The Kupol’ gold-silver deposit is located approximately 220 km SSE of Bilibino in the Chukotka Autonomous Okrug, Russia. The Okhotsk-Chukotka Volcanic Belt is a Late Cretaceous Andean-type (continental margin) predominantly andesitic magmatic arc. Kupol’ is located in a caldera established on Jurassic sedimentary basement. The 1,300 m thick bimodal volcanic sThe Kupol low sulphidation gold deposit is located 220 km from the town of Bilibino and 400 km NW of Anadyr, in the Chukotka Autonomous Okrug of the Far East Region of the Russian Federation. The satellite Dvoinoye mine is 90 km further to the NNW.
The deposit lies within the 3000 km long Cretaceous Okhotsk-Chukotka volcanogenic belt which is interpreted to be an Andean volcanic arc, with the Mesozoic Anui sedimentary fold belt representing a back-arc to the northwest of the Kupol region. The Kupol deposit is centered on a 10 km wide caldera, along the northwestern margins of the 100 km wide, Upper Cretaceous bimodal nested volcanic complex, known as the Mechkerevskaya volcano-tectonic 'depression'.
The 1300 m thick volcanic succession is comprised of a lower sequence of felsic tuffs and ignimbrites, a middle sequence of andesite to andesite-basalt flows and fragmentals capped by felsic tuffs and flows, all of which are cut and discordantly overlain by basalts of reported Paleogene age. The volcanic rocks unconformably overlie and intrude folded Jurassic sediments.
There are a series of well-defined caldera ring structures within the region, each of which range from 7 to 12 km across, nested within less well-defined volcanic complexes. Felsic (rhyolite dominant) units predominate to the northeast, east and southeast of the Kupol, while intermediate rocks predominate to the west and northwest. The boundary between the felsic- and intermediate-dominant volcanic units is defined by a strong north-south trending lineament which is believed to represent a deep-seated fault structure (the Kaiemraveem fault), based on the bouguer gravity and lithology contrast across the lineament, which lies immediately to the west of the Kupol deposit. The Kupol deposit lies on a north-south splay of the Kaiemraveem fault known as the Kupol structure. The magnitude of displacement, if any, along the Kupol structure is unknown. The Kaiemraveem fault has been interpreted to have intersected a volcanic subsidence ring structure (Kovalevsky caldera) within the Kupol deposit area. The Kaiemraveem fault and Kupol structure are the locus for felsic dome and dyke intrusions.
The Kupol deposit area is underlain by shallow eastward dipping andesite fragmentals, feldspar-hornblende porphyry andesite and andesite-basalt (trachytic andesite) flows. The andesitic volcanic units are intruded by massive to weakly banded rhyolite dykes, rhyolite and dacite flow-dome complexes, and basalt dykes.
The Intermediate and Mafic Volcanic Rocks comprise:
Porphyritic andesite flows, which are of two principal types: a feldspar phyric (crowded porphyry) unit and a porphyritic andesite unit. The feldspar phyric unit outcrops predominantly to the west of the Kupol vein. It is believed that it represents a thick flow unit or sub-volcanic sill and is distinguished from the porphyritic andesite units by a higher percentage (40-60%) of 1-4 mm euhedral feldspar phenocrysts, the presence of clinopyroxene and biotite, and weak magnetism. The units are laterally continuous and massive. The andesite flow is characterised by a grey-green fine-grained to aphanitic matrix that is weakly to moderately sericite-chlorite-carbonate ± clay altered. This unit is likely the altered equivalent of trachytic andesite (andesite-basalt). The presence of minor fragments led to some misinterpretation of the flows as pyroclastic units; therefore, any feldspar phyric volcanic with greater than 1% fragments is classified as a pyroclastic. The presence of fragmented and chaotic crystals in some of the flow units suggests that they may actually be crystal tuffs; however, to preserve continuity the so called crystal tuffs were logged as flows.
The amygdaloidal andesite flows occur as units with 1 to 15 m thickness within the Big Bend, Central, and North zones. They have similar character as the andesite described above, and contain 5-20% of 1 to 4 mm amygdules that are commonly filled with calcite or chlorite. They are likely either discontinuous or not easily distinguished from the main andesite flow units.
