The Santa Elena mine project gold and silver deposits form as prominent east–west-trending veins and associated breccias in sub-aerial felsic volcanic rocks. The Santa Elena Main Vein is delineated by drilling along a 1,950 m strike length and 750 m down dip.

Silver and gold mineralization at the Santa Elena mine is hosted in quartz veins and stockworks displaying typical epithermal textures, including banded quartz, vuggy quartz, and brown–black bladed calcite (pseudomorph to quartz) with many of these textures intermixed with hydrothermal breccia.

Other gangue minerals include calcite, adularia, chlorite, and fluorite. Rhodonite has been noted at approximately 530 m vertical depth. Bonanza ore shoots appear to be locally present but have not been delineated in detail. A trend of higher grades and thicker veining is apparent with a plunge of approximately 25° to the east. Up to 200 m of a pyrite and calcite matrix breccia in the hanging wall andesite proximal to the Main Vein has been intersected.

Sulphide abundance is generally low within the veins but can be as much as 5–30%. The sulphides are dominantly pyrite and pyrrhotite with minor galena, sphalerite, and chalcopyrite. Gold occurs typically as native gold, electrum, and silver occurs as electrum, minor acanthite, and rare native silver.

Alteration within the Santa Elena mine deposit is widespread. The volcanic units in the immediate vicinity of the veins exhibit pervasive propylitic to silicic alteration. Widespread argillic alteration and silicification proximal to quartz veining is common. Chloritic alteration increases away from the mineralized zones. The permeable nature of the fractured zones has allowed partial oxidation to occur to depths of 400 m below the surface in selective fractured zones. Limonite within the oxide zone is brick-red in colour and is associated with brown goethite and local yellow jarosite. Manganese occurs locally as pyrolusite and minor psilomelane near the surface. Kaolin and alunite occur primarily along deeply weathered and oxidized structures and along the fractured contacts.

Metal zonation appears to correspond to northwest-trending structures that crosscut the Main Vein forming high-grade gold and silver mineral shoots. Vertical zonation shows gold content consistent with depth and silver content increasing.

The Santa Elena mineral deposits vary in dip, thickness, and geotechnical conditions along strike and dip. Multiple underground mining methods are required to achieve efficient extraction of mineralized material. The mining methods commonly used in the Santa Elena mine are sublevel longhole stoping (Longhole), longhole avoca (Avoca) and Cut-and-Fill. The method assigned to a vein depends on the vein characteristics and rock mass characteristics (for example width, dip, and rock competence, among others). Depending on the selected mining method, the production rate is adjusted to reflect the various productivities.

Longitudinal Longhole
Longitudinal longhole mining is suitable where the dip of the orebody is 45° or greater, and the mineralized material is of sufficient width and grade that the estimated dilution does not eliminate the profitable recovery of the ore. Longitudinal longhole mining consists of an undercut level and an overcut level, each accessed from the main ramp or a transportation drift. Each sill is accessed perpendicular from the ramp, and then developed along strike of the vein to the economic extents of the ore. Once sill development is completed on each level, a longhole rig drills production holes between the sills, which are then blasted in retreating vertical slices until the stoping panel is completed. Stope panel lengths are based on a hydraulic radius calculation considering the geotechnical conditions of the area.

Once the maximum panel length is achieved (up to 40m), stability can be maintained by either the placement of unconsolidated backfill or a rib pillar. Once a sufficient stope length has been extracted and backfilled, mining can progress either up-dip for stoping with fill, or down-dip for stoping with pillars. Stopes are designed between 15–20 m in vertical height (floor of the undercut to the floor of the overcut level) and longitudinal longhole with backfill is the primary method used in the Alejandra Alto, Americas, Tortuga Veins, and Alejandra Bajo, below the 425 Level. Longitudinal longhole with pillars is the primary method used in the Alejandro Bajo, above the 425 Level.

Avoca With Backfill
Avoca mining uses longhole stoping techniques to extract the mineralisation. The method is similar to longitudinal longhole stoping, but Avoca also uses a footwall drift that runs parallel to the strike of the orebody offset by 15-20 m (predominately in waste) to access the mineralisation at regular intervals. Secondary crosscuts are then driven into the sill at 40 m intervals, which gives independent access to each stoping panel and allows filling and extraction to occur at different locations along the strike of the mineralisation. The operation at Santa Elena has demonstrated that in favourable conditions, Avoca mining is a safe, flexible, and cost-effective mining method.

The method has been used for the extraction of the Main Vein, where existing footwall development is excavated. Avoca uses a sublevel spacing of 15–25 m between the overcut and undercut levels and uses 76 mm production blastholes. As the deposit width narrows, Avoca will be replaced by longitudinal stoping with unconsolidated backfill.

Cut-And-Fill
The cut-and-fill mining method is selected for areas where the dip of the orebody is less than 50°, to minimise dilution associated with blasthole deviation and irregular footwall angles, and to minimise mining loss associated with blasted muck not reporting to the extraction drift. Cut-and-fill is mined bottom-up (i.e., up-dip or overhand) with unconsolidated fill placed after the level is completed. Ground support is installed as each round is mined.

