Los Filos is an operating open-pit and underground mining operation located in the state of Guerrero, Mexico. It is operated by Equinox Gold, a gold producer based in Canada.
Open-pit mining at Los Filos commenced in 2005, while underground operations commenced in 2007, with the first gold pour occurring the same year.
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An updated feasibility study for the Los Filos gold mine was released in October 2022 with updated mineral resources and reserves including an expansion for the addition of a new 10,000 tonnes per day (tpd) carbon-in-leach (CIL) processing plant.
The addition of the CIL plant along with the continued development of the Bermejal underground deposit will extend the Los Filos’ mine life to 14.5 years.
The expansion will also increase the average annual production of the mine to up to 280,000 ounces (oz) of gold. The life of mine production is estimated at 3.97 million oz (Moz) of gold with peak production between 2025 and 2030 expected to average at 360,000oz of gold per year.
The new CIL processing plant is expected to commence operations in the third quarter of 2024.
The gold mine’s location
The Los Filos mine complex is located approximately 180km southwest of Mexico City in the Municipality of Eduardo Neri, Guerrero State, Mexico. The Los Filos mine property encompasses 30 exploitation and exploration concessions covering an area of 10,433ha.
The Los Filos mine complex comprises three open-pit areas namely the Los Filos open pit, Bermejal open pit and Guadalupe open pit and three underground mines Los Filos South and Los Filos North, and Bermejal underground mine.
Geology and mineralisation
The Los Filos property sits in the Guerrero Gold Belt, near the centre of the Morelos-Guerrero sedimentary basin, which is dominated by the Cretaceous Morelos Formation overlain by the Mezcala Formation. Tertiary granodiorites intrude the sedimentary units of the basin and are associated with sills ranging from diorite to granodiorite.
The Los Filos deposit mineralisation at linked with early Tertiary granodiorite stocks and an associated sill arranged in carbonate rocks, which then led to the formation of high-temperature calc-silicate and oxide metasomatic skarn assemblages.
Mineralisation at the Bermejal and Guadalupe deposits takes place along the contact of the Bermejal-Guadalupe stock and is linked with carbonate rocks which come from the Morelos Formation.
Reserves at Los Filos
The proven and probable mineral reserves at Los Filos gold mine are estimated at 193.2Mt grading 0.86g/t gold and 6.9g/t silver with a total metal content of 5.3Moz gold and 42.5Moz silver, as of June 2022.
Mining methods
Open-pit mining operations involve conventional drilling and blasting along with loading by excavator and haulage by trucks. The open pits use 9m-high production benches with 18m double benches as safety berms for every second bench. The mining fleet consists of shovels and front-end loaders, supported by 136t trucks.
The standard haul road is two-way with a width of 27.4m and narrowed at the lowest benches of the pit to allow only one-way traffic.
The overhand cut-and-fill mining method is used at the narrow areas of LFUG, while the overhand drift-and-fill method is used for the wider areas. The long-hole open-stoping method is also used in areas with good rock conditions and vertical ore-body continuity.
Underground mining operations at LFUG are accessed by multiple portals outside of the current open-pit operations. The main ramps measure 4.5m x 4.5m to accommodate ten-wheel, 14m3 highway dump trucks. They are located on the hanging wall, 60m to 100m away from the ore.
At the Bermejal underground mine, the overhand cut-and-fill mining method accounts for approximately 91% of the tonnages, while underhand drift-and-fill is used for the remaining stopes.
The underground mine design is based on trackless mobile equipment with twin ramp access. Main access declines connect each ore zone to form a network of ramps. Production levels are driven from the footwall or hanging wall side of the ore body in 20m vertical intervals.
Processing at the gold mine
The run-of-mine (ROM) ore from the open-pit and underground operations undergoes crushing in a primary jaw crusher followed by further crushing in two secondary cone crushers. The crushed ore is treated with cement and lime prior to being transferred to the heap leach pads.
