Mineralization consists of polymetallic, high-temperature, intrusion-related carbonate-replacement and minor skarn-hosted deposits. Mineralization occurs as tabular veins, mantos, massive lenses, breccia pipes, and irregular replacement zones.

The deposits were grouped into four geological zones: the Prieta complex, the San Javier–Milagros complex, the Vein systems, and Tailings Deposit No. 4.

Discordant, near-vertical deposits with irregular elongate shapes proximal to main intrusions are referred to as chimneys and breccia pipes. These include the San Javier, Milagros and Prieta breccia deposits.

Tabular sub-vertical replacement deposits are referred to as veins and can contain richer mineral shoots or small chimneys at the intersection of northwest-trending faults and fractures. Steeply dipping, tabular deposits of the Vein systems have a northeast orientation, and are commonly distal to main intrusions.

Massive lens replacement zones of the Prieta complex are proximal to a granodiorite source intrusion and formed adjacent to skarn alteration. Contact metamorphic features (recrystallization to marble, development of hornfels and skarnoid) normally occur peripheral to the skarn zone.

Mineralization consists of secondary oxide minerals including silver, iron, zinc, lead, copper oxides and native silver. Native silver and oxide minerals also occur with sulphides in skarn and carbonate replacementzones where sulphides are partially converted to oxide minerals. The sulphide minerals acanthite, pyrite, magnetite, marmatite (iron-rich sphalerite), galena, chalcopyrite, and covellite occur in the Prieta and the San Javier–Milagros complexes.

The Tailings Deposit No. 4 consists of cyanidation circuit paste tailings from previously processed ore that has been stacked on the surface close to cyanidation Plant No. 2.

The silver mineral deposits at La Encantada are high-temperature polymetallic replacement deposits hosted in sedimentary carbonate rocks related to felsic intrusions and controlled by local and regional structures. Carbonate replacement deposits are characterized by irregular shaped pods, lenses and massive lens, and roughly tabular masses of oxides. Some replacement deposits are associated with skarn alteration and mineralization also hosted by the sedimentary carbonate rocks.

La Encantada Silver Mine included two main underground silver mines: the La Encantada and the El Plomo mines. Mining activities are conducted by both First Majestic and contractor personnel.

The deposits vary in dip, thickness, and geotechnical conditions along strike and dip. Multiple mining methods are required to achieve the maximum efficient extraction of mineralization. Three well established methods were selected:
• Inclined and sublevel caving;
• Long hole stoping;
• Cut-and-fill.

Inclined and sub-level caving is well suited for the bulk tonnage deposits at La Encantada such as the San Javier–Milagros breccia and the Ojuelas deposit. Longhole stoping is being used for near vertical structures that are relatively consistent along strike and length and have competent wall rock. The minimum planned mining width is 1.4 m, based on a minimum vein width of 1.0 m plus an allowance for 0.2 m on the hangingwall and footwall. The stopes are drilled using a basic two-hole pattern when the width is <1.5 m, increasing the number of drill holes per section as the vein increases in width. Cut-and-fill is performed using jackleg drilling and is used for vein deposits that are irregular in nature and commonly possess poorer geotechnical conditions. The minimum mining width is 1.3 m, based on a minimum vein width of 1.0 m and an additional 0.15 m was added to both the hangingwall and footwall as planned dilution using the stope optimizer.

Ground conditions throughout most of the La Encantada mine are considered good. In contrast, the breccia and massive lens-type deposits form weak, soft material that lends itself to caving mining methods. All development by caving into the mineralized breccia deposits is supported with a primary coat of 2” fibre shotcrete with bolt and mesh followed by a secondary 2” coat of shotcrete to prevent unravelling of the weak rock matrix. The vein deposits possess fair rock quality and are hosted in competent limestone. Waste pillars will be left where necessary to increase stability in longhole stoping.

All working areas are above the water table which is at 1,424 masl. The main water inflow of comes from surface filtration during the rainy season. Mine water is pumped from the lowest elevation of 1,509 masl to surface. Ventilation for the Prieta complex is primarily supplied through the Esperanza ramp and the Maria Isabel shaft (113 kcfm) and is exhausted through the 660 vent raise (Robbins) via the 1600 tunnel to the old Milagros workings. For the La Encantada mine, fresh air enters through the old Plomo workings and the Guadalupe audit and is exhausted through the Spendrup and Escalera vent raises.

The process plant is divided into two areas: Plant No. 1 and Plant No. 2. Plant No. 1 consists of the crushing and grinding circuits, while Plant No. 2 comprises the leaching circuit.

The installed plant capacity of the processing plant is 3,000 tpd for the crushing/grinding area.

Plant No. 1:
- Crushing: three-stage crushing circuit consisting of a primary jaw crusher, followed by two parallel vibrating screens and a secondary crusher and a closedcircuit tertiary crusher and two parallel vibrating screens;
- Grinding: one ball mill in closed circuit with hydrocyclones.

Crushing
The oversize material of the grizzly, -12” to +4”, is fed into a 24” x 36” primary jaw crusher and reduced to -3” to -3½”. This crushed product is combined with the grizzly undersize and transported by a 30” wide belt conveyor to the two primary vibrating screens.

These screens are single deck and the sieve aperture is 0.375” x 0.375”. The undersize of the screens contains material with 80 to 90% passing ¼”.

The oversize of the vibrating screens flows into a CH430 Sandvik secondary crusher which reduces the size to -1”. Product from the secondary crusher discharges on a 30” wide conveyor.

The lower discharge of the vibrating screen is conveyed and discharged into the fine-ore bin, constructed of steel plates with storage capacity of 3,000 t. The fine-ore material contains 80 to 90% passing ¼” with average moisture of 3–4%.

The crushing plant has a capacity of 3,000 tpd and operates for 18 hours per day.

Grinding
The grinding section consists of one 12’ D x 24’ effective grinding length Metso ball mill, equipped with an 1,800 HP motor with variable frequency drive. The grinding circuit includes a bench of D-26” Krebs cyclone classification system and two 10” x 8” 250 HP pumps, one in operation and one in standby.

The fine ore is discharged through three chutes into a 36” wide conveyor which is equipped with a Ramsey cell that is used to record the mill feed tonnage.

The ball mill media are prepared with three different sizes of balls: 2-½”, 2” and 1-½”.

The average solids percentages that are handled at each point of the circuit are: mill discharge 78%, coarse ore cyclone 81%, and fine ore cyclone 35%. The final ground size is a product with approximately 75% passing 200 mesh, equivalent to a P80 of 90 µm. The grinding circuit product is pumped to Plant No. 2, using a 8” x 6” 200 HP pump, and fed into the primary thickener.

The nominal mill capacity is 3,000 tpd.

The processing plant at La Encantada has been operating for several years and has continuously improved silver metallurgical recoveries. The process is based on cyanide tank leaching and MerrillCrowe of ground ROM ore to produce silver doré bars. The installed plant capacity is for 3,000 tpd for the crushing and grinding area, and 4,500 tpd for the leaching circuit. Throughput levels averaged 2,500 tpd in 2019 and 2020. In the same period, the average plant-feed contained head grades of 154 g/t Ag.

The process plant is divided into two areas: Plant No. 1 and Plant No. 2. Plant No. 1 consists of the crushing and grinding circuits, while Plant No. 2 comprises the leaching circuit.

Leaching circuit comprises:
o Cyanide leaching: 17 agitated tanks;
o Counter-current decantation (CCD) system: Four CCD thickeners working in series;
o Merrill-Crowe, precipitate handling and smelting;
o Tailings management: three press-filters and tailings handling con ........