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The Vasilkovskoye intrusion-related gold deposit is located 17km north of the city of Kokshetau (Kokchetav) in Akmola Oblast, northern Kazakhstan (#Location: 53° 26' 14"N, 69° 14' 59"E).
The deposit was discovered in 1963, and a pilot open pit mining project undertaken from 1980 to 1986. Mining commenced in 1995 until the end of 2007, mining the oxide ores which were treated by heap leach. Production to 2011 was 14 Mt of ore.
The Vasilkovskoye deposit is within the western part of the NW-trending Shatskaya metallogenic zone of the Altai-Sayan orogenic belt. It lies within a Proterozoic metamorphic basal complex, intruded by Ordovician granitoids, and overlain by later Palaeozoic sediments. The area has been subjected to a Mesozoic weathering regime, and is overlain by Cenozoic sand and clay sediments.
In the Vasilkovskoye deposit area, the Altybaysky intrusion occurs as a diorite/gabbrodiorite and a granite/granodiorite suite, part of the larger the Zerendinsky Complex. Sharp planar contacts between the two suites are common, although not always the case. The diorite/gabbrodiorite suite, which is restricted to the northern section of the deposit, and is generally only weakly mineralised, represents the transition between the metamorphic country rocks and the Altybaysky intrusion granite rocks. The granite/granodiorite suite occupies the majority of the deposit area and hosts the bulk of the mineralisation. In the southern part of the deposit, at depths of 400 to 500 m, quartz-diorites, diorites and gabbro-diorites have been intersected which may be related to the Stepnyaksky Complex which is related to other gold deposits in northern Kazakhstan.
The Altybaysky intrusion is cut by numerous faults, most trending either NW and NE, parallel to two major first order structures, the NE-striking Vasilkovskoye fault, and the NW-trending Dongulagashsky fault. The deposit is located in the northern quadrant formed by the intersection of these two structures, within the thermal aureole caused by the granitic intrusion. These and other structural trends can be summarised as follows:
i). North Western (300 to 320°) which are related to the regional Dongulagashsky fault, have variable dips, occurring as widespread, isolated fractures and zones of jointing. They are 2 to 10 cm wide and rectilinear, containing ferruginous fault gouge and fine schistosity along vein margins. They were pre-ore but reactivated during the ore forming event;
ii). North Western (330 to 350°), dipping steeply to the NE, and less commonly to the SW;
iii). North (350 to 10°), vertically dipping, and rarely steeply to the NE and SW. These are numerous and occur as thick (5 to 7 m) zones of schistosity, lenses of brecciated rocks and narrow zones of fault gouge. They trends are curvilinear and were reactivated into shear zones;
iv). North-North Eastern (15 to 30°) dipping steeply to vertically (65 to 90°), predominantly to the SE. This is the dominant structural direction, and may represent part of a large regional fault zone. Steeply dipping (75 to 90°), narrow fractures (0.5 up to 25 to 30 cm) control the distribution of ore veins. The longest of these veins is 143 m, although most pinch out between 1 to 2 m and tens of metres. Quartz-arsenopyrite and arsenopyrite veinlets (0.5 to 3 cm thick) are developed at the contacts of quartz veins. The frequency of the veins/veinlets varies from one per 5 to 7m to hundreds per metre, determined by the density of intersecting fractures. Clusters of veinlets merge into veins along NW fractures at the intersection of the NE veins and veinlets with NW structural dislocations;
v). North Eastern (40 to 60°), dipping 30 to 55°, predominantly believed to be part of a large, thick of regional fault zone. Isolated 5 to 10m thick zones of schistosity zones are evident;
vi). East (80 to 100°), which are uncommon, and have various dips, predominantly ~45°
vii). North Western (290 to 310°), gently dipping at 10 to 15° to the NE. These veins may be up to 30 cm wide and are related to the ore phase.
Gold mineralisation is spatially associated with a stockwork of hydrothermal quartz and quartz-arsenopyrite veins and veinlets that form a zone that flattens towards the surface, and steepens with depth, dipping SW, persisting to 1000 m below the surface, before pinching out.
