Summary:
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.