt @ 3.1 g/t Au for 0.50 t of gold;
TOTAL Resources - 9.460 Mt @ 8.1 g/t Au for 76.40 t of gold.
This summary is also partly derived from "Fluor, 2018 - Kekura Gold Project Feasibility Study, Section 1, Executive Summary; prepared for CJSC Bazovye Metally by FLUOR", available from Highland Gold website, 74p.
The most recent source geological information used to prepare this summary was dated: 2020.
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.
Nagornaya, E.V., Baksheev, I.A., Selby, D. and Tikhomirov, P.L., 2020 - The latest Aptian/earliest Albian age of the Kekura gold deposit, Western Chukotka, Russia: implications for mineralization associated with post-collisional magmatism: in Mineralium Deposita v.55, pp. 1255-1262.|
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