The Mulatos mineral deposits are large epithermal, high-sulfidation, disseminated, gold deposits hosted within a mid-Tertiary dacitic dome complex. Gold mineralization is closely associated with silicic alteration within extensive areas of argillic and advanced argillic alteration. The Mulatos deposit proper is composed of the contiguous Estrella, El Salto, Mina Vieja, and Puerto del Aire Mineral Resource areas. The Escondida deposit is the faulted extension of the Mina Vieja and El Salto subdeposits and is believed to be continuous to the northeast with the Gap, El Victor and San Carlos mineralized areas. Although zones are often bounded by post-mineral faults, together they form a trend of 2.7 km of gold mineralization starting at the north end of the Estrella pit to the San Carlos deposit.

Within the larger Mulatos Group of Concessions, and generally within 20 km from the Mulatos deposit, geologically similar high sulfidation gold deposits, occurrences, or prospects are known. The principal ones, some of which are in the process of being evaluated and/or drill-tested, are: El Carricito, El Halcon, Las Carboneras and El Jaspe.

Gold deposits of the Mulatos district are considered to be high sulphidation-state epithermal systems. Epithermal precious metal systems may be classified as high, intermediate, and low sulphidation styles. They are characterized by the sulphidation state of the hypogene sulphide mineral assemblage, and show general relations in volcano-tectonic setting, precious and base metal content, igneous rock association, proximal hypogene alteration, and sulphide abundance. Ore in all occurrences is of the type formed under epizonal conditions, that is, generally within 2 km of the paleo-surface.

Precious metal mineralization at Mulatos is associated with intense silicic alteration (mostly vuggy silica), advanced argillic alteration, and the presence of hydrothermal breccias. The original protolith (dacite porphyry flow/tuff, coarse grained volcaniclastic rocks, breccias), as indicated by surface mapping and core drilling, may have contained in the order of 2-3% sulphide as pyrite with various amounts of enargite and tetrahedrite. The principle gold bearing host rock is interpreted as favoured for mineralization due to relatively high primary porosity and its intense fracturing.

Gold mineralization within the Mulatos deposit occurs primarily within areas of pervasive silicic alteration of the volcanic host rocks, and to a lesser extent, within advanced argillic alteration assemblages proximal to silicic alteration. The gold-bearing advanced argillic zones are dominated by pyrophyllite or dickite alteration. Silicic rocks host approximately 80% of the contained gold within the deposit. There are three main mineralization assemblages. From oldest to youngest they are: 1) quartz + pyrite + pyrophyllite + gold; 2) quartz + pyrite + kaolinite + gold + enargite; 3) kaolinite + barite + gold. Free gold is commonly found in hematite-filled fractures. Gold also occurs in pyrite, as gold/silver telluride minerals, and possibly as a solid solution in some copper sulphide minerals. Supergene oxidation and perhaps remobilization and secondary enrichment of gold have been ongoing since the post-mineral volcanic cover was removed (in those specific deposits where it has been removed).

Mining operations at the Mulatos Mine consists of the Mulatos open pit complex, the Cerro Pelon open pit, and the Yaqui Grande open pit currently under development. Mining in the Mulatos open pit started in 2005. The open pit is a standard loader and truck operation running 24 hours per day, seven days per week. Mining is currently undertaken by a contractor and prior to 2014 mining was undertaken by Mulatos personnel. It is anticipated that the contractor will remain for the life of the open pit. The current Mulatos open pit complex consists of the main Mulatos pit and the adjacent Victor and San Carlos pits. Based on Mineral Reserves as of December 31, 2020 (including stockpiled ore) the Mulatos open pit complex has a remaining life of approximately three years, excluding Cerro Pelon and La Yaqui Grande.

Development for the San Carlos underground was initiated in early 2014 and production stoping commenced in late 2014. Mining at the San Carlos underground was completed in the third quarter of 2018.

The Company is currently developing the La Yaqui Grande open pit operation. La Yaqui Grande will have its own dedicated crushing facility and leach pad. Loaded carbon will be transported to the Mulatos ADR for further processing. The La Yaqui Grande remains on track for commercial production in the third quarter of 2022.

There are two primary recovery processes at MON. The 17,500 tpd heap leach facility is for the lower grade ore and the recently installed 1,000 tpd mill processing plant is for high grade ore.

Crushing for Heap Leach Process
Run-of-mine ore is delivered to the primary crusher feed hopper by rear-dump haul trucks. A static grizzly screen above the hopper limits the top size of rock fed to the crusher. Below the hopper, a vibrating grizzly feeder transfers ore. Grizzly screen oversize feeds the primary jaw crusher. Grizzly screen undersize joins the crusher product on the primary crusher discharge conveyor which feeds the primary crushed product to the second stage of crushing. The rock breaker is installed to serve the static grizzly. Tramp iron is removed from the crushed product by way of the magnet mounted above the discharge of the discharge conveyor.

Primary crushed product is fed to the secondary cone crusher. Secondary crushed product is discharged onto a conveyor and delivers ore to the scalping screens by conveyors. Screen oversize reports to the screen oversize conveyors which feed the radial stacker which discharges into the coarse ore stockpile. Screen undersize reports to the conveyors which discharges onto conveyor which ultimately directs ore to the fine ore stockpile.

The tertiary crushed product is discharged onto conveyor and fed to a vibrating screen. Screen oversize is transferred by conveyors and is crushed in the single quaternary cone crusher. The quaternary crushed product is discharged onto the crusher conveyor and re circulates back to the vibrating screen.

With the depletion of the San Carlos underground in 2018, the high grade mill has been placed on care and maintenance.
Crushing for High Grade Mill (Gravity Plant)
High grade ore which has been crushed to P80 75 mm, is diverted to a stockpile located near the gravity circuit. Ore is then withdrawn from the stockpile at the rate of 50 tph by one of three vibrating feeders located along a reclaim tunnel. The reclaim conveyor delivers ore to the gravity primary feed conveyor.

The primary feed conveyor is equipped with a weightometer and a belt magnet to remove tramp metal. The ore is delivered to the cone crusher, operating in open-circuit. The ore is discharged onto another belt conveyor where it is carried to the double-deck wet primary screen.

The wet primary screen oversize ore is transported via a conveyor to feed the vertical shaft impactor (VSI). The screen undersize reports to the rougher IPJ feed pump box. The VSI discharge is conveyed back to the primary screen feed conveyor.

The Mulatos processing facilities consist of a heap leach pad with an associated crushing plant, and a high-grade mill.

Run of mine heap leach ore from the open pit is crushed in a four stage plant to 100% passing -3/8”. A run of mine stockpile is in front of the primary crusher and additional surge capacity is situated between the secondary and tertiary crushing plants, and after the quaternary crusher. Following quaternary crushing, the ore is transported via a 1.7 km overland conveyor to the leach pad. At the leach pad, cement is added via two agglomerators and the ore is then transported via grasshopper portable conveyors to a stacker where it is stacked in 7 m lifts.

Cyanide leach solution is applied to each lift for approximately 90 days. The gold bearing solution reports to one of two “pregnant” solution ponds via gravity. Pregnant solution is pumped to the ADR where gold is recovered through the carbon absorption columns, carbon stripping and electrowinnin ........