Pillar Recovery at Excellon Resource’s Platosa Mine

2019

Samuel Freitag, Excellon Resources Inc. (TSX:EXN; OTC:EXLLF); Denis Flood, Excellon Resources; Francisco Rodriguez, Excellon Resources

La Platosa Mine is a silver-lead-zinc carbonate replacement (CRD) deposit located in Durango, Mexico, 60 kms NW of Torreon. The orebody geometry is highly variable and occurs as interconnected mantos emplaced in carbonates. Since starting production in 2005, these horizontal mantos have been mined using a modified room and pillar method. The ore was accessed from above and benched down to the bottom of the manto, while delineating 4-meter diameter pillars with a maximum spacing of 8 meters and a height range between 4 and 15 meters. In many areas of the mine, valuable ore has been left behind in pillars. Some areas are currently backfilled with unconsolidated waste material while other areas are accessible, supported by only the pillars. Platosa recognized the opportunity to re-enter these areas and recover ore left in pillars. Historical production and grade sampling data was reviewed to determine the location of economic pillars, and stress modeling in Map3D was conducted to determine the feasibility of removing pillars from previously mined areas. Cable bolts are installed around the pillars to provide support after the pillar is removed, with extensometers to monitor movement in the roof and provide warning of any convergence. The convergence includes some expected relaxation of the ground and any additional movement due to separation along joints, faults, or bedding planes. Backfilling the areas with unconsolidated waste material was initiated in order to provide confinement and afford access to the roof for additional support installation. Pillar recovery is ongoing at Platosa, beginning in one manto, containing ten pillars. Two pillars were successfully recovered in 2018, with the six more planned for 2019. Additional pillar recovery in other previously mined mantos is being assessed for subsequent years.

Keywords: geotechnical, rock mechanics, pillar recovery, room and pillar, monitoring, modeling, mexico, silver, lead, zinc, underground