The vein-hosted mineral deposits within the San Dimas Mine district are considered to be examples of silver- and gold-bearing epithermal quartz veins that formed in a low-sulphidation setting. Epithermal veins are typically localized along structures but may also form in permeable lithologies. Upward-flaring ore zones centred on structurally controlled hydrothermal conduits are typical. Large to small veins and stockworks are common. Vein systems can be laterally extensive, but the associated ore shoots have relatively restricted vertical extent. High-grade ores are commonly form within dilational faults zones near flexures and fault splays. Textures typical of low-sulphidation quartz vein deposits include open-space filling, symmetrical and other layering, crustification, comb structure, colloform banding and complex brecciation.

Within the San Dimas district, the mineralization is typical of epithermal vein structures with banded and drusy textures. Epithermal-style veins occupy east–west-trending fractures, except in the southern part of the Tayoltita Block where they strike mainly northeast, and in the Santa Rita area where they strike north–northwest.

The silver- and gold-rich quartz veins formed in two different phases. The east–west striking veins developed first, followed by a second system of north–northeast-striking veins. Veins pinch and swell and commonly exhibit bifurcation, horse-tailing, and sigmoidal structures. The veins have been followed underground from a few metres in strike-length to more than 1,500 m. One of these veins, the Jessica Vein, extends for more than 1,000 m in the Central Block.

Three major stages of mineralization have been recognized in the district: (1) early stage; (2) ore forming stage; and (3) late-stage quartz. The minerals characteristic of the ore-forming stage consist of white, to light grey, medium to coarse grained crystalline quartz with intergrowths of base metal sulphides (sphalerite, chalcopyrite and galena) as well as pyrite, argentite, polybasite, stromeyerite, native silver and electrum. The veins are formed by filling previous fractures and typical textures observed include crustification, comb structure, colloform banding and brecciation.

The San Dimas mine includes five main underground gold and silver mining areas: West Block (San Antonio mine), Sinaloa Graben Block (Graben Block), Central Block, Tayoltita Block, and the Arana Hanging-wall Block (Santa Rita mine). In 2020, 68% of ROM production came from the Central Block, 29% from the Sinaloa Graben and 3% from the Tayoltita Block and other areas.

Access to the mining areas is achieved by adits and internal ramps. The Central Block and Sinaloa Graben rely solely on truck haulage, whereas Tayoltita ROM material is transported to the surface stockpile via rail. Main accesses are typically driven at 5 m wide by 5 m high, with accesses to the stopes at 3 m wide by 3 m high. Typical rail haulageway dimensions are 3.5 m wide by 3.5 m high.

Internal ramps connect stopes from both the hanging wall and foot wall, and often, when two or more veins are in close proximity, single ramps can provide access to multiple veins.

The predominant mining methods at the San Dimas mine are mechanized cut-and-fill and longhole mining. Longhole mining was introduced in 2012 and is becoming increasingly important.

Cut-and-fill mining is carried out using jumbo or jackleg drills and load-haul-dump (LHD) machines. Minimum mining widths of 2.5 m and 0.8 m for jumbo and jackleg mining, respectively, may be attainable. Waste rock is used as fill material and provides both wall support and a working base from which to take subsequent cuts after the initial sill cut.

Typically, an initial 3.5 m high sill cut is taken followed by a second 3.0 m cut. Waste rock is then used to fill the void to about 1.0 m from the back, so as to form the working floor for the next cut. The next 3.0 m cut is then breasted down on top of the fill. When this mineralized material is mucked out, filling occurs again to within about 1.0 m of the back. The process is repeated until within about 4.0 m of the next sill cut. Sills beneath waste fill are mined using uppers. The general mining recovery factor is about 95%, and that for sill mining is about 75%.

Longhole mining consists of drilling production holes in the pillar between two mineralized drifts. A minimum mining width of 1.2 m is envisaged for the method. A drop raise or an inverse raise is drilled and blasted at the extremity of the mining block. The length of the block is determined relative to the geotechnical condition of the exposed walls. Stopes can be mined either with upholes or downholes, with respective maximum heights of 12 m and 15 m. The longhole mining method offers increased productivity, lower unit operating costs, and reduced waste dilution in veins of consistent geometry.

Twelve-metre long upholes are drilled from the lower drift. In this example, the blast holes are stopped 4 m from breakthrough into the upper drift to maintain a sill pillar. Where possible, holes are drilled along the contact of the vein, and typical overbreak on the hanging-wall and footwall is approximately 0.3 m. San Dimas has regularly mined veins less than 1 m in width with success using this method.

