The Piskanja deposit is of continental lacustrine type, typical of many global boron deposits, and is considered to have formed within a closed basin with abnormally high salinity. The boron mineralization is most likely to have been sourced from local volcanic rocks, from which it was leached by hydrothermal fluids. Boron minerals were then deposited in sedimentary successions in lacustrine conditions through the processes of evaporation and chemical precipitation. The presence of laminated dolomitic rocks and claystone in association with borate mineralization indicates sedimentation in the deeper parts of a lake.

Most borate minerals are highly soluble in water which restricts the areas in which they form, and more importantly, are preserved. The majority of known global borate deposits have formed in lacustrine or playa lake environments in closed basins that opened up in active extensional setting near subductive plate boundaries. Rock types associated with the deposits generally include calc-alkaline extrusive rocks, tuff, limestone, marl, claystone, gypsum, continental silts and sands. The source of boron is not always the same and can be derived variously from leached marine sediments, magmatic fluids from subducted crust or from volcanic material (tuff).

The boron deposits in the USA and Turkey (which together account for around 80% of world production), are associated with continental sediments and show a continuum between hydrothermal spring, playa lake and lake deposits. Borate minerals precipitate once they become saturated in the fluids circulating these basins, either through evaporation of the basinal waters or addition of borate rich fluids from hydrothermal springs and circulating meteoric waters. Different borate minerals form at different levels of acidity; for example, borax (sodium borate) precipitates at a higher pH than ulexite, and in comparison, colemanite forms at a lower pH and in warmer fluids. Due to cycles of basin refill and sediment input, there may be numerous layers of borate mineralization interbedded with barren sedimentary horizons.

The Piskanja mineralization is likely to have been deposited in a restricted inter-montane basin occupied by a perennial saline lake. Boron-rich fluids in these environments usually emanate from geothermal springs with a volcanic input (Garrett 1998).

The main mineralization is concordant with stratigraphy and Erin’s staff have noted that the borate mineralization correlates laterally with carbonate horizons, consistent with a syn-depositional or syn-diagenetic origin anticipated for evaporitic deposits.

The mineralized horizons show a range of textures comprising different growths of the borate minerals colemanite, ulexite and howlite predominantly.

Massive mineralization which appears within buff, laminated carbonate rocks. Occasional muddy irregular laminations and inclusions occur within the borates which appear to represent displaced soft-sediment (pre-lithification). These zones are interpreted to have developed at or just below the lake bed.

Minor veinlets parallel to the stratigraphy. These have mineral fibres oriented steeply, indicating the veins opened vertically, consistent with growth at low overburden pressures (very shallow depths). The veins have sub-angular tips, suggesting they developed when the sediments were only partially lithified (semi-coherent).

Breccias hosted by siltstones and claystones, with textures ranging from clast-supported jigsaw breccias through to matrix-supported chaotic breccias with a fine clast size. These clast-size variations appear vertically stratified; tentatively suggesting variations in sediments (porosity and cohesion) influenced the style of deformation. Overall, these breccias are interpreted to represent over-pressuring by hydrothermal fluids in the shallow-subsurface.

Both massive and vein borates contain two types of vuggy hollows:
• Open vugs with ingrowing crystals of borate, which represent holes present during the growth or remobilisation of borate minerals;
• Vugs which appear to have undergone some mineral dissolution, which are commonly stained by minor hydrocarbons. These are interpreted as minor permeability networks where aggressive (acidic) fluids associated with the maturation of hydrocarbons have accentuated existing porosity. The hydrocarbons are likely to be locally sourced in organic rich units in the sediments.

Overall, there is very little textural evidence preserved for an active tectonic component controlling mineralization, with most textures consistent with a syngenetic to diagenetic origin for the mineralization.

The thicker accumulations of borate mineralization appear to occur within a broadly NW-SE orientated corridor some 200 m to 250 m wide. This suggests a potential geological control on the deposition, such as faster subsidence due to faulting or the presence of a hydrothermal vent sourcing the B-bearing hydrothermal fluids, both of which point to the presence of one or more faults.

The geometry and depth of the mineralization identified at Piskanja lends itself to an underground mining method. This section of the report presents the results of work completed to date to determine how the mineralization will be most appropriately worked and extracted. While more detailed analysis is required and will be undertaken in due course it is the assumptions presented here that form the basis of the PEA presented later in this report.

It is envisaged that mining will be by cut and fill method and that the key underground infrastructure will comprise:

• Twin access declines from surface to the deposit: i) main haulage decline (further addressed as MHD) from surface to the floor of KZONE1 and ii) main ventilation decline (further addressed
as MVD) from surface to the roof of KZONE3;
• An underground spiral ramp connecting MHD and MVD and enabling access to all levels;
• A shaft connecting MHD and MVD to serve as an ore pass and temporary (if needed) ore stock;
• Footwall drives located below seam horizons of KZONE1, KZONE2 and KZONE3; and
• Level drives and ventilation connections between three footwall drives.

