Epithermal Gold Systems
Epithermal gold deposits are a type of lode deposit that contain economic concentrations of gold, silver and in some cases base metals including copper, lead and zinc. Gold is the principal commodity of epithermal deposits, and can be found as native gold, or alloyed with silver. As a lode deposit, epithermal deposits are characterized as having minerals either disseminated through the ore-body, or contained in a network of veins. Beyond epithermal deposits, other classic examples of lode deposits include Greenstone Belts, VMS and Intrusive Related Gold. Epithermal deposits are distinctive from low-grade bulk tonnage deposits such as porphyries in that they are typically high-grade, small size deposits.
A few characteristics distinguish epithermal deposits. These deposits are found near the surface and mineralization occurs at a maximum depth of 1 km, but rarely deeper than 600 m. Due to their shallow depth, it can also be noted that epithermal gold deposits form under moderate crustal temperatures of 50-300oC, and under medium pressure. These deposits commonly occur in island arcs and continental arcs associated with subduction. However, they can also be found in shallow marine environments and associated with hot springs. Due to their shallow-depth location, epithermal gold deposits are more susceptible to erosion; accordingly, these deposits represent a high-grade, easily mineable source of gold.
The Baley deposit in Southern Siberia reprepsents a bonanza epithermal vein system with well developed sinter within Cretaceous conglomerates, sandstone siltstone sequences. Historical mine of major proportions having produced 450-650t Au at average grade of +15g/t.
Three major veins developed both in granite (main Baley pit) and metasediments (Taseevskoye pit) developed on discrete fracture systems. 60% of the gold produced came from the Taseevskoye pit.
Mineralisation is well developed in the Cretaceous conglomerates with associated stockwork vein system with christmas tree-like geometry as mineralising fluids spread laterally along porous and permeable sandstone horizons
Mineralisation still present at +50g/t over 700m deep. Major porphyry body developed at around 450m vertical at contact between the granites ad the metasediments. Significant stockwork mineralisation is developed beneath the porphyry.
Current resource estimates suggested up to 160t of gold at +3 g/t remain but considered marginal economics
Significant potential for additional satellite resources as exploration was curtailed post discovery of Baley.