Ore from the three mine areas, Central Block, West Block, and Sinaloa Graben, is hauled to the mill site via three main tunnels know as the San Luis, San Fernando, and San Francisco tunnels. The mine is setup with a series of ore passes that connect the stopes areas to the haulage levels for the material to flow and to be loaded and hauled to surface and the plant stockpiles. From there, the ROM material is hauled to the mill site and dumped in the stockpile patios or directly into the crusher. The ROM material from the Tayoltita mine sector is transported by rail to the mill.

Ore is hauled from the underground mine to the surface by means of 14 m3 conventional trucks. Most truck haulage at San Dimas is carried out by contractors, however First Majestic has 12 trucks
used for material transfers inside the mine.

The waste material from the main underground infrastructure development is used as backfill for longhole and cut-and-fill stopes. The rock size distribution is adequate to stabilize the cavities and no cement is needed. The back-filling process is carried out with the same equipment used for mucking and hauling.

Crushing
The crushing plant is divided into two parts: primary crushing and secondary crushing. The coarseore bin has a lower discharge chute that discharges into a vibrating feeder. The -12” + 4” material is fed into a 20” x 36” primary jaw crusher and reduced to a minus 3” to 3-½”. This product is transported by a belt conveyor to a 450 t capacity secondary bin, which subsequently feeds an 8’ x 16’ double-deck vibrating screen.

The upper section has 1” x 1” openings, while the lower section has ½” x ½” openings. The underflow of the screen contains material from 90–95% minus 5/16” (8 mm), as well as an average of 70% minus ¼” (6.4 mm).

The upper discharge of the vibrating screen flows into one of two 7’ Symons short head secondary crushers that reduces the size to minus 5/16”. There are two secondary crushers, one working and the other on standby. The secondary crusher product becomes the circulating load which returns to the screen feed.

The lower discharge of the vibrating screen (underflow) is transported through a belt conveyor and a pipe conveyor and unloaded into two ore bins (number 3 & 4), which have a capacity of 1,300 t each.

The fine mineralized material is considered 90–95% minus 5/16” with average moisture content of 3–4%. The overall crushing plant capacity is 145 t/hr.

Grinding
The grinding section consists of three ball mills circuit. The three mills have the same dimensions: 12’ diameter x 14’ long equipped with 1,500 HP drives and D-20” gMax Krebs cyclones. Every ball mill is equipped with a cyclone classification system and a pair of pumps (one in operation the second on stand-by). The mills only use 3” diameter balls.

The crushed ore from bin #3 feeds ball mill #1, while bin #2 feeds ball mills #2 and #3.

The reagents added in the mills are as follows:
• Cyanide: grinding is carried out by adding semi-pregnant solution, which has a cyanide concentration of 1,000 ppm;
• Lime: added only in ball mills #1 and #2 in dry form. Lime consumption is 0.7 kg/t;
• Litharge (lead oxide) added in all three ball mills (35 g/t).

The average pulp densities that are handled at each point of the circuits are as follows:
• Ball mill discharge 1.85 kg/L (75% solids);
• Cyclone underflow 1.90 kg/L (77% solids);
• Cyclone overflow 1.20 kg/L (27% solids).
The final ground product is approximately 70% minus 200 mesh, equivalent to a P80 of 90 µm. The product of the three grinding circuits is pumped to the two primary thickeners (hi-capacity) with dimensions of 55’ x 12’ and 48’ x 12’.

The processing plant at the San Dimas has been successfully operating for several years and continuously achieved high levels of recoveries for silver and gold. The process is based on cyanide tank leaching and Merrill-Crowe precipitation of ground plant-feed to produce silver–gold doré bars. The installed plant capacity is for 3,000 tpd. However, the current throughput levels are around 2,000 tpd. The average feed contains head grades in the order of 300 g/t Ag and 3.6 g/t Au.

The plant consists of the following operating units:
• Crushing – a two-stage crushing circuit with a primary jaw crusher followed by two cone crushers, one in operation and one in standby, in closed circuit with dry vibrating screens;
• Grinding – three ball mills, each paired with hydrocyclones in closed circuit working in parallel;
• Cyanide leaching – plant-feed is leached with cyanide in a series of agitated tanks;
• Counter current decantation (CCD) – two CCD thickeners worki ........