The dominant sulphide is arsenopyrite, which occurs as sulphide veinlets, with quartz veins and as disseminations, with native gold, minor pyrite and trace amounts of chalcopyrite, bismuth sulphides and bismuth tellurides. Lesser sphalerite, galena, molybdenite, stibnite and tennantite are also recorded. Uranium is spatially associated with the gold, but is younger than the main gold event. Minor gold also occurs within carbonate and silicates. Uranium mineralisation, locally as high as 100 ppm is present within the deposit, with rare, narrow higher grade intervals. Outside of the pit envelope, uranium grades >1% occur.
All of the sulphides are found in both the diorite/gabbrodiorite and granite/granodiorite suites, although pyrite and chalcopyrite are more common in the mafic rocks. Extensive weathering in the upper levels of the deposit converted the sulphides to oxide minerals.
Higher grade gold values are found in microcrystalline grey quartz veins and veinlets, which are commonly relatively thin (<3cm), and oriented at a high angle, either en echelon or slightly oblique to, the direction of the main faults. The concentration of veinlets in the ore zone varies from 1 to 5, to as much as >10 per metre. Gold grains ranges from 1 to 63µm, averaging 2.5µm in the gabbro, and 4µm in granodiorite, mostly occurring as < 10µm inclusions within arsenopyrite.
Mineralisation has been divided into a series of stages, all of which include some sulphides, particularly arsenopyrite:
Pre Ore Phase - corresponds to the earliest sulphide introduction, comprising chalcopyrite-pyrrhotite-pyrite, arsenopyrite-pyrite and Au assemblages, which are best developed within the gabbro suite, especially adjacent to the contact with the granites, and in strongly fractured areas. The chalcopyrite-pyrrhotite-pyrite assemblage is generally ore when closely associated with rocks rich in mafic minerals.
Early Ore Phase - dominated by the introduction of quartz, carbonates, sericite and pyrite (i.e., carbonate and phyllic alteration).
Main Ore Phase - essentially a quartz, carbonate and chalcophile mineral assemblage, accompanied by the introduction of complex copper, bismuth, antimony, arsenic and tellurium minerals.
Post Ore Phase - representing a waning of the system with the introduction of silica, carbonate and minor complex sulphides, as well as the deposition of uranium.
Alteration occurs as steeply dipping lenticular zones, including: chlorite; chlorite-albite; albite; carbonate; quartz-sericite; quartz-K feldspar; K feldspar; and argillic alteration. The most intensive alteration is confined to areas of increased fracturing, and drops of in intensity with distance from the ore. Syn-mineral alteration accompanying the sulphide-quartz auriferous stockwork and veins is principally the quartz-sericite (phyllic) assemblage, essentially comprising grey quartz, infilling of vesicles and minor recrystallisation of rock minerals to sericite and carbonate. The alteration accompanying the post mineralisation phase is largely argillic alteration comprising 2 to 5 cm veins of predominantly of carbonate.
JORC compliant mineral resources at 1 Jan, 2011 (Wardell Armstrong 2011, for Kazzinc Glencore) were:
0.4 g/t Au cut-off
Measured + indicated resource - 186.80 Mt @ 1.72 g/t Au for 320 t of gold;
Inferred resource - 99.08 Mt @ 1.77 g/t Au for 175 t of gold;
0.9 g/t Au cut-off
Measured + indicated resource - 128.49 Mt @ 2.22 g/t Au for 286 t of gold;
Inferred resource - 68.63 Mt @ 2.27 g/t Au for 156 t of gold;
1.5 g/t Au cut-off
Measured + indicated resource - 78.49 Mt @ 2.90 g/t Au for 227 t of gold;
Inferred resource - 39.74 Mt @ 3.07 g/t Au for 122 t of gold;
JORC compliant ore reserve at 1 Jan, 2011 (Wardell Armstrong 2011, for Kazzinc Glencore), using a 0.48 g/t Au cut-off were:
Proved + reserve - 33.3 Mt @ 1.95 g/t Au for 65 t of gold;
Probable + reserve - 90.7 Mt @ 1.94 g/t Au for 176 t of gold;
TOTAL reserve - 123.97 Mt @ 1.94 g/t Au for 240 t of gold.
The information in this summary is largely sourced from: Newell et al., 2011 - Competent Person's Report for the Assets held by Kazzinc Limited
in Kazakhstan and Russia; prepared by Wardell Armstrong International, Truro, UK.
The most recent source geological information used to prepare this summary was dated: 2011.
Record last updated: 17/5/2013
This description is a summary from published sources, the chief of which are listed below.
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