Basalts exposed in the deposit area are fine-grained, black to dark grey, massive and moderately to strongly magnetic. Two generations of basaltic units appear to be present in the deposit area. The older unit occurs as narrow microdiorite dykes throughout the main deposit area and an irregular northeast trending stock or dyke encountered in drilling in the Big Bend area. This weakly carbonate- and/or chlorite-clay altered unit cuts the veins and is in turn cut by rhyolite dykes and faults. The younger unit occurs as flows that are exposed on the western valley slope above the Kaiemraveem Valley and as narrow dykes in the South and the North Extension zones. These units are not carbonate-altered and commonly contain 5-7% olivine.
Trachytic andesites (basalt-andesite) have a higher colour index (depending on level of alteration) than the andesites, are weakly to strongly magnetic, and are composed of 40 to 50% plagioclase, 5 to 15% K-feldspar, 7 to 15% biotite and 2-4% orthopyroxene or hornblende and 5-10% clinopyroxene. They are most prevalent in the northern portion of the deposit, where they are intercalated with andesite fragmental units. In outcrop, these units tend to have more friable weathering, in contrast to the blockier weathering pattern of the porphyritic andesite flows.
The andesitic pyroclastic or fragmental units have been sub-divided into the following units based on the dominant textural character within a horizon:
Ash tuff (grain/fragment size <2 mm) + lapilli tuff (fragment size 2-64 mm); Lapilli tuff (fragment size 2-64 mm); Volcanic containing >1% fragments in a feldspar phyric matrix (fragments <64mm); and Agglomerate tuff (fragment size >64 mm)
These units occur as intercalated, continuous to discontinuous layers or horizons. Some of the more continuous and distinctive tuffaceous horizons, in particular ash tuff horizons, are useful for correlation. One variably hematitic and commonly clay-altered ash tuff bed, referred to as the upper marker unit, has been traced northward for 2.3 km and unit varies from 5 to 20 m in thickness (locally to 30 m). The lapilli and agglomerate tuffs are commonly comprised of fragments of porphyritic andesite. The prevalence of lapilli and agglomerate tuffs in the deposit area suggest that it is close to a volcanic centre.
The felsic volcanic rocks have been subdivided into two principal groups, namely Dome complex related lithologies and Dykes and related contact lithologies, each of which is further subdivided based on occurrence, texture, and composition. Each of these subdivisions is described below.
Rhyolite flows and pyroclastics - the larger rhyolite to rhyodacite bodies occur within the Kupol structure and are distinguished from the dykes by their size, heterogenic character and by apparent layering with fragmental beds. The composition of these units appears similar to the dykes. These units are believed to be flow dome complexes and small eruptive centres. In the far north, the rhyolite flows and pyroclastics form a 50 to 75 meter thick lens that is conformable with stratigraphy.
Dacite (Undifferentiated dykes and pyroclastics) are exposed as a single mass in the northeastern portion of the deposit area, to the west of the Kaiemraveem River valley and appear to be related to a subaerial eruption. The dacitic body has an apparent dip of approximately 20° to the east and unconformably overlies basaltic flows.
Polymictic breccias are intimately related to the rhyolite dykes throughout the deposit area and most commonly occur in the footwall of the dykes, transitional to host rocks. They comprise a mixture of angular rhyolite, obsidian, quartz vein and andesite fragments in a dark, clay-rich felsic matrix. The breccias are irregular in outline (in some cases pipe-like and/or conformable with contacts) and are believed to represent explosive breccia bodies or breccia; in part reflecting interaction of the rhyolitic magma with groundwater.
Rhyolite to rhyodacite dykes transect and bisect the Kupol vein in a 100 to 400 m wide NNE trending corridor in the Central, Big Bend and South zones, where they comprise 10-25% of lithologies. They have two main orientations: NNE with steep east or sub-vertical dips, roughly paralleling the vein system and NNE with steep westerly dips, commonly occurring as splays off the first set. Individual dykes reach widths of up to 70 m. Rhyolite dykes do not appear to displace the Kupol vein system and tuff marker horizons, suggesting that they are of dilational in nature and only fill extensional structures. The most common felsic dyke is aphanitic, with a weak to strong flow-banded texture, whuile the secondary type is weakly porphyritic.
