Deposit Type
The San Agustin Project does not fit entirely into an epithermal classification. The San Agustin deposit appears genetically and spatially related to a quartz monzonite stock with intense phyllic alteration and local tourmaline breccias. These factors may point towards a telescoped system associated with a deeper porphyry centre. This is supported by broad zones of potassic alteration that are overlapped by pervasive phyllic alteration; however, locally on the surface and in some drill holes, boiling textures, suggestive of an epithermal system do occur. Mineralization is mainly associated with pyrite that fills fractures, is disseminated, and occurs in the matrix of hydrothermal breccias. These form an extensive system of sulphide stockworks and disseminated mineralization dominated by pyrite.

San Agustin is interpreted to be a porphyry-style gold system related to Eocene aged intrusions emplaced into Cretaceous clastic and carbonate sedimentary rocks in an extensional tectonic setting. Gold mineralization occurs throughout the magmatic-hydrothermal system in space and time and is spatially related to early potassic development and an overprint of phyllic alteration. Supergene alteration, formed as a product of acid leaching, resulted in argillic-quartz alteration assemblages within the oxide zone of the deposit. The main gold event is associated with magmatic hydrothermal fluids corresponding to phyllic alteration. The gold system was overprinted by a younger structurally controlled epithermal system dominated by silver and zinc. The difference in style of mineralization from the nearby El Castillo deposit is possibly due to San Agustin having undergone less erosion than El Castillo thus preserving a larger volume of this late epithermal overprint. In support of this, late-stage high-level tourmaline breccias are prevalent at San Agustin but are not exposed at El Castillo.

Mineralization
The host rocks for mineralization at San Agustin are quartz monzonite-dacite bodies and the sedimentary sequence they intrude. Mineralization is emplaced through a strong and widespread system of sulphide rich veins, veinlets, and fissure fillings that make the system similar to a disseminated deposit. Fracture systems follow two main project-scale trends that run northeast and northwest. Locally mineralization can be observed following lithological controls in the sedimentary rocks, especially where they run parallel to sediment-intrusive rock contacts. Mineralization is also observed in the flow facies of the intrusion and is usually characterized by disseminated pyrite and in parallel veinlets. A component of the pyrite is thought to be premineral and associated with early phyllic alteration. The mineral system has very little silica and is more related to sulphide fracture filling. Epithermal boiling textures were observed locally such as bladed textures, coliform silica, or drusy quartz. These epithermal textures are not common. Some structures with cryptocrystalline jasperoid have also been found in deeper drill intercepts within sulphide zones. Two late phases of mineralization were identified with one carrying sphalerite and pyrite, and the other, galena and sphalerite.

The Main Fault, an important northwest striking and westerly dipping post-mineral fault, bisects the mineralized area showing differences in mineralization on either side. On the hanging-wall (west side) it is common to find structures rich in manganese and barite that are not observed in the footwall. The hanging-wall block also has higher silver and lead grades than the footwall block.

The sulphide boundary is located within a range of 30 m to 170 m below the surface with an average depth of about 65 m. The boundary is reached when the rock colour turns grey and disseminated pyrite becomes visible. The transition zone is commonly less than 1 m wide. The boundary’s surface is undulating and erratic across the deposit, due to the many faults and fractures controlling ground water in the area.

The predominant alteration type is phyllic alteration characterized as an assemblage of sericitequartz-pyrite mineralization. In some areas it appears that the host rock was pervasively altered, destroying the original texture and converting biotite and feldspars to sericite. The matrix also shows the presence of sericite, silicification, and disseminated pyrite. In some areas veinlets of jarosite and alunite are observed and thought to be products associated with acid leaching of pyrite as opposed to hydrothermal alteration.

A phase of early potassic alteration was observed but is less common. These zones are characterized by the presence of moderate to pervasive secondary biotite associated with veinlets of quartz-magnetite and disseminated magnetite. Phyllic alteration is superimposed on this early potassic alteration with the latter being closely associated with mineralization.

In the areas more distal to mineralization, the intrusion is typically phaneritic with a coarse porphyritic texture with only propylitic alteration shown by moderate chlorite replacement of ferromagnesian minerals.

The San Agustin deposit is roughly 1,500 m long by 800 m wide. The average depth of oxide material is 65-100 m below surface. Gold mineralization is found along faults and fractures within the host igneous and sedimentary rocks and as disseminations in halos across the deposit. Sulphide mineralization extends, where drilled, down to an average depth of about 200 m with the deepest tested areas extending to 400 m below surface.

San Agustin is a contract-operated mine and Owner-operated process facility using conventional equipment and conventional mining methods.

The San Agustin mine is a relatively low-grade gold deposit that benefits from a low strip ratio and disseminated mineralization that is amenable to bulk mining activities and good heap leach recoveries. Situated in a semi-arid environment surrounded by moderate topography, the oxide material that hosts the mineralization is relatively shallow with no major impediments to mining.

