This paper discusses the main causes for unplanned ore dilution in longhole mining and possible mitigation measures. It reports on the results of a case study of an underground mine that uses the longitudinal retreat method for the extraction of a narrow gold-silver orebody. The study is conducted with finite difference software FLAC3D. The modelling technique tracks and fills the cavities created by progressive overbreak with cemented rockfill in bottom-up sequence. In this work, a strategic measure of ore dilution control is explored by incorporating a sill pillar in the mine plan. Comparison of two mine plans - with and without sill pillar - reveals the benefits of sill pillar on unplanned ore dilution control, especially in the three mining levels immediately above the sill pillar. The findings of this work could be equally applicable to similar mining systems with one or more sill pillars in each mine plan.
Longhole mining is a popular method for the extraction of steeply dipping ore deposits. In this method, the orebody is divided into sublevels and each sublevel is further divided into stopes. The method, also known as sublevel stoping, is widely practiced in Canadian mines due to its inherent safety and efficiency. Narrow and tabular orebodies are usually mined with longitudinal retreat along the orebody strike whereas wider deposits are extracted with transverse stoping along the orebody thickness in a primary-secondary stope sequence (Henning & Mitri 2007). Stopes are drilled and blasted from an overcut, and blasted ore is mucked from an undercut access. Production drilling patterns may vary depending on the thickness of the orebody. For narrow veins, a ring of two or three holes is adopted whereas in tabular orebodies, a fan pattern is employed. Drilling rings are repeated along the stope strike with a specified burden. Backfilling is used in different forms depending on the selected type. The most common types of backfill are paste-fill, cemented rockfill, sandfill, and uncemented rockfill. The latter is simply waste rock. The choice of backfill material and system depends on the selected mining method, stope sequencing, availability of raw materials, and capital and operating cost of the backfill system. More information about backfill practices in sublevel stoping systems is found elsewhere (Emad et al. 2015). Despite their popularity, longhole mining methods are notorious for unplanned ore dilution, i.e., the failure or overbreak of the stope walls into the stope cavity during production blasting. This is due to the nature of the method which creates non-entry stopes making it difficult to support the stope walls, unlike methods like cut-and-fill where the stope walls can be directly accessed and reinforced from the cut. Unplanned ore dilution can have a significant negative impact on the economics of a mining operation as the overbreak material is either of low economic value (below the cut-off grade) or even waste rock. Therefore, the subject of ore dilution in longhole mining has attracted the attention of many researchers over the last 30 years. Past research has identified the main factors influencing the occurrence of overbreak in longhole stopes. These are discussed below.
