Porphyry Project Descriptions
Due to rain-induced construction delays, full mining operations could not commence in Q4 of 2016 as planned, and are expected to ramp-up to full production levels by March 2017. PT BSI remains confident of producing an estimated ore mining rate of approximately 4 Mtpa on the basis of the 2016 Ore Reserve estimate of 36 Mt, averaging 1.03g/t Au and 22g/t Ag3).
Mine operations started in December 2016 after pre-mining works including clearing and development of the mine access road was completed. Ores will be mined from five open pits over the initial nine-year mine life. This will primarily involve the mining of near surface oxide gold and silver resources, which have been estimated to produce average annual outputs of 100,000 ounces of gold and 300,000 ounces of silver. At these rates, total production over the initial nine year mine life is estimated to reach over 900 thousand ounces of gold and 2.7 million ounces of silver.
Mining will initially start in the three highest grade pits that will be developed using standard drill & blast, excavation and truck haulage methods. The two lower grade pits will subsequently be brought into production as part of the Life of Mine plan. This plan mines a total of 36 Mt of ore that will be trucked to the ore preparation plant where the ore is crushed through a primary jaw crusher and secondary cone crusher to produce a 75mm product. Water and cement are then added to this material to produce agglomerate that is then trucked to the heap leach pad.
Agglomerated ore is stacked by truck dumping onto a series of five leaching bays across the leach pad. Ore is stacked at 10 meters lift height in a sequence of 14 lifts that results in a total leach pad height of 140 meters and total capacity of 36 Mt of stacked ore. Areas are brought under irrigation using very diluted sodium cyanide solution that is sprayed by a piping and sprinkler system onto the ore. As this solution percolates through the leach pad gold and silver is recovered and this solution is then sent to the Adsorption Desorption and Recovery (ADR) processing plant. At the ADR processing plant, gold and silver are recovered using industry standard methods of metal adsorption onto carbon, desorption off carbon and recovery through an electrowinning and refining circuit. Gold and silver are smelted to produce rough or doré bars that are transported offsite for final refining.
All infrastructure works that pertain to environmental safety of the mining operations had been completed as of year-end 2016. Waste material that is mined to access the ore will be contained within a Central Waste Dump next to the open pits. This waste dump will be constructed in three sections with an engineered rock wall at the toe of each section to provide structural stability along with a series of lined spine drains to remove water from the waste dump. All sediment produced within the mining area will be fully contained within a series of sediment sumps that will be pumped back to the environmental control dams.
At the heap leach pad and ADR processing plant all solution is fully contained and is recirculated through the leaching and processing circuits to maintain an overall neutral water balance. All rain water run off from the leach pad is contained within a fully lined storm water pond and this water will be used for irrigation of new areas on the leach pad. When further water needs to be added to the leaching and processing circuit it will be drawn from the raw water storage facilities and permitted water bores at site. When excess water needs to be discharged from the storm water pond it will be processed through a detoxification circuit at the ADR plant to remove any residual traces of cyanide solution and neutralized prior to discharge into the environmental control dams.