The mineralized material is accessed via an attack ramp located in the footwall of the orebody and accesses the orebody perpendicularly. The first lift is driven at minimal grade (elevated only to allow dewatering), then silled out from the hanging wall to the footwall in drifts either 4.0 m high (wide veins) or 2.5 m high (narrow veins). Once the sill is driven to the extents of the orebody, the vein is outlined in the roof of the drift and fired into the open void at either 5.0 m high (wide veins) or 2.5 m high (narrow veins). The mineralized material is then mucked, and the waste is slashed into the open void. Successive lifts are driven on top of the waste pile from the start of the attack ramp, where the brow is then slashed into allowing a ramp to be driven to access the next drift. This process is repeated until the attack ramp is too steep (>15%) to allow for haulage, and then the orebody is accessed from another sublevel.

Where it is not available from underground development, trucks backhaul fill material from waste rock stockpiles on surface, with fill being placed inside the stopes utilizing scoop trams.

The plant is currently fed with the Santa Elena blend consisting in a mixture of fresh underground ore and spent-ore from the heap leach. The ore from the heap leach is fed directly to the primary screen of the crushing circuit, to crush any oversize material (+1/4 inch) prior to feeding the grinding circuit. The underground ROM material delivered from the mine is dumped on an intermediary stockpile and then hauled to feed a coarse ore bin upstream of the crushing circuit. The coarse ore bin is equipped with a vibrating grizzly screen at the top. The grizzly has openings from 1” to 4”, allowing fine material to bypass the primary crusher.

The crushing circuit has a nominal capacity of 240 t/hr and the flowsheet consists of three reduction stages: primary, secondary, and tertiary crushing. The ore from the intermediary stockpile is fed into a coarse ore bin, which is fed to a 20” x 36” primary jaw crusher and reduced to -3” to -4”. This product is transported by a conveyor belt to an 8' x 16’ double deck vibrating screen (scalper).

The over size of the vibrating screen flows into the secondary crusher, an XL400 FLSmidth Raptor Crusher, which reduces the size to ~1”. The discharge from this crusher feeds a closed tertiary circuit, which has two XL400 FLSmidth Raptor crushers and a 10 'x 20' double deck vibrating screen.
The upper deck has ¾” x 2?” openings, while the lower section has ¼” x 1” openings. The undersize of the screen contains material from 90–97% minus ¼” (6,350 µm). The oversize of the vibrating screen flows back into the tertiary-crushing closed circuit. The undersize product from both vibrating screens is transported by a belt conveyor and discharged into the fine ore stockpiles. The particle size in the fine ore stockpiles has approximately 95% minus ¼” and average moisture content of 3–4%. The existing crushing circuit at the Santa Elena plant will be used for both Ermitaño material and the Santa Elena blend.

The crushed ore from respective stockpiles, is fed through a conveyor belt to a primary grinding circuit equipped with a 15’ D x 21-½’ (diameter by length) FLSmidth ball mill operating in closed circuit with an hydrocyclone classification system and a pair of pumps, one operating and one stand- by. Cyanide solution and lime are added to the primary grinding circuit. The final ground product contains approximately 65% -200 mesh particles, equivalent to 106 µm P80. The product from the primary grinding circuit is pumped to the secondary grinding circuit.

The secondary grinding circuit consists of an Outotec High Intensity Grinding Mill (HIG-Mill) with a hydrocyclone classification system, and an agitated buffer tank. The product from the ball mill reports into the agitated buffer tank called rougher feed tank. A first stage of classification is fed from this tank, which produces an overflow of 85% -20 µm separating the slimes. The underflow is re-classified in another cyclopack, that operates in closed circuit reporting the overflow back to the rougher tank. The underflow from the second cyclopack reports to a pumpbox which feeds the secondary grinding HIG-Mill, reducing the particle size to a range of 70-75% -45 µm.

The secondary grinding circuit produces two products: slimes (30% by weight) and HIG-Mill product (70% by weight). Both products go to two different thickener tanks which their underflow slurry reports to the leaching circuit.

Santa Elena has been processing what is known as the Santa Elena blend which consists of high-grade underground mineralized material blended with spent-ore from the existing heap leach pad. The process is based on cyanide tank leaching and MerrillCrowe smelting of fine-ground ore to produce silver–gold doré bars.

The nominal plant capacity is for the processing of 3,000 tpd with the possibility of higher throughput rates depending on ore hardness, targeted final grind, and leaching residence time. Plant throughput averaged 2,500, 2,150 and 2,450 tpd in 2019, 2020 and the first half of 2021, respectively. Plant feed head grades averaged 90 g/t Ag and 1.48 g/t Au from January 2019 to June 2021.

With the introduction of mineralized material from Ermitaño, the plant will continue to process the Santa Elena blend in campaigns alternating with fresh material from Ermitaño. For the achievement of optimum levels of metal recoveries and the corresponding maximum metal p ........