Deposit Type
- Breccia pipe / Stockwork
- Vein / narrow vein
- Orogenic
- Mesothermal
Summary:
The Kumtor Mine area is dominated by several major thrust sheets with an inverted age relationship. The dominant structural direction is northeast southwest (D 4), with moderate dips to the southeast. Each thrust sheet contains older rocks than the sheet it structurally overlies. Four major structural sheets (zones), stacked upon one another, and bounded by long-lived faults that were active repeatedly, have been identified, as follows:
* Zone 0 consists of Cambro-Ordovician limestone and phyllite, thrust over Tertiary sediments of possible continental derivation that in turn rest on Carboniferous clastic sediments with apparent profound unconformity;
* Zone 1 constitutes the KFZ, whose upper limit is the Upper Kumtor Fault. The KFZ is generally a dark-grey to black, graphitic gouge zone. The KFZ strikes northeasterly, dips to the southeast at moderate angles and has a width of up to several hundred metres. The adjacent rocks in its hanging wall are strongly affected by shearing and faulting for a distance of up to several hundred metres;
* Zone 2 includes the mineralization which is hosted by Vendian meta-sediments, grey carbonaceous quartz-sericite-chlorite schists or phyllites that are strongly folded and schistose, with a large proportion of faulted and sheared rocks. Zone 2 is delimited in the footwall by the Upper Kumtor Fault and in the hanging wall by the Lysii Fault. It appears that the mineralizing event, itself multi-phase, has healed some of the earlier brittle features within Zone 2;
* Zone 3 consists of phyllites, also of Vendian age, that show several phases of folding. Zone 3 is sub-divided into three units based on the orientation of the foliation. The dip of the schistosity is shallow to steep to the southeast in the Zone 3A and 3C rocks, and steep to the northwest in the Zone 3B rocks. The subsequent brittle deformation is less strongly developed compared to Zones 1 and 2. Zone 3 is sub-divided into three units based on the orientation of the foliation. Zone 3 is important for the pit slope stability discussed in Section 16 because of the development of the D3 and the D4 back-thrust faults within the zone.
It is important to note that most fault structures in the area are persistent, with thick gouge or tectonic breccia and are therefore potential failure surfaces. This is attributed to D 4 reactivation of pre-existing faults.
The main structures of the Central Deposit are also present in the Southwest and Sarytor Deposits. The main thrust faults at Sarytor strike E-W, and the faults and S 1 schistosity along with the mineralized zones have a shallow southerly dip. The structures are truncated in the west, with Zone 0 juxtaposed against a steep NNE SSW trending fault. This fault is strike-slip in nature indicating a D 3 origin, and is likely the over-steepened continuation of the Lower Kumtor Fault, which separates Zones 0 and 1 in the Central Pit area. The E-W orientation of thrust faults with a southerly dip is a function of D 3 deformation, with limited overprinting by the D 4 structural event.
Structures start to swing to a NE-SW trend as they continue eastward into the Southwest Deposit area, where there is a strong D 4 structural overprint. This D 4 overprint can be traced northward into the Central Pit and thus the structural make up of the Southwest Deposit is more like that of the Central Pit than the Sarytor Deposit. These observations indicate that the D 3 structures were rotated into a NE-SW trend by the D 4 structural event.
Gold mineralization of economic importance occurs where the Vendian sediments have been hydrothermally altered and mineralized, an event that may have taken place in Permian time as discussed above, based on structural considerations (Ivanov et al., 2000) and as reported by Mao et al. (2004). Gold mineralization is developed over a strike distance of more than twelve kilometres. The Central Deposit is the most important accumulation identified to date and has considerable dimensions with a strike length of 2.4 kilometres, a vertical extent of one kilometre, and a width of up to 300 metres. Other known occurrences along the mineralized trend are the Southwest Deposit and Sarytor Deposit as shown on Figure 7-2. Figure 7-8 provides a composite of the main ore accumulations of the Central, Southwest, and Sarytor, Deposits.
According to Ivanov et al., 2000, mineralization in the Kumtor Mine area took place in four main pulses. An initial pulse resulted primarily in pervasive quartz carbonate-albite-chlorite-sericitepyrite alteration, with little gold of economic consequence being deposited. However, this early alteration may have “stiffened” the host rocks sufficiently to make them susceptible to the intensive veining, stockwork and hydrothermal breccia development during the next two pulses that deposited all of the economically significant gold. A few typical examples of mineralized drill core are in Figure 7-9.
The temperature of formation of the second stage veins was 310 ± 15°C, according to Ivanov & Ansdell, 2002. The mineralogy during the main phases includes early K-feldspar followed by later albite and variable amounts of carbonate (calcite, dolomite, ankerite and siderite), quartz, pyrite, sericite and chlorite, in addition to small amounts of chalcopyrite, haematite, barite, strontianite and accessory magnetite, scheelite, ferberite, rutile, cassiterite, sphalerite, galena, native gold and tetrahedrite, as well as a number of silver-gold, lead and nickel tellurides. The feldspars combine to comprise nearly 20% of the ore, the carbonates collectively 25 to 30%, pyrite 15 to 20%, quartz 5 to 10%, and the remainder are host rock inclusions.
The mineralization is most intense, and the gold grade is the highest, where the metasomatic activity was continuous through mineralization phases two and three. This is the case for the Stockwork and SB Zones, and explains their higher-than average gold grades. The last pulse created planar carbonate-pyrite metasomatic rocks that are associated with zones of intense deformation of previously altered phyllites.
Native gold and the gold-silver tellurides are intimately associated with pyrite to the extent that gold grade and pyrite content are “positively correlated” (Ivanov et al., 2000). The gold and the gold-bearing minerals occur as very fine inclusions in the pyrite, with an average size of only 10 microns. This, together with the poor cyanide leach response of the gold tellurides, accounts for the partly refractory nature of the ore. The refractory characteristics are reflected in the relatively low historic and forecast gold recovery of around 80%, despite the very fine grind applied to the pyrite flotation concentrate from which most of the gold is recovered. However, the fine grain size of the gold also renders assaying of this mineralization relatively reliable, with only a small minor nugget effect.
Most of the mineralization takes the form of veins, veinlets, and breccia bodies in which the mineralization forms the matrix. In the more intensely mineralized areas, the surrounding host rock has also been altered. Post-ore faulting is generally parallel to, or at low angles with, the mineralized sequence. These faults often carry significant quantities of graphite, and other carbonaceous components which constitute the sources for the preg-robbing character of some of the mineralization.