The Las Chispas mine is an underground silver and gold mining development in Sonora, Mexico. It is fully owned by SilverCrest Metals through its Mexican subsidiary, Compañía Minera La Llamarada.
A preliminary economic assessment (PEA) of the Las Chispas project was published in February 2019 while the environmental impact statement was approved by the Secretaria de Medio Ambiente y Recuros Naturales (SEMARNAT) in July 2019.
A feasibility study for the silver and gold mine was published in February 2021 with an estimated life of mine of 8.5 years. Las Chispas is one of the highest-grade primary silver mining projects in the world. SilverCrest secured a $120m financing facility from RK Mine Finance for the project in January 2021.
Las Chispas mine location and geology
The Las Chispas mine is located approximately 180km north-east of the city of Hermosillo within the Arizpe Mining District in north-western Mexico. The mining property encompasses 28 mineral concessions over approximately 1,400ha.
The mine is situated within the subduction of the Farallon Plate beneath the North American Plate. The host rocks of the Las Chispas property are dominated by pyroclastic, tuffs, and rhyolitic flows, which are members of the Lower Volcanic Complex forming a gentle syncline and anticline complex across the project. Local volcanic pyroclastic units include rhyolite, welded rhyodacite tuff, lapilli (lithic) tuff, and volcanic agglomerate.
Mineralisation and reserves at Las Chispas mine
Mineralisation at the Las Chispas mine is classified as gold and silver occurring in low to intermediate sulphidation epithermal systems. The majority of the mineralisation is hosted in hydrothermal veins, stockwork, and breccia. The mineralisation is 0.10m-10m in true width, encompassing a central quartz ± calcite mineralised section with narrow veinlets within the adjacent fault damage zone.
The proven and probable mineral reserves at the Las Chispas mine were estimated at 3.35 million tonnes (Mt) grading 4.81 grams/tonne (g/t) of gold (Au) and 461g/t of silver (Ag), as of January 2021.
Mining methods at Las Chispas
The Las Chispas mine will use long-hole stoping and cut-and-fill mining methods via several access drifts and ramps. Long-hole longitudinal retreat stoping and Avoca methods will be included as part of the long-hole stope mining methods in mining areas where vein thicknesses are greater than 1.5m.
The variations of cut-and-fill methods involving cut-and-fill with uppers will be used in areas where vein thickness is greater than 1.5m. Cut-and-fill with breasting will be used in areas with adverse ground conditions, and vein thickness of more than 1.5m while resue mining will be used areas with vein thickness of less than 1.5 m.
Three access portals will be constructed, namely the Santa Rosa, Babicanora Central, and San Gotardo portals. Mining fleet will include 1.2t, 3t, 3.5t and 10t load–haul–dump (LHD) vehicles. A 30t diesel truck fleet will transfer ore to a surface transfer pad. Ore from the pad will then be transferred to the process plant by surface mining trucks or via loader.
The mine will support a nominal throughput rate of 1,250tpd. The run of mine (ROM) ore will undergo crushing in a primary jaw crusher and a semi-autogenous (SAG) mill operating in closed circuit with a cyclone cluster. The cyclone underflow is returned to the SAG mill while the overflow is transferred to a trash screen.
The cyclone overflow will feed the bulk rougher flotation circuit where the bulk of the gold and silver will be recovered from the concentrate. The flotation concentrate will undergo leaching in a high concentration cyanide solution to dissolve gold and silver and produce a concentrate slurry.
The slurry will be thickened and the thickener overflow, a pregnant solution, will be transferred to a Merrill Crowe circuit. Zinc powder will be added to the pregnant solution to form a precipitate of gold and silver. The precipitate will be filtered and mercury will be removed by treating it in a retort furnace. The treated concentrate will then be smelted to produce the final gold and silver doré bars.
The Las Chispas project area is accessible from the community of Arizpe, located 12km away, and Banamichi, located 25km away, both by the paved Highway 89 from where approximately 10km-long, secondary gravel roads lead to the site.
Power required for the operational phase of the mine is expected to be sourced from the national grid power via an overhead power line. The power supply provision is currently in the permitting process, with construction anticipated in 2021.
Contractors involved in Las Chispas project
The feasibility study for the Las Chispas mining project was prepared by Ausenco Engineering Canada. Ausenco was also awarded the engineering, procurement and construction (EPC) contract for the construction of the processing plant.
Tetra Tech Canada was engaged to prepare the preliminary economic assessment for the project.