Ventilation
For the purposes of this PEA, it is envisioned that the mining operation will not utilize blasting, that all equipment will be electric (powerline or battery) and that maximum number of workers per shift in the underground facilities will not exceed 50. Having this in mind required air intake is calculated upon the number of workers as 6 m3/min/worker. This results in a minimum air intake of 300 m3/min.

The experience in Serbian coal mines, with similar annual production, and operating in methane conditions, with drill and blast operations, show that air intake does not exceed 33 m3/s.

On the other hand, a continuous miner will produce significant amounts of dust so dust emission and required air quantity for dust removal need to be considered. With two continuous miners in operation, with dust suppressors installed, the required air amount should not exceed 50 m3/s. Dust emission from continuous miner operations and the required amount of air for dust removal needs to be assessed in detail in the future studies.

On the basis that the decline is the main air intake to the mine, with planned cross-sectional area of 14.6 m2, maximum air intake into the mine should not exceed 90 m3/s to comply with legislated maximum air speed of 6 m/s in the travel way.

Backfill
It has been assumed that 85% of the mined void would be filled. As discussed, backfill design and materials selection will be the subject of future investigation into the availability and characterization of materials from which a plan to provide backfill before the mining operation can be developed.

To minimize airgaps left after backfilling, which could result in potentially damaging subsidence, the backfill should be pumped in placed. This would require hydraulic transport of the backfill material as a slurry. There is a potential to transport the dry backfill material underground and mix it into slurry on site and then use a concrete pump to build it into place which would require backfill transport routes to be free of equipment, leaving the only possible route to be within the main ventilation decline.

Having analysed the former, the Author determines the best option is for backfill delivery to working area to be by a pipeline. The pipeline could be placed inside the ventilation decline but a more viable option is to drill a hole from the surface and construct an underground delivery pipeline. The backfill mix could then be prepared at the surface, hauled to the delivery drill hole, lowered underground and distributed through the mine.

The system can be used to deliver either wet or dry prepared backfill mixture. For the purposes of the PEA, a delivery of dry backfill mix has been conceptualized.

The cement slurry system would be containerized, the arrangement including hopper, mixing tank and slurry pump, and would be located underground near to the area being backfilled. The equipment would be electrically powered and skid mounted to enable relocation in the mine.

Materials handling
Materials handling is considered to be by conveyor direct to surface and a surface storage facility adjacent to the mine portal. This will be located in the mine haulage decline and will run parallel with other mine traffic. A materials handling study will be required to identify the most appropriate way to deliver mined material from stopes to surface.

Decline construction
As the only ventilation intake, a 14 m2 decline heading will be required. This size of development will also help accommodate the materials handling conveyor use to take excavated material from the mine.

The geotechnical conditions in which the decline will be constructed need investigation and a ground control plan created. However, it is expected that weak, bedded siltstone/mudstone will be the predominant host rocks. As a main travel way, the heading will need to be supported along its length; and in areas of poor and very poor ground conditions that might be expected around structural features additional ground support will be required.

A decline approximately 1,300 m long is required to access the orebody. For the purposes of this cost estimate, 60% of the decline length is classified Fair, 10% Good, and the remaining 30% as Poor. Since Piskanja shares similar geology and geologic structures with the nearby underground mine, Pobrde, the experience from that mine was utilized to assess which type of ground control might be employed at Piskanja. The drifts and declines in Pobrde are supported in steel so the same support is planned for Piskanja. The distance between steel frames and the properties of steel support will be a matter of future analyses.

In addition, 15% of decline length for development is included to account for pumping, materials handling, and ventilation requirements.

Mining equipment
Two continuous miners with a pair of shuttle cars or haulers each are envisaged as being required for mine production. A productivity study will be required once mine layouts are completed for the mine design to determine the fleet make. However, although it is considered that two continuous miners are likely to be heavily under-utilized, two machines are likely to be required to ensure there are sufficient working places available to maintain production and to mitigate against the risk of production stoppages due to unavailability of equipment.

Additional testing needs to be done to determine the optimal method for upgrading the Piskanja mineralization to what is considered to represent a minimum marketable concentrate grade. However, the process of B2O3 ore beneficiation does exist and is widely used in Turkish boron mines.

According to Burat (2008), concentration of colemanite ores in Turkey is carried out by disintegration, washing and classification in the size fractions. In large sizes, colemanite concentrate is obtained through attrition tumbling and hand sorting. While in fine sizes (–6 mm), attrition scrubbing and classification in the size fractions are carried out. At Emet Colemanite Concentration plant at a capacity of 600,000 tpa a colemanite ore of 27% B2O3 content is treated to produce a concentrate having 43% B2O3 at 300,000 tpa.

It was assumed that a process can be utilized to upgrade the mineralization to satisfactory levels of B2O3, to the following criteria:

• A colemanite ........