The Las Chispas processing plant will utilise a Merril-Crowe recovery circuit. Image courtesy of Shelia Sund.
The Las Chispas underground silver-gold mining project is located in the state of Sonora, Mexico. Image courtesy of Daniel Mennerich.
The Las Chispas property is located approximately 220km away from Hermosillo within the Arizpe Mining District of Sonora, Mexico. Image courtesy of Pmoroni.

The Las Chispas is an underground silver-gold mining project by  SilverCrest Metals in the state of Sonora, Mexico. SilverCrest Metals will develop and operate the mine through its wholly-owned Mexican subsidiary Compañia Minera La Llamarada.

Two-thirds of surface rights for the mine area is owned by Silver Crest while a 20-year lease agreement was signed with the local Ejido population for land access and exploration activities on the remaining one third of the surface rights.

A preliminary economic assessment (PEA) of the project with an estimated mine life of 8.5 years was published in February 2019 while a feasibility study is scheduled for completion in the first quarter of 2021.

SilverCrest secured approximately £88m ($120m) financing for the project in January 2021. It also announced the award of the engineering, procurement, and construction (EPC) contract for the processing plant of the project in the same month.

The construction works are expected to be started in the first quarter of 2021 with the start of operations expected in the second quarter of 2022.

Project location and geology

The Las Chispas silver-gold mining property is located approximately 220km away from Hermosillo within the Arizpe Mining District in the State of Sonora, Mexico. The property comprises of 28 mineral concessions covering a total area of approximately 1,400ha.

The Las Chispas ore body is located on the magmatic arc volcanism of the Farallon Plate beneath the North American Plate. The host rocks are predominantly pyroclastic, tuffs, and rhyolitic flows of the Lower Volcanic Complex. The underlying volcanic pyroclastic units include rhyolite, welded rhyodacite tuff, lapilli (lithic) tuff, and the volcanic agglomerate that form a gentle syncline and anticline complex across the project area.

Mineralisation and reserves

The deposits at the Las Chispas property occurs as a deeply emplaced, low-to-intermediate-sulphidation system, and mineralisation occurs in hydrothermal veins, stockwork, and breccia. The mineralisation encompasses a central quartz-calcite mineralised zone with narrow veinlets within the adjacent fault damage zone. The structurally controlled hydrothermal conduits host the stockwork and breccia zones.

The Las Chispas property was estimated to hold approximately one million tonne (Mt) of indicated resources grading  6.98 g/t of gold and 711g/t of silver, and approximately 3.46Mt of inferred resources grading 3.42g/t of gold and 343g/t of silver as of February 2019. The stockpile inferred resources were estimated to be 174,500 tonnes grading 1.38g/t gold and 119g/t silver.

Mining methods

The Las Chispas underground mine is envisaged to use a combination of mechanised cut and fill and resue mining methods for the ore extraction.

The vertical space between the stopes will be 15m with the base of the stope to be accessed via a pivot ramp. The base of the stope will be breasted out to the stope limits with subsequent cuts made using drilling and blasting uppers in 2m lifts.

Mineral processing

The Las Chispas processing plant will be a conventional process plant using the gravity concentration, cyanidation, and the Merrill Crowne processes, operating at a nominal throughput rate of 1,250 tonnes per day (tpd).

The run-of-the-mine (ROM) ore will undergo three stages of closed circuit crushing which will feed the grinding circuit equipped with hydro-cyclones. The grinding circuit will feature two centrifugal concentrators in parallel configuration to recover coarse gold and silver from the cyclone underflow. The gravity concentrate will undergo intensive cyanide leaching and electrowinning processes to recover cyanide dissolved gold and silver.

The cyclone overflow will be thickened and leached in a conventional cyanide leaching circuit and the leach residue will undergo washing in a four-stage counter current decantation (CCD) circuit to produce a gold and silver bearing pregnant solution.

A Merril-Crowe recovery circuit will be used to recover precipitates of gold and silver from the pregnant leach solution is planned to be smelted on-site along with the electrowinning sludge from the intensive leach circuit to produce gold-silver doré bars.

Infrastructure facilities

The Las Chispas property can be accessed via an existing 10km access road from the paved dual-lane, all-weather, Highway 89.

The power requirement for the project will be supplied from three of the four 1.2 MW diesel generator sets with one of them acting as a backup generator. The construction of a power transmission line to draw power from the national grid is also envisaged in the future to reduce the generator load.

The make-up water is expected to be pumped from a well in the nearby valley via a 10km-long, short-diameter pipeline.

Contractors involved

Ausenco, which is working on the feasibility study for the project, was engaged as the EPC contractor for the process plant in January 2021. The value of the EPC contract is estimated to be approximately £56m ($76.5m).

The independent preliminary economic assessment for the Las Chispas project was completed by Tetra Tech Canada in February 2019.