The Bolivar deposit is classified as a high-grade Cu-Zn skarn and exhibits many characteristics common to this type of deposit. The term ‘skarn’ refers to coarse-grained calcium or magnesian silicate alteration formed at relatively high temperatures by the replacement of the