The El Castillo Complex includes two operating mines, El Castillo and San Agustin.

El Castillo
Deposit Type
El Castillo is interpreted to be a porphyry-style gold system related to Eocene granodiorite– diorite porphyries that intrude Cretaceous clastic and carbonate sediments in an extensional tectonic setting. Gold mineralization occurs throughout the magmatic-hydrothermal system in space and time and is related to sulphide mineralization spatially associated with early potassic development and genetically related to an overprint of phyllic alteration. Supergene alteration, formed as a product of acid leaching, has resulted in argillic-quartz alteration assemblages within the oxide zone of the deposit. The main gold event is believed to be associated with magmatic hydrothermal fluids corresponding to phyllic alteration.

The El Castillo gold system is similar to that found at Andacollo, Chile (Reyes, 1991 and Oyarzun, et al., 1996). Andacollo is a Cretaceous diorite to granodiorite porphyry copper-gold system with central porphyry copper-gold mineralization related to a classic porphyry alteration assemblage and a distal sediment-hosted (manto) low sulphidation and epithermal-level gold satellite system. Fluid inclusion work by Oyarzun et al. (1996) indicates that the manto epithermal gold deposits may in fact be related to another intrusive that has not been recognized at the surface due to high temperature fluid inclusions (about 365ºC) that are found about 5 km from the porphyry centre.

Mineralization
The moderately dipping granodiorite–diorite sills, siltites and argillites are the most favourable host lithologies. Argillic–quartz alteration is often closely associated with the intrusive contacts and can be indicative of higher-grade zones of mineralization.

The dominant controls on the gold mineralization include structural channeling along contacts between intrusive sills and metasedimentary units and a broad zone of northeast-striking, steeply-dipping faults and fractures that acted as conduits to help spread mineralization. Gold precipitation may be somewhat dependent on a chemically-favourable environment within the sediments but does not appear to be strongly influenced by rock composition. The hydrothermal fluids and their contained metals are believed to have been derived from a magmatic source and are a primary volatile component of the porphyry intrusion that is host to much of the mineralization.

There is typically a transition zone of partially oxidized mineralization that lies between the fully oxidized material and lower non-oxidized, sulphide material. The transition zone varies from 5 m to 50 m thick and is generally influenced by degree of fracturing and level of erosion.

The sulphide zone is generally identified by the presence of pyrite mineralization. The occurrence of sulphides, either fracture-related or disseminated, is usually a good indicator of gold mineralization. The sulphide veinlets are most commonly 0.5 cm to 4.0 cm wide.

There are two preferred trends to mineralization. The most obvious of these reflects the generally stronger mineralization within the sedimentary units. The favourable permeability related to increased fracturing within the sediments enhanced the distribution and broader geometry of mineralization. The second trend of mineralization is to the northeast and reflects the dominant structural controls to mineralization. These structures are considered to be important conduits that helped channel the mineralizing system. The combination of these geologic controls resulted in a northeast-elongated gold zone that measures approximately 1,800 m by 1,500 m.

San Agustin
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.

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 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.

The El Castillo Complex includes two operating mines, El Castillo and San Agustin.

El Castillo
El Castillo is an Owner-operated mine using conventional equipment and conventional mining methods.

The El Castillo 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. The operation is situated in a semi-arid environment surrounded by moderate topography.

There are three mine phases remaining in the LOM plan:
• The Phase 10.2 design is approximately 300 m long by 100 m wide with 36 m depth. Total Mineral Reserves contained in this phase are 1.1 Mt at an average grade of 0.25 g/t Au with a strip ratio of 0.21;
• The Phase LB design is approximately 700 m long by 250 m wide with 170 m depth. Phase LB contains 3.8 Mt of Mineral Reserves grading 0.27 g/t Au and a strip ratio of 1.39;
• The Phase WP design is approximately 700 m long by 400 m wide with 150 m remaining depth. Phase WP contains 6.6 Mt of Mineral Reserves grading 0.44 g/t Au with a strip ratio of 0.61.

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

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 at the pit bottom after stripping requirements were met.

Pit Design Parameters:
Inter-ramp Angle: 34°, 41°, 45°, 50°;
Bench Face Angle: 36°, 60°, 67°;
Berm Width: 4, 5, 7;
Bench Mining: Height 6;
Berm Placement: 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. The mining widths for the El Castillo pits will be generally greater than 50 m between each phase except when temporary ramps of 25 m are mined.

Waste Rock Storage Facilities
Argonaut designed three main waste rock storage facilities (WRSFs); one that is in-pit and two that are external to the pit designs. The concept of in-pit storage has been used successfully at El Castillo where low-grade sulphide material has been covered with waste. Areas of higher-grade sulphide material were not back-filled due to the potential for mill processing of the mineralization at higher gold prices.

Argonaut designed WRSFs with the capacity to hold 9.5 Mt of waste rock that are currently defined in the LOM production schedule. Several of the WRSFs have the ability to be expanded vertically, and the opportunity for additional in-pit storage is available. As a result, there is a low risk that WRSF space is a limiting factor of any future mine production.

San Agustin
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.

San Agustin
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.

El Castillo
Until January 2020, Argonaut processed gold ore through a conventional heap leaching operation that included two-stage crushing of higher-grade ore (>0.25 g/t Au) to -1 ½ inch (38 mm) and coarse crushing of lower grade ore (<0.25 g/t Au) to -4 inch (100 mm). Fine crushed ore was leached on the East Heap Leach Pad and coarse crushed ore was leached on the West Heap Leach Pad.

During the latter half of 2019 the mine transitioned to a ROM only processing operation. After December 2019 crushing was discontinued, and all ore was processed as ROM, with haul trucks directly dumping ore onto the leach pads.

Both the East and West leach operations are essentially the same, processing a combined total of approximately 24,000 t/d ROM ore, generally targeted as 50% of the ore to each leach pad. The ROM ore is 24 inch (600 mm) top size and is truck-stacked on the leach pads in 8 m lifts. The ore is leached with a cyanide solution (250 ppm NaCN) for a 90-day le ........