The Bolivar Mine is an historic underground copper-zinc skarn deposit owned by Sierra Metals Inc. and located in the Piedras Verdes District of Chihuahua State, Mexico.
In January 2009, Dia Bras Exploration Inc. (Sierra Metals after December 2012) decided to construct a new mill at the mine. The mill commenced operations in October 2011. Bolivar has been producing commercially since 2012.
From 1,000 tonnes per day (tpd), the capacity of the mill increased 100% to 2,000tpd as announced in September 2013.
Two technical reports were filed by Sierra Metals in April 2017 and July 2018 followed by an updated Preliminary Economic Assessment (PEA) of the mine filed with an effective date of 31 December 2019 and a reporting date of 21 September 2021. The updated PEA is based on a PEA with a reporting date of 19 October 2020.
According to the updated PEA, Bolivar has a life of mine operating period of 14 years, a life of mine copper payable production of 551 million pounds, and a life of mine and sustaining capital cost of $345m.
As of 30 September 2023, the project produced 26,392 copper equivalent pounds and had an all-in-sustaining cost of $3.24 per copper equivalent pound sold.
Bolivar Mine Location and Site Details
The Bolivar Mine is located in the municipality of Urique and the mountainous terrain of the Sierra Madre Occidental in Mexico with elevation levels varying from 600m to 1,200m above sea level.
The mine is situated approximately 400km southwest of Chihuahua City and approximately 1,250km northwest of Mexico City.
The Copper Canyon Mine is situated 50km to the northeast of the Bolivar Mine. El Fuerte River lies 18km to the south of the Bolivar and Piedras Verdes Village is located 5km to the north of the project.
The Bolivar Mine is surrounded to the northwest by the Cieneguita Town at 12.5km.
The mine can be accessed by travelling along a 325km paved road and 70 to 80km unpaved road to the Piedras Verdes or Cieneguita villages.
The transportation from the villages to the site is through private and company vehicles.
The site area has a semi-arid climate and a mean annual temperature of 25oC with an average annual precipitation of 758mm.
The mine consists of 14 mining concessions covering an area of 6,800 hectares. The mine is a contiguous part of the Bolivar Property land package of 15,217 hectares in the Urique Municipality.
Bolivar Mine Exploration History
Since the Spanish colonial period, the Sierra Madre belt of Northwestern Mexico has been historically mined, prospected, and explored for polymetallic copper-zinc-lead-silver-gold deposits. The belt has been subjected to small-scale mining since Spanish colonial days.
Minera Frisco explored the Piedras Verdes District for porphyry copper deposits between 1968 and 1970. Minera Frisco conducted mapping, sampling, and drilling in the district.
Around 3,00,000 tonnes of mineralised material were mined between 1980 and 2000. At that time, the Bolivar Mine was controlled by the Bencomo Family. The Detailed production records are not available.
From December 2003 to the present, Sierra Metals explored the Bolivar property with regional geological mapping which included detailed mapping, geochemistry sampling, topographic surveying, and diamond drilling. A total of 1,414 diamond drill holes were drilled on 2,74,321m of land. The information has been provided by the exploration department of Sierra Metals as of 30 September 2019.
Bolivar Geology and Mineralisation
The Bolivar deposit contains copper, zinc, and many precious and base metal deposits of the Sierre Madre Belt. The belt trends north-northwest across Chihuahua, Durango, and Sonora states of northwestern Mexico.
The Bolivar deposit lies within the Guerrero composite terrane and this terrane constitutes the bulk of western Mexico.
Proposed to have been formed in the Late Cretaceous, the terrane consists of submarine and lesser subaerial volcanic and sedimentary sequences. The sequences range from Upper Jurassic to Middle Upper Cretaceous in age and rest on deformed and partially metamorphosed early Mesozoic Oceanic sequences.
The Piedras Verdes District consists of Cretaceous andesitic to basaltic flows and tuffs and contains greywacke, limestone, and shale beds. The copper-zinc skarn mineralisation lies in carbonate rocks and shows strong stratigraphic control.
Two stratigraphic horizons, an upper calcic horizon and a lower dolomitic horizon, contain the bulk of mineralisation. The upper calcic horizon is rich in zinc and the lower dolomitic horizon is rich in copper.
Mineral Resource Estimate
As per the updated PEA, the total indicated mineral resource estimate is 19.4Mt containing 149,116t copper at a grading of 0.77%, 9.4Moz silver at a grading of 15.1g/t, 127.8koz gold at a grading of 0.21g/t, and iron at a grading of 13.8%.
