The Santo Niño ore body at Pinos Altos Mine has been in production since 2009 using both open pit and underground mining methods. Between 2015 and 2017 an extensive study was conducted to evaluate extraction of the crown pillar which remained between the open pit and underground mine workings. The project evaluation considered numerous factors influencing mine safety, operations and economics. This case study paper will describe the geomechanical aspects of the project evaluation and execution.
The Santo Niño ore body is one of several mineralized zones at Agnico-Eagle Mines’ Pinos Altos property. Pinos Altos is located in the state of Chihuahua in northwestern Mexico within the Sierra Madre mountain range (Fig. 1).
Gold and silver production at Pinos Altos started in 2008 with the extraction from the Santo Niño and Oberon de Weber open pits. The Pinos Altos Complex produces over 2Mt per year from the main Santo Niño orebody and nearby satellites. Milling and heap leach are used to treat the underground and open pit ores. Underground mining was initiated simultaneously with the open pit production at the Santo Niño orebody with the first pyramid extending from Level 20 to Level 16 (Fig. 2). The mining strategy consisted of excavating and backfilling the upper most stopes up to Level 16 before the open pit reached its final depth. The recovery of the crown pillar between the pit bottom and Level 16 was not fully detailed in the initial mine planning. Following the technical and economic evaluations in 2015-2016, the crown pillar ounces between the pit bottom (elev. 2023 masl) and the top of the underground mining (elev 2000 masl) were incorporated into the Life of Mine Plan and contribute to the replacement of higher grade ore to the mill following depletion of the large Santo Niño open pit.
Initial thoughts on crown pillar recovery focused on simply mining the open pit in the usual 7 m benches down to the top of backfill on Level 16. Open pit mining is less costly than underground extraction and geotechnical studies focusing on open pit slope stability indicated that deep slope instability was not expected with the removal of the crown pillar. Nonetheless, further analyses demonstrated that there were advantages to recovering the crown pillar from the underground; economic evaluations showed that resource recovery could be increased which made underground recovery more profitable (60% by open pit versus at least 85% from underground), slope deformations could be reduced hence the risk of localized slope instability was decreased and exposure of personnel in the pit ramp could be significantly reduced. A thorough risk analysis and review process was implemented by a team of key personnel from pit and underground operations, mine and corporate technical services, health and safety and mine management. The objectives of the exercise was to develop measures to control identified risks to personnel and machinery while optimizing the recovery of the reserves. This paper documents the geomechanical analyses and monitoring measures implemented to support the design and recovery of the high grade Santo Niño crown pillar.