Following introduction of long hole stoping during the June quarter at Sumatra Copper & Gold’s Belinau underground gold-silver mine in Indonesia to accelerate ore production, significant unplanned dilution has been experienced above Level 4 in an area of poor ground conditions.
This has resulted in undercutting of the cemented rock fill on Levels 2 and 3 breaching to the surface and a localised collapse into a stope on Level 4. The area was being monitored and the safety of the underground crew was not compromised at any time by the incident and no lost time injuries were recorded.
The Belinau mine forms part of the Tembang Gold-Silver Project in South Sumatra.
Sumatra says the remaining void above Level 4 will be backfilled with competent rock and is not expected to impact on any long-term infrastructure as the decline, escape ways and return air rises are all located in the hanging wall, away from the ore body.
An interim ore extraction method by ore drives between Levels 4 and 5 will be implemented to attain a competent sill pillar through this zone above Level 5. This will provide long-term access to the extremities of the ore body on Level 5 and allow the decline to advance so that a more appropriate stoping method to suit the geotechnical conditions in this area can resume.
The Level 5 ore drive was expected to be established by early August, with ore development continuing as planned along the Level 4 West drive.
The modification will result in a slightly lower ore tonnage at a higher grade. Increased ore production from the open pits is anticipated due to favourable weather conditions during the current dry season making up for any tonnage shortfall from underground.
Sumatra’s managing director David Fowler said, “We have recently appointed Rob Gregory as chief operating officer and also added two mining engineers with significant underground experience to enhance our planning, design and implementation capability.”
An updated production and cost guidance is expected to be provided later this month following a revision of the mine schedule.
During the June quarter the Tembang mine produced 6387 ounces of gold and 91,012 ounces of silver at an all-in sustaining cost of US$1317 per ounce. This compared to 5406 ounces of gold and 121,569 ounces of silver in the previous quarter.