The ore undergoes 120 days of leaching the resultant pregnant leach solution (PLS) is transferred to the adsorption-desorption-recovery (ADR) plant.
The ADR plant consists of seven trains of four carbon columns to adsorb gold from the PLS into loaded carbon. Three carbon-stripping circuits strip gold from the loaded carbon using a hot alkaline-cyanide solution.
The concentrated gold strip solution is passed through four electrowinning (EW) cells where gold is precipitated onto stainless-steel cathodes as sludge, which is removed by high-pressure water. The metal-rich EW sludge is dewatered and passed through a mercury retort and an electric induction furnace to produce gold doré product.
New carbon-in-leach facility details
The proposed 10,000tpd CIL facility will treat ore from the open pit and underground operations at the Los Filos gold mine.
The ROM ore will be crushed in a jaw crusher and fed to a semi-autogenous (SAG) mill and an overflow ball mill. The undersized SAG mill product will feed to a hydrocyclone. The cyclone underflow will be transferred to the gravity concentration circuit consisting of a gravity scalping screen, a gravity concentrator, an intensive cyanidation unit (ICU) and a dedicated EW cell.
Concentrate from the gravity concentrator will be sent to the ICU for intense cyanidation followed by leaching with sodium cyanide, sodium hydroxide and a leach-aid to produce a PLS. The PLS will be pumped to the EW cell to recover gold and silver onto stainless steel cathodes. The gold and silver sludge will be washed and collected in a hopper and will later be filtered and dried before being smelted into doré ingots.
The ground ore from the SAG and ball mill will be thickened to 51% solids in the pre-leach thickener prior to being pumped to the CIL circuit comprising nine mechanically agitated tanks operating in series.
Loaded carbon from the CIL process will be recovered and washed in an ADR plant, which will consist of a 10t acid wash and a 10t elution column, a 1,639kW strip solution heater, two heat exchangers, two EW cells, the gold room and associated tanks and pumps.
Loaded carbon will gravitate to the acid-wash column to undergo acid wash prior to being transferred to the elution column. Gold and silver will be stripped off the loaded carbon in the elution circuit to produce a pregnant eluate, which will then undergo electrowinning to produce a gold metal sludge that will be filtered, dried and smelted in the gold room to produce doré.
The gold mine’s infrastructure details
The Los Filos mine complex is accessible by road, helicopter or fixed-wing charter flight from Mexico City, Toluca or Cuernavaca. The property can be accessed from Mexico City via National Highway 95/95D south to the town of Mezcala, from where an 18km paved road leads to the mine site. The mine also has a 1,200m-long paved private landing strip.
Fresh water is sourced from the Rio Balsas River through multiple inlets to a concrete storage container adjacent to the river. Water is then conveyed to the mine site by four pumping stations via a 15km-long pipeline. The site has three potable-water treatment facilities to produce potable water.
Power for the mining operations at Los Filos is supplied by CFE, Mexico’s electric utility, from the 600MW Caracol hydroelectric station. The power supply is received at 115kV from the Mezcala main substation, located 8km from the site by two 10MWA transformers. A 115kV to 13.8kV substation will be commissioned for the planned CIL process plant.
Contractors involved at Los Filos
The October 2022 updated feasibility study was prepared by Equinox Gold, AMC Mining Consultants (Canada), Lycopodium Minerals Canada (Lycopodium), Paul M. Sterling and Struthers Technical Solutions.
Canadian mining consultants AMC provided the mineral reserve estimates and mine plans and reviewed the geotechnical work for the open-pit and underground deposits, including the ultimate pit designs and waste dumps.
Lycopodium, an engineering and project management consultancy, along with Equinox Gold, AMC and Paul M. Sterling were responsible for the capital expenditure and operational expenditure estimates of the mine.
Lycopodium also performed a trade-off study to compare the benefit of detox versus the installation of a sulphurisation-acidification-recycling-thickening plant.
Struthers Technical Solutions, an engineering services provider for electrical power systems, was responsible for the power supply and electrical infrastructure for the project.
