Summary:
The Vasilkovskoye deposit belongs to the group of intruded-related Au deposits. It is located in the western part of the Shatskaya metaliogenic zone of the North Kazakhstan province, with a northwest trend.
The deposit is located in the endo-exo contact of the Altybaysky granite intrusion (part of the Zerendinsky Complex) with the Precambrian metamorphic rocks. Within this contact occurs a large gold gas chemical halo, which represents a thermal aureole caused by the granitic intrusion, and where the gold enrichment took place.
The Altybaysky intrusion is crossed by a considerable number of faults, with main directions being NW and NE. These two main faults are the Vasilkovskoye fault (NE strike) and a major northwest trending structure parallel to the Dongulagashsky fault (NW strike). The deposit is situated in the northeast quadrant of this intersection.
The Altybaysky granite intrusion is represented by two major units at the Vasilkovskoye deposit; a diorite/gabbrodiorite unit and a granite/granodiorite unit. Planar and sharp contacts between the two units are common, although sometimes contacts are different.
The diorite/gabbrodiorite unit is located in the northern part of the deposit, and shows generally weak mineralisation. This unit represents the transitional zone between the metamorphic host rocks and the Altybaysky intrusion granite rocks. The Granite/Granodiorite Unit occupies the majority of the deposit and hosts the mineralised body.
In the southern part of the deposit, at depths of 400-500m, quartz diorites, diorites and gabbro-diorites have been intersected which may be related to the Stepnyaksky Complex (the main gold deposits of North Kazakhstan province are paragenetically related to this complex).
The deposit’s area is characterised by the presence of intense faulting and sheared zones. First, second and third order faults can be distinguished, with the main first order faults represented by the NW major structure parallel to the Dongulagashsky fault and the NE trending Vasilkovskoye fault. The rest of the faults trend in various directions such as N, NW or NE.
Seven dislocation orientations have been identified:
• North Western (NW 300-320°) of variable dip, widespread and occur as isolated fractures and zones of jointing. They are 2-10cm wide, rectilinear, contain ferruginous fault gouge and fine schistosity along vein margins. They are related to the Dongulagashsky regional fault. They are pre-ore but were reactivated during the ore forming event;
• North Western (NW 330-350°) dipping steeply NE and SW (relatively uncommon);
• North (N 350-10°) vertical and rarely dip steeply to the NE and SW. These are numerous and occur as thick (5-7m) zones of schistosity, lenses of brecciated rocks and narrow zones of tectonic fault gouge. They are curvilinear and were reactivated into shear zones;
• North Eastern (NE 15-30°) dipping steeply to vertical, predominantly SE (65 to 90°). These are predominant and may be part of a very thick zone of a large regional fault. Steeply dipping (75 to 90°), narrow fractures (0.5 to 25-30cm) control the distribution of ore veins of the deposit. The longest of these veins is 143m, but they generally pinch out between 1-2m and tens of metres. Quartz-arsenopyrite and arsenopyrite veinlets (0.5 to 3cm) develop at the contacts of the quartz veins. The frequency of the veins and veinlets varies from one per 5- 7m to hundreds per metre. The frequency is determined by intersecting fractures. The highest frequency occurs in blocks bounded by submeridional dislocations. At the intersection of the NE veins and veinlets with NW dislocations, clusters of veinlets merge into veins along NW fractures;
• North Eastern (NE 40-60°), dipping 30 to 55°. They are predominant and may be part of a very thick zone of a large regional fault. The thickness of isolated schistosity zones is up to 5- 10m. Rectilinear surfaces, considerable extension and do not enclose productive assemblages;
• East (E 80-100°) various dips predominantly to the north (45°). Uncommon; and
• North Western (NW 290-310°), gently dipping to the northeast at 10 to 15°. They are up to 30cm wide and are related to the ore phase.
Style of Mineralisation
The mineralisation at Vasilkovsky gold deposit is spatially associated with a stockwork of hydrothermal quartzarsenopyrite veins. The main mineralised structures comprise gold-quartz-dominated veins locally merged on stockwork/breccia zones.
Vasilkovskoye represents a stockwork located at the junction of the gabbro/gabbrodiorite and the granite/granodiorite rocks, recognising that the ore control was the intersection of faulting parallel to the NW trending Dongulagashsky fault with the NE trending Vasilkovskoye fault. The mineralisation forms a flattened zone, pinching out at depth.