There are seven mine phases remaining in the LOM plan and the final pit dimensions are approximately 1.6 km long (east-west) by 1.1 km wide (north-south) and up to 140 m deep. The final pit contains approximately 36.6 Mt of ore and 28.7 Mt of waste for an overall strip ratio of 0.78 (waste:ore) including mining dilution and ore losses applied in the mine schedule. Production is expected to be limited to 11 Mt/year for ore over the course of the mine life. As a result, it is expected that 344 koz of gold will be placed on the heap leach pad from August 2021 through the end of 2024 for a remaining mine life of 3.5 years.

The mining method consists of traditional drill-and-blast operations followed by excavator loading of rigid-body haul trucks (100 t class) for ore transport to the crusher and waste transportation to designated WRSF locations.

Approximately 16.7 Mm3 of waste storage space was defined outside the current and future mine phases which will accommodate approximately 30 Mt of waste.

Pit Design
Pit designs include a ramp width of 25 m (truck factor of 3.5), which can safely support Cat 777 (or equivalent) sized mining trucks. One-way access of 15 m was applied for the six bottom pit benches after stripping requirements were met.

Pit Design Parameters:
Interramp Slope: 45°;
Bench Face Slope: 67°;
Berm Width: 7;
Bench Mining Height: 6;
Bench Height: 12;
Ramp Width: 2 Ways 25;
Ramp Width 1: Way 15;
Ramp Gradient: 10%.

Phase Design
Phase designs were largely driven by the effective mining widths and the influence of the designs on access to the mineralization. The same design parameters used in the final pit design were incorporated into the phase designs.

Waste Rock Storage Facilities
Argonaut designed three main waste rock storage facilities (WRSFs). The South WRSF is located south of the pit limit with approximate dimensions of 700 m x 600 m; the North WRSF is located northeast of the pit and fills a portion of the mined pit with approximate dimensions of 500 m x 300 m; and the West WRSF is located northwest of the pit limit with approximate dimensions of 700 m x 350 m. The maximum lift height design was 100 m. The capacity of the WRSFs is 30 Mt which is sufficient to provide storage for all waste defined in the production schedule.

Crushing and Conveying
The Plant 1 crushing circuit includes a primary jaw crusher followed by secondary crushing with two cone crushers operated in open circuit to produce a final crush size of P80 22 mm. A hopper in front of the primary crusher is fed directly by haul trucks, or by a front-end loader from stockpile. From the dump hopper, ore is fed across a vibrating grizzly with oversize being directed to the jaw crusher and the undersize directed to the primary crushed ore stockpile.

The primary crushed ore is reclaimed from the primary stockpile and conveyed to two doubledeck vibratory screens. Screen oversize is combined and conveyed to a surge bin, which feeds two secondary cone crushers. The secondary crushing circuit operates in open circuit with the crushed product combined with the undersize material from the vibrating screens and directed to the crushed ore stockpile. Plant 1 produces approximately 17,000 t/d of P80 22 mm crushed ore product.

The Plant 2 crushing circuit is nearly identical to the Plant 1 crushing circuit, except the equipment is slightly smaller and the secondary circuit uses one only one vibrating screen and cone crusher rather than two. Plant 2 produces 13,000 t/d of P80 22 mm product.

The two streams of crushed product join at a common crushed product stockpile. Ore from the crushed product stockpile is conveyed 0.5 km to the heap leach pad on two overland conveyors and then onto the heap leach pad with a system of 24 grasshopper field conveyors and stacked at 30,000 t/d in 8 m high lifts with an index conveyor and mobile radial stacker.

The ore is belt-agglomerated with 2 kg/t to 3 kg/t cement (2019 excluded) and 5.6 kg/t lime as required for improved percolation. Since April 2021 an agglomeration polymer (DustTreat) has been used in place of cement at an addition rate of 0.05 kg/t.

The San Agustin Project is an open pit mine with a heap leach operation using a multiple-lift, single-use leach pad. There are two crushing plants at San Agustin: a 17,000 t/d plant and a 13,000 t/d plant. The ore is crushed (P80 22 mm) at a total rate of approximately 30,000 t/d, stockpiled, reclaimed, agglomerated, and stacked on the leach pad with a mobile conveyor stacking system. The stacked ore is leached with a low-grade cyanide solution and the resulting pregnant solution is processed through a 1,200 m3/hr gravity-cascade carbon adsorption circuit to extract gold and silver. A small 250 m3/hr Merril Crowe plant was added in 2020 to treat pregnant solutions for some gold extraction and additional silver recovery from the overall circuit. The loaded carbon is trucked to Argonaut’s La Colorada mine for carbon stripping and smelting. Precipitate from the Merrill-Crowe plant is also shipped to La Colorada for smelting.

Heap Leaching
The existing heap leach pad is a m ........