Today, the UBC Mining Grad Research trip toured the Morococha mine, owned by Pan American Silver. The group woke up bright and early, headed off at 6:00 am, made it to the mine by 8:00 am, missed the turnoff, and ended up at the Toromocho mine. The bus promptly turned around and after a bit of searching, made it to its proper destination. It turns out it is very easy to end up at the wrong mine when there are 3 operations immediately next to one another. The group was beginning to understand the extent of the mineralization in this region of the Andes.
The neighbouring Toromocho Mine
The group was warmly greeted by Compania Minera Argenta S.A., the subsidiary that operates the mine. The group was brought into the lunch hall and an excellent breakfast was served. Safety precautions for the visit were discussed over breakfast.
Next, the group was given a presentation by the heads of the ventilation and geology departments on the geology and mining methods of Morococha. Morococha is a polymetallic deposit consisting of silver, copper, lead, and zinc and is located 142 km from Lima. The mine consists of three major zones and has an overall geology very similar to other mines in the region. The mine has approximately 6.4 million tonnes of reserves. The average head grades of the reserve are 159 g/t of silver, 0.45% copper, 1.29% lead, and 3.62% zinc. The deposit is extracted underground by sub-level stoping, with backfilling of the mined-out stopes. The backfill material used depends on the stope size. Smaller stopes are backfilled with rockfill, while cemented fill is required for larger stopes. Significant effort goes into preventing overbreak in drilling and blasting operations. Each hole is carefully surveyed to ensure that they are properly collared. Each hole is planned in detail, including hole length, diameter, linear metres of charge, explosive type, burden, spacing of the holes, and more. Additionally, relief holes are drilled to control overbreak and fracturing. Peewee instrument control is used to measure hole deviation; normal deviation is within 2 - 3%. The Geological Strength Index (GSI) is used to characterize geotechnical parameters, which are used for ground support design. Within most of the mine, rock bolts and mesh screens are used as the primary method of ground support; however, shotcrete is used within the stopes. Ventsim software is used for ventilation design. The mine has an excellent ventilation system which actually supplies more air than is needed for operations. The mine uses modern mobile equipment, including the CAT R1600 Scooptram, the Raptor 44-XP stoper, the Simba S7D drill, and Mercedes 25 tonne haul trucks.
Following the presentation, a health assessment was performed by the on-site medic. Based on previous medical assessments, several higher risk individuals within the group were identified. Their vitals were evaluated and each of them received special instructions to ensure their safety and well-being. The assessments reinforced the company’s commitment to health and safety for both mine employees, as well as visitors.
The team at Morochocha was committed to ensuring the health and safety of all guests
Personal protective equipment was distributed to every member of the group including hard-hats, cap lamps, safety glasses, hearing protection, respirators, gloves, coveralls, miners’ belts, and steel-toed muckers. Water bottles were also distributed. The group was loaded onto the underground transport bus, with some of the higher risk individuals joining the medic in their vehicle. The group then headed to the portal to begin descent into the mine.
The first stop on the tour was the underground maintenance shop. Each of the three zones of the mine has its own shop. Each shop is capable of maintaining all mobile equipment within their respective zone.
An R1600 scooptram being serviced in the underground shop
The second stop was 500 m below surface. This level of the mine is significant as it at the phreatic surface of the mine. All the water use for the mine operations is drawn from this level. One drainage drift within this level is over 100 years old.
The group then proceeded to a production level of the mine. First, the group approached a mined-out stope (DE 80-100). The vein being mined at that area was only about 40 cm wide; due to this the stope was also very narrow. The grades of this stope were 38 g/t of silver, 0.45% copper, 0.31% lead, and 4.48% zinc. Fifty millimeters of shotcrete was applied to the walls of the stope and rock bolts were applied at the foot of the drift to control overbreak. Some of the remaining casing of the drill holes for the stope could be seen. The casing of the blast holes were painted yellow, while the casing of the relief holes, which control overbreak, were painted red. The guide explained that typical stope dilution is within 8-9%; however, without the aforementioned controls, dilution could reach as high as 20%.
A mined out narrow stope at Morochocha
Next the group proceeded along the same level to where a new stope was being drilled. The drill was in the process of drilling a 12 m hole at a 50 degree angle. The hole was planned to be blasted with ANFO with a powder factor of 0.78 kg/t. The guide explained that both ANFO and dynamite are used on site for blasting operations. Dynamite is primarily used; however, ANFO is used for harder rock. For every 25 meters of stoping, a 3-5 m pillar is left behind to support the excavation. The width of the pillar depends on the competency of the rock. These pillars are not recovered later in the mine plan.
The group walked along to a new drift, which had not yet been stoped. Along the way, the group passed several information boards describing general safety precautions; mine-area information; and gas measurements, including, O2, CO, CO2, and NO2. These signs are posted in mine workings.
Information boards are posted at all mine workings
The new drift had not yet been shotcreted at the end; therefore, mineralized veins were still visible. Within the drift, three veins were visible. The guides explained that it is important to ensure that the drifts are developed such that the veins stay within the centre of the back, so that they will stay within the stope that will eventually be recovered. Fragments of ore could be found along the floor of the drift. Chalcopyrite, sphalerite, galena, and argentite mineralization could be readily identified within the fragments.
The group then returned to surface for lunch. Again, the company was very hospitable and accommodating.
Following lunch, the group toured the shotcrete and cemented fill plant. The plant has a capacity of 50 m3/h. Fine aggregate is supplied to the plant from local businesses, while coarse aggregate is taken from waste rock extracted during the mining operations. For shotcrete, the wet mix design includes aggregate, cement, water, stabilisers, superplasticiser, and steel fibres. Accelerator is added at the mobile sprayer. The plant is fairly unique in its design. Aggregate is loaded into a hopper by a loader. A conveyor belt brings the aggregate to a weigh scale. The weigh scale then loads another conveyor which brings the aggregate to the mixer, where cement and additives are also added. The plant operates in a batch process. When a transmixer is ready to be filled, it positions itself next to the mixer. The mixer is then raised with pistons, allowing it to tip the wet mix into the transmixer. A rigorous quality control program is followed by the plant. Moisture contents are measured from the aggregate pile twice a week. Sieve analysis is conducted on aggregate samples three times per day. Point load tests are conducted 4 times per day on cured samples to determine their hardness. Uniaxial compressive strength tests are also conducted for 3-day, 7-day, and 28-day breaks. Samples for UCS breaks are taken both from cast cylinders and cored from cured panels. Extensive quality control is required for shotcrete, as drift development proceeds only 4 hours after shotcrete has been applied.
The shotcrete and cemented fill plant at Morococha
Following the tour of the plant, the group returned their PPE and said their goodbyes to their hosts. As a thank you, the guides were presented with UBC Mining apparel. The group then proceeded to Lima to rest before beginning their next adventure.