The main deposit strikes north-south and has been divided into six contiguous zones, from north to south these are: North Extension, North, Central, Big Bend, South, and South Extension.
Mineralisation is associated with the north-south trending Kupol splay structure of the Kaiemraveem regional fault. The Kaiemraveem structure terminates 25 km to the north at the east-west trending strike-slip structure, the Maly Anui River fault and 22 km to the south at the Mechkereva River 'caldera'. To the north, west and northeast of the Kupol area Jurassic sediments host orogenic style vein lode and placer gold deposits of which the Mayskoye deposit, approximately 400 km to the northeast, is the best known.
Two principal mineralised systems have been identified at Kupol, with the bulk of the mineralisation hosted in a north-south trending dilatant splay off the large regional Kaiemraveem fault structure of similar orientation. The main Kupol deposit consists of one or more polyphase quartz-adularia quartz veins of an epithermal low sulphidation character that are sporadically cut by rhyolite dykes. Gold and silver mineralization is primarily associated with sulphosalt-rich bands and pods within colloform, crustiform and brecciated veins
The main deposit has been divided into six contiguous zones: South Extension, South, Big Bend, Central, North, and North Extension. Mineralisation has been defined within these zones over a strike length of 3.9 km. The Big Bend zone shows the highest grade and most continuous mineralization.
The main vein system in the Kupol structure strikes north-south and dips steeply to the east at 75 to 90°. It is a linear fissure structure that contains local dilational jogs, sinusoidal sways, branches, anastomosing vein sets, and sigmoidal loop structures. The jogs often correspond to primary and second order dilational zones with resultant thickening of the veins and development of higher-grade shoots. The thickest portions of the vein, or local thickening, often occur at north-northeast and north trending left bends in the vein, defining sinistral jogs and resultant development of some of the second order steeply plunging ore shoots. Localisation of the jogs may be a function of intersection of the vein structure with pre- to syn-mineralization structures. The highest concentration of precious metals in the main deposit occurs in the Big Bend zone, a dilational jog in the Kupol structure where the vein swings from an azimuth of due north to 10 to 22°. The ore shoot in this area is approximately 700 m in strike length and plunges toward the South zone at a shallow angle where it continues for greater than 300 m. The vein host structure shallows to 65 to 70° at depths of >450 m in the South, Big Bend and Central zones.
There is an apparent displacement of the stratigraphy across the Kupol structure, although the magnitude of the displacement is uncertain because there are no distinctive markers that can be correlated across the structure. Based on structural studies it is believed that the vein emplacement occurred in a near pure extensional environment and thus the displacement across the main structure is likely a reflection of pre-mineralization tectonics. The occurrence of pre- and syn-mineral faulting is suggested by narrow (0.5-10 cm) zones of silicified, foliated cataclasites that parallel the Kupol vein within 10 meters of it; these may be silicified fault gouges. Pre-mineralization structural events have been largely overprinted by the vein(s) and dykes. It is inferred that the Kupol structure has a significant component of east-side-down normal movement. Several post-mineral faults with significant displacements have been identified. Syn- to post-mineral faults that cross the main vein at an oblique angle commonly display an element of dextral diffraction across the Kupol structure of 20 to 30 m. This diffraction is accompanied by a jarostic vein breccia and gouge zone. A similar element of diffraction is evident in plan where the rhyolite dykes cross the vein; however, in section the sense is opposite: sinistral-reverse.
Much of the alteration associated with the Kupol structure shows up as a broad, up to 400 m wide, zone of magnetite destruction as an up to 3500 nt anomaly, with the bulk of the alteration associated with the structural hanging wall of the main zone. There is a zonation of the alteration within the deposit area with distal propylitic alteration grading into proximal silicification, argillic alteration and potassic alteration. At the upper levels of the deposit, and in particular in the north above the vein zone, the alteration is predominantly argillic.
Weak to moderate propylitic alteration (chlorite-calcite+sericite+pyrite+epidote) occurs within 400 m of the Kupol structure, particularly in the hanging wall to the vein. Epidote is very rare. Calcium carbonate (calcite) alteration is common, usually occurring pervasively in the matrix of the fragmental and flow units. Iron carbonate (siderite and ankerite) is present in limited amounts. Dolomite occurs within the vein and locally as a wall rock alteration in the northern portions of the deposit. Clay alteration is often accompanied by pervasive and fracture-filling calcium carbonate + disseminated pyrite. To the north, the clay alteration is particularly intense and is interpreted to be a steam-heated alteration (advanced argillic) zone at the top of the Kupol hydrothermal system; zones of vuggy silica, textural leaching and localized accumulations of massive pyrite accompany this alteration. The clay-acid sulphate (jarosite-gypsum rich) alteration zone continues to the south; this is indicated by a broad zone of intense, sulphate-rich, pyritic, clay alteration. Clay type varies by location with smectite-kaolinite dominant to the north and at shallower levels and illite-montmorillonite-smectite more prevalent in the hangingwall in the Big Bend zone. The rhyolite dykes are commonly weakly to moderately clay-altered. There is a broad area of weak to moderate clay + sulphate alteration southwest of the main deposit. Potential acid sulphate alteration occurs as jarosite-rich zones but more commonly as discontinuous, abundant gypsum stringer veins to 15 cm wide. Clay alteration (smectite-kaolinite) is more prevalent in the pyroclastic units in this area.
Alteration adjacent to the veins consists of silica, adularia and pervasive illite in the hangingwall and, to a lesser extent, the footwall volcanic units. In selected areas, the silicification extends up to 40 m from the vein. Near the surface, the silicification-adularization (K-feldspar alteration) is commonly accompanied by a strong late sulphate-rich, yellowish coloured (jarosite) anomaly.
There is a broad chloritic zone at depth within the North, Central and Big Bend areas of the deposit. Within this zone, chlorite-pyrite+magnetite-rich bands and clots are present within the banded quartz veins. There is an apparent replacement of original sulphosalt bands and sulphidic breccia matrices with chlorite-pyrite and a partial re-crystallization of the fine colloform and crustiform quartz bands.
Gold and silver mineralisation at Kupol occurs within colloform- to crustiform-banded quartz-adularia veins and polyphase breccias. The vein types are classified by texture into the following:
Massive veins are comprised of massive to sugary, very fine to fine-grained quartz. Similar, more massive veins occur in the footwall or hanging wall host rocks. This style often cores the colloform- to crustiform-banded veins and contains fragments of the sulphosalt-rich colloform-banded veins. Comb- textured amethyst is a relatively common component in the core of these veins.
Banded colloform and crustiform veins which have well developed cyclic banding of quartz + sulphides/sulphosalts with cryptocrystalline (chalcedonic) to fine grained quartz. Cockade and lattice structures are common. Banded quartz, brecciated and healed by a lighter coloured quartz phase is included in this style.
Vein Breccia, which is comprised of brecciated quartz vein, where the matrix is composed of rock flour, sulphides, and/or vein fragments.
Quartz breccia, which is brecciated quartz vein with the matrix comprised of dark sulphide-rich (pyrite with rare sulphosalts) quartz. This unit is principally a quartz-healed tectonic breccia.
Stockwork-style vein mineralisation contained either within the main vein or in the hangingwall or footwall of the system, comprising areas with multiple generations of crosscutting veining, with the veinlets commonly <10 cm wide.
Stringer veining, which consists of sheeted, non-crosscutting veinlets and forms haloes up to 55 m wide within and/or adjacent to the main vein system and may contain veinlets or veins of colloform, crustiform, and breccia character.
Wall rock breccia, which is breccia in which veins contain >25% wall rock fragments and/or puzzle breccias of wall rock healed by quartz veins. The quartz infill commonly shows cockade, crustiform to colloform textures. Sulphosalt concentrations are generally very low.
Yellow siliceous breccia is a brecciated vein and/or banded vein with fractures and rock flour filled with jarosite + quartz that give the rock a distinctive yellow hue. Jarosite commonly makes up 3 to 10% of the matrix. The unit was differentiated because it is common in the Big Bend and Central zones. It occurs down to a maximum depth of approximately 250 meters.
Hematitic breccia, which is a brecciated vein with hematite rich fracture and breccia infill.
A multitude of different vein textures and degrees of brecciation are present within vein intersections.
Polyphase brecciation within the vein system is believed to be principally hydrothermal and phreatic, with only minor, later, tectonic brecciation. Tectonic breccia occurs as rock flour and minor gouge zones within or along the margins of the veins. As a generalisation, there are low- and high-sulphide (<2 and 2-7% sulphides respectively) veins and brecciated veins present in the system. The high sulphide veins carry the highest 'bonanza' grades. Multiple cycles of sulphosalt mineralization are present in the vein system as evident in the sulphosalt-rich banding.
Later cycles of quartz, including amethyst, commonly occur as open space filling and often have cockscomb, cockade to dogstooth textures. Quartz pseudomorphs of bladed calcite (lattice texture) are present throughout most of the deposit but are more prevalent in the north and near surface. Vuggy, drusy and frothy textures, representing a near surface environment, are present in the North Zone between the Premola and North faults.
The predominant gold and silver minerals are electrum, native gold, silver-rich tetrahedrite (freibergite), acanthite, and a variety of sulphosalts. Arsenic and antimony-rich end members of a variety of mineral groups are present reflecting different solution chemistry in the evolution and/or zonation in the deposit. Stephanite and pyrargyrite are the dominant sulphosalts. Traces of selenium-bearing sulphosalts and naummannite are present. Coarse bladed stibnite was observed locally, in the South Extension zone. Arsenopyrite is reported in petrography and in some of the logging but is generally very fine grained and not readily recognizable in core or hand samples.
In May 2006, reserves and resources were (Garagan, Bema Gold, 2006):
Probable mineral reserves - 8.225 Mt @ 16.8 g/t Au, 205 g/t Ag;
Inferred resource - 3.9 Mt @ 13.7 g/t Au, 177 g/t Ag.
In December 31, 2011, reserves and resources were (Kinross Gold, 2012):
Proven + probable mineral reserves - 9.561 Mt @ 9.73 g/t Au, 120.8 g/t Ag;
Measured+indicated+inferred resource - Nil
Reserves and resources at the Dvoinoye mine, 90 km to the NNW, the ore from which is being treated at Kupol was (Kinross Gold, 2012):
Proven + probable mineral reserves - 1.95 Mt @ 17.8 g/t Au, 21.8 g/t Ag;
Measured+indicated resource - 0.243 Mt @ 17.98 g/t Au, 12.3 g/t Ag;
Inferred resource - 0.155 Mt @ 12.82 g/t Au, 12.6 g/t Ag;
(Source: Porter GeoConsultancy, http://www.portergeo.com.au/, 2011)uccession has a lower division of felsic tuff and ignimbrite overlain by middle andesitic basalt flows and fragmentals, also trachyandesite. This was followed by caldera development with rhyolite and dacite dome and dike swarm and hydrothermal activity. This area is in the present permafrost zone, but was not extensively glaciated so there is no thick till cover and some relics of pre-Quaternary regolith have been preserved.
The Late Cretaceous to Lower Tertiary (“Laramide”) hydrothermal mineralization is of the low sulfidation epithermal Ag>Au type and is controlled by N-S dilatant splays off a large regional fault. The ore zone is 4.1 km long, 50 m wide, followed to depth of 725 m. The multistage steeply 75-90° east dipping quartz-adularia fault and fissure veins, with breccias, are low in sulfides (pyrite > arsenopyrite, chalcopyrite, galena, sphalerite) and the precious metals reside in fine-grained black bands of freibergite, acanthite, stephanite, pyrargyrite > naumannite, electrum, native gold.
Production 2008-2011 totalled 2 M oz Au (62 t Au) and 20 Mt Ag (620 t); Reserves in 2012: 9.561 Mt @ 9.73 g/t Au, 120.8 g/t Ag (Kinross Ltd). Total endowment: ~5 Moz Au, 57 Moz Ag.
(Source: Peter Laznicka, September 2012)