The Irokindinskoye deposit is comprised of quartz veins occurring in three main fracture systems which are flat dipping (25° to 45°) to the west. The veins are typical of other quartz vein deposits, with the veins pinching and swelling in both the strike and dip directions and in which, more often than not, the gold distribution is very irregular and the economic mineralization is located in shoots of varying strike lengths.

According to the recent classification of the gold deposits (Robert, F. et al, 2007), the Irokindiskoye deposit can be classified as an intrusion related, low sulphidation epithermal gold deposit. The characteristics for this deposit model are: high Au:Ag ratio and low base metal content. High grade low sulphidation gold deposits are structurally controlled and usually they are hosted within volcanic units or their basement. The hydrothermal wallrock alterations show lateral and vertical zoning and include silicification, sericitization, pyritization and propylitic alteration.

The Zun-Holba deposit is located within a northwest trending belt of metamorphosed and tightly folded volcanic and sedimentary rocks of Proterozoic age, which contain several broadly conformable quartz-sulphide veins. Six en echelon subvertical veins contain most of the gold within a 1.3 km long mineralized zone. The width of the veins varies from 80 cm to 15 m.

According to the recent classifications of gold deposits, the Zun-Holba mine can be classified as an orogenic quartz-carbonate vein gold deposit (Robert, et al, 2007).

The gold deposits on the Zun-Holba Property can also be classified as intrusion related, low sulphidation epithermal gold deposits. The distinguishing characteristics for this deposit model are high Au:Ag ratios and a low base metal content. High-grade low sulphidation gold deposits are structurally controlled and usually they are hosted within volcanic units or their basement rocks. The hydrothermal wall rock alterations show lateral and vertical zoning and include silicification, sericitization, pyritization and propylitic alteration.

According to the Russian classification system for mineral deposits, the Zun-Holba and Irokindinskoye deposits corresponds to a vein deposit of the third category of complexity. The third category of complexity or “Group 3 Deposits” are defined in the latest Russian “reserve” classification guidelines dated December 11, 2006 as:

“Group 3. Deposits (subsoil areas) of complex geological structure with large and average size orebodies, having strongly dislocated bedding, characterized by very variable thickness and inner structure, sometimes immature quality of the minerals, and very uneven distribution of the basic valuable components. The peculiarities of the structure of the deposits (subsoil areas) are determined by the possibility of development of the “reserves” of categories C1 and C2 in the process of exploration”.

IROKINDA MINE:

Due to the large separations between the veins, which may reach several km, and the mountainous relief, the development of individual veins is typically conducted through one or more isolated adits. In some cases, however, a single adit provides access to more than one vein.

Development of the No. 30, No. 35 and Laguernaya veins has been incorporated into a single layout; comprised of an adit and several mine levels. In this case, the mining development included the sinking of two blind shafts to a depth of 150 m.

Since 1996, the room-and-pillar mining method has been used, with the room locations following the dip of the vein. This method is best suited for mining veins which have an extended strike length and are located in stable rock. The mining is done by extracting chambers 8 m wide, using jacklegs and stopers to drill short blast holes. Delivery of the broken material from the block to the sublevel (sill) drift is accomplished by both slushers and the initial explosive force. The mined-out space is supported by temporary pillars and, if necessary, by props.

Recently (2008) tests have been conducted on the use of a mining method that does not require leaving pillars, with faces advancing on the retreat. This system includes driving a broad face along the strike of the vein and benching along the slope or dip of the vein, with the mined-out area being left behind the current mining face. This method is best suited to areas in which the rock stability is good and to veins of any dip angle with widths up to 3 m. Extraction is accomplished by making cuts of 25 to 30 m width along the strike of the vein, with the extraction of the mineralization accomplished by taking benches of 8 to 10 m in width. The space at the face is supported using hydraulic props with screen (polymer networks) installed on the props nearest the face, in order to isolate the mined-out area for final vacuum-cleaning. After vacuum-cleaning, the mined-out area is closed off from the current mining operations and the hydraulic props are dismantled. The minimum mining width accepted for this type of stoping is 1.1 m. Although this method is being used, it is not as widely applied as the room-and-pillar method.

All of the mining methods used at Irokinda are labour intensive. Jacklegs and stopers are used to drill the blast holes. Extraction of the broken material is accomplished primarily by using slushers to move the material into the ore and waste passes. From the ore and waste passes, the broken material is loaded into rail cars either from a chute or by mucking machine (shrinkage stoping).

ZUN-HOLBA MINE:

Due to both the geotechnical factors and the geological features of the mineralized bodies within the deposit, the following mining methods are used at Zun-Holba:
- Shrinkage stoping with short blast holes (up to 6% of production).
- Timbered stoping method (up to 3% of production).
- Cut-and-fill method (up to 42% of production).
- Shrinkage stopping with backfill (up to 50%% of production)

The primary mining methods are shrinkage stoping and conventional cut-and-fill, with up to 92% of the present mining being conducted using these methods. Two different variations of the cut-and fill-method are used at Zun Holba:
- Bottom-up (overhand) cut-and-fill method.
- Top-down (underhand) cut-and-fill method.

