Deposit Type
- Epithermal
- Vein / narrow vein
- Breccia pipe / Stockwork
Summary:
The Kupol deposit is considered to be an example of a low-sulphidation epithermal deposit (e.g., Panteleyev, 1996). Low-sulphidation epithermal deposits are high-level hydrothermal systems, which vary in crustal depths from about 1 km to surficial hot spring settings. Host rocks are extremely variable, ranging from volcanic rocks to sediments. Calc-alkaline andesitic compositions predominate as volcanic rock hosts, but deposits can also occur in areas with bimodal volcanism and extensive subaerial ashflow deposits. A third, less common association is with alkalic intrusive rocks and shoshonitic volcanics. Clastic and epiclastic sediments in intra-volcanic basins and structural depressions are the primary non-volcanic host rocks.
Mineralization in the near surface environment takes place in hot spring systems, or the slightly deeper underlying hydrothermal conduits. At greater crustal depth, mineralization can occur above, or peripheral to, porphyry (and possibly skarn) mineralization. Normal faults, margins of grabens, coarse clastic caldera moat-fill units, radial and ring dyke fracture sets, and hydrothermal and tectonic breccias can act as mineralized-fluid channelling structures. Through-going, branching, bifurcating, anastomosing, and intersecting fracture systems are commonly mineralized. Mineralization forms where dilatational openings and cymoid loops develop, typically where the strike or dip of veins changes. Hanging wall fractures in mineralized structures are particularly favourable for high-grade mineralization.
Deposits are typically zoned vertically over about a 250 m to 350 m interval, from a base metal-poor, Au-Ag-rich top to a relatively Ag-rich base metal zone and an underlying base metal-rich zone grading at depth into a sparse base metal, pyritic zone. From surface to depth, metal zones grade from Au-Ag-As-Sb-Hg-rich zones to Au-Ag-Pb-Zn-Cu-rich zones, to basal Ag-Pb-Zn-rich zones.
Silicification is the most common alteration type with multiple generations of quartz and chalcedony, which are typically accompanied by adularia and calcite. Pervasive silicification in vein envelopes is flanked by sericite-illite-kaolinite assemblages. Kaolinite illite-montmorillonite±smectite (intermediate argillic alteration) can form adjacent to veins; kaolinite-alunite (advanced argillic alteration) may form along the tops of mineralized zones. Propylitic alteration dominates at depth and along the deposit margins.
The mineralization typically includes pyrite, electrum, gold, silver, and argentite. Other minerals can include chalcopyrite, sphalerite, galena, tetrahedrite, and silver sulphosalt and/or selenide minerals. In alkalic host rocks, tellurides, roscoelite and fluorite may be abundant, with lesser molybdenite as an accessory mineral.
Features that classify the Kupol deposit as a low-sulphidation epithermal-style deposit include:
1) Vein was emplaced in a predominantly extensional environment, vein is associated with regional through-going structure;
2) Presence of chalcedonic and opaline quartz (low temperature cryptocrystalline to colloidal quartz);
3) Mineralization is hosted in multiphase colloform- to crustiform-banded quartzadularia veins and polyphase breccias; well-developed cyclic banding of quartz and sulphides-sulphosalts with cryptocrystalline (chalcedonic) to fine-grained quartz; cockade and lattice structures are common;
4) Gold occurs within or is rimmed by sulphosalts and free within the quartz;
5) Sulphide assemblages are dominated by pyrite. Russian studies indicate the presence of very fine-grained arsenopyrite, stibnite, silver-rich tetrahedrite (freibergite), acanthite, stephanite, and pyrargyrite;
6) Zonation of the alteration within the deposit area with distal propylitic alteration grading into proximal silicification, argillic alteration and potassic alteration; above the deposit in the north area an advanced argillic cap has developed;
7) Fluid inclusion studies that show homogenization temperatures for vein samples that range from 160 to 260°C;
8) Silver-gold ratio of 12:1.
Gold and silver occur as native gold, the gold-silver alloy electrum, in acanthite and silver-rich sulphosalts (stephanite and pyrargyrite dominant). Gold and these minerals occur with pyrite and minor amounts of arsenopyrite, chalcopyrite, galena and sphalerite predominantly in bands within chalcedonic quartz, quartz and quartzadularia colloform and crustiform veins and breccias. The predominant gold and silver minerals of the Kupol deposit are electrum, native gold, silver-rich tetrahedrite (freibergite), acanthite, and a variety of sulphosalts. Stephanite and pyrargyrite are the dominant sulphosalts. Traces of selenium-bearing sulphosalts and naummannite are present. Visible native gold or gold-silver amalgams are common throughout the deposit but rarely exceed 3 mm in size.
Pyrite and marcasite are ubiquitous, and are accompanied locally by chalcopyrite. Base metals occur throughout the Kupol vein; however, there is not a noticeable transition from precious to base metal-rich mineralogies at depth.
Polymetallic mineralization present in the veins to the southwest of the main vein system (Vtoryi veins) may reflect a different source of hydrothermal fluids or lateral zonation of fluid chemistry out from the main structure. These veins have silver-gold ratios that range from 1:1 to 1,500:1.
The Kupol deposit is located in the 3,000 km long Cretaceous Okhotsk-Chukotka volcanogenic belt. This belt is interpreted to be an Andean volcanic arc type tectonic setting, with the Mesozoic Anui sedimentary fold belt in a back-arc setting to the northwest of the Kupol region. Tthe Kupol deposit area is centred within a 10 km wide caldera, along the northwestern margins of the 100 km wide Mechkerevskaya volcano-tectonic “depression”, an Upper Cretaceous bimodal nested volcanic complex. The volcanic succession in the area is 1,300 m thick and consists 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.
The property is underlain by shallow eastward-dipping andesite lithic tuffs, feldsparhornblende 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 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.
The volcanic stratigraphy of the Kupol deposit comprises rocks of andesitic composition that have been divided into three principal groups (flows, volcaniclastics, and epiclastics) based on composition, textures, and depositional environment:
Each group is further subdivided based on composition and/or texture. The volcanic rocks are intruded by dykes of basalt and rhyolite composition. The rhyolite dykes are most abundant and extensively intrude faults and the vein zone. Locally, zones of polylithic breccia with a clayey or glassy matrix occur adjacent to dykes and in the vein zone.
The Kupol vein system dips steeply to the east at 75° to 90° and describes a broad arc varying between azimuth 22° and 350°. The vein is a fissure structure that contains local splays, anastomosing vein sets, and cymoidal loop structures. The cymoids correspond to thickening of the veins and development of higher-grade shoots.
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 000° to 010-022°. 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.