The total inferred mineral resource estimate of the mine is 21.4Mt containing 167,077t copper at a grading of 0.78%, 9.8Moz silver at a grading od 14.2g/t, 145.6koz gold at a grading of 0.21g/t, and iron at a grading of 13.5%.
The total (indicated and inferred) mineral resources of the mine are 40.8Mt.
As of August 2023, the total (indicated and inferred) mineral resource estimate of the Dulce Zone is 5.17Mt containing 4,717t copper at a grading of 0.91%, 7,034 copper equivalent (CuEq) and 15.5Mlb CuEq at a grading of 1.360%, 9.5Koz gold at a grading of 0.57g/t, and 412Koz silver at a grading of 24.8g/t.
Mining Methods and Processing of Ore
Currently, the Bolivar Mine is being mined by underground room and pillar mining methods. Before this, the minerals were sometimes mined using the long hole stope mining method in the areas where the mineralised zones have a steeper dip angle.
Currently, the mineralised production is from the El Gallo Inferior Zone, Chimenea 1 and 2 Zone, and the Bolivar West Zone.
The development waste rock is stored underground in historic openings. The mineralised material is brought to the surface by adits or declines and then dumped onto small storage pads lying outside the portals.
From the storage pads, the mineralised material is loaded into rigid-frame trucks and received at the Piedras Verdes Mill.
The recovery of minerals is done by Piedras Verdes processing plant operating since 2011. The plant is operated by Sierra Metals.
The mineralised material enters the crushing circuit which crushes the material to a size of less than 3/8 inch. This reduced-size material is transferred to two silos each with a 1,000 tonnes capacity and one new silo with a capacity of 2,000 tonnes.
The grinding circuit receives the small-sized material from the crushing circuit. The grinding circuit includes conventional ball mills operating in a closed circuit with hydrocyclones.
To improve plant stability, the hydrocyclones were changed from D26 to D20. The material size in the hydro cyclones ranges between 34% and 48% passing 75 micrometres and an average size of 43.5% passing 75 micrometres.
The overflow from hydrocyclones enters the flotation circuit. For further size reduction, the underflow stream enters back to the ball mills.
The flotation circuit includes three identical parallel flotation lines each with a 12*12-foot (ft) flash flotation tank and three DR 300ft3 rougher scavenger flotation cells.
The flotation circuit produces a final copper concentrate and the tails from the rougher scavenger cells become the final tails of the processing plant.
The concentrate undergoes thickening in thickeners of 40*10ft and 50*10ft respectively. The solids get thickened to greater than 40-45% solids by weight in the thickener underflow. They are filtered by a Clever Filter Press with 30 plates 1,200*1,200mm.
The filtrate from the filter is sent back to the thickener for recycling. The thickener overflow is recycled back into the process.
The resulting filtered copper concentrate has approximately 9%-10% moisture. The final flotation tails are sent to the tailing’s storage facility and the process water is reclaimed from the tailings water pond for reuse in the process plant.
A magnetite concentrator recovery circuit receives the tailings stream from the copper flotation circuit to reclaim old tailings.
The concentrate undergoes cleaning in the recovery circuit and the final magnetite concentrate gets thickened and filtered by a vacuum drum filter. The filtered product is stored on-site for transport off-site by trucks.
The infrastructure of the Bolivar Mine includes well developed access roads, camp facilities, service roads, operations and support facilities, a new ore delivery tunnel, process support facilities, and water supply facilities.
The potable water is supplied by the Piedras Verde community from local springs via pipes at a rate of 40,000-50,000 litres per day.
The supply water for the process plant is supplied from Piedras Verdes dam owned by Dia Bras. Propane is used to heat water and other heating processes on the site.
The other facilities include communications, security, logistics, waste handling and management facilities, an existing tailings storage facility, and a backfill plant.
Power Supply and Transmission
The mine receives power via a 33kV high voltage power line from Comisión Federal de la Electricidad (CFE), the state-owned utility.
The mine has a substation which feeds the mine and the process plant via a secondary distribution line. The connected load is approximately 4MW on the site and the system operates at 2MW typical load.
The backup power is supplied by a diesel-powered generator set of capacity 2,000kVA located at the location of the process plant.
SRK Consulting (Canada) prepared the updated PEA of the Bolivar Mine.
Sierra Metals contracted Flopac Ingenieria to conduct geotechnical evaluation, design, costing, and construction of the tailings storage facility expansion program allowing for more production of ore than reserves stated in the PEA.
The Pre-Feasibility Study (PFS) of the mine was prepared by Gustavson Associates, of May 2013.
Quantec Geoscience of Toronto, Ontario conducted major geophysical survey of the mine in August 2010.
Quantec International Project Services Ltd. Conducted Induced Polarisation (IP) geophysical surveys on the property in May 2010.