All mining methods are labour intensive, with the use of both jacklegs and stopers to drill the blast holes. Extraction of the broken material is accomplished primarily by using slushers to move the material into the ore and waste passes. From the ore and waste passes, the broken material is loaded into rail cars using either a chute (for the cut-and-fill and timbered stoping methods) or a mucking machine (shrinkage stoping).

By the time Buryatzoloto became involved with the Zun-Holba mine, in 1991, the upper portion of the deposit had already been developed with the adits spaced 40 m to 50 m apart vertically. This development included: adits # 1 and #2 (the adit entrances at an elevation of 2,256 metre above sea level (m)), adits # 3 and # 4 (2,213.8 m), adit # 4 bis (2,173 m), adit # 5 (2,134.4 ml), adit # 5 bis (2,083.4 m), adit # 11 (2,032.2 m), adit # 8 (1,820 m) and adit # 12 (1,740 m).

In the upper part of the deposit Buryatzoloto developed adits # 14 (adit entrance located at the 1,982.3 m), # 15 (1,935.0 m) and # 16 (1,883 m). Also, the 1790 m sublevel was developed between adits # 8 and # 12. “Reserves” of the deposit located below the adit # 12 level at a depth of 350 m are exposed by two vertical blind shafts. These shafts are the Glavny (Main) shaft developed from the 1790 m level and the Vspomogatelny (Secondary) shaft developed from the 1840 m level. Using a 50 m distance between active levels at the mine, the “reserves” below adit # 12 are accessed by five shaft levels, and two further shaft levels (1440 m and 1390 m) are in the process of being developed. The # 12 adit is the primary tramming level for both the upper and the lower (shaft) levels. The ore of the upper levels is transported via the ore passes to the adit # 12 level and then out of the mine. The ore from the lower levels is hoisted in the Glavny shaft to the 1790 m level where it is reloaded via the ore passes to the # 12 adit level and then transported out of the mine. The Vspomogatelny shaft is used to transport the ore and waste rock from the lower levels to the # 8 adit level, from which the waste rock is transported out of the mine and dumped to the waste rock stockpile. The ore is dumped through an ore pass to the # 12 adit level and then transported out of the mine, where it is dumped onto the ore stockpile.

The Irokinda and Zun-Holba mines are both relatively mature underground operations. Both contain a processing plants, each with crushing, grinding, gravity and flotation circuits. The Zun-Holba plant also contains a CIL circuit and related facilities. For processing, there are two crushing stages and two grinding stages using ball mills, followed by gravity separation, which produces concentrates for both the gravity and flotation circuits. The gravity circuit is used to recover free gold, after which smelting produces doré bars. Flotation is used to recover finer gold particles into a flotation concentrate.

Ore is sourced from underground operations using conventional mining methods involving drilling and blasting, followed by load and haul to the run of mine (ROM) stockpiles where it is either directly fed to the primary crusher or stockpiled. The ore is crushed to a P80 15 mm using a two or three-stage crushing circuit with screening. The crushed material is then conveyed to a grinding circuit consisting of deferent size ball mills.

Both Irokinda and Zun-Holba mines use a gravity circuit to recover free gold before ore is milled finer for floatation. Primary concentration takes place in a centrifugal concentrator followed by cleaning of concentrates using several stages of shaking tables. This allows the plant to produce a gravity concentrate ready for direct smelting. Gravity gold recovery account for up to 20% at Zun-Holba and 60% at Irokinda.

Flotation is a method of separating minerals depending on their ability to attach to air bubbles. The flotation process consists of producing a mineral concentrate through the use of chemical conditioning agents followed by intense agitation and air-sparging of the agitated ore slurry to produce a mineral rich foam concentrate. At Zun-Holba and Irokinda after grinding and gravity recovery circuit all the ore goes to flotation. The flotation concentrates from the Irokinda is shipped to the CIP plant at the Zun-Holba mine for further treatment by cyanidation and sorption.

At Zun-Holba mine flotation concentrate is carried out in a carbon in pulp (CIP) circuit. During the CIP or CIL stage, pulp flows through several tanks where sodium cyanide and oxygen have been added to dissolve gold into solution. In the absorption stage, this pulp flows through several tanks containing activated carbon. Gold then attaches onto the activated carbon, which is moved counter current to the pulp, while screens separate the barren pulp from the gold-loaded carbon.

The gold loaded carbon is removed and washed before undergoing desorption. Desorption is achieved by introducing the gold loaded carbon to a solution of sodium cyanide at a high temperature (>110°C) and pressure. This results in the reversal of the attachment process, with the gold detaching from the carbon back into solution. This produces a small volume of solution with a high gold concentration.

The gold loaded eluate solution that emerges from the desorption process is passed through electrowinning cells, where gold is deposited onto the cells cathodes.

The cathodes then undergo a smelting process, further refining the gold and leading to the eventual production of doré bars for transport to a refinery. Dore bar is an alloy of gold, silver, copper and other metals containing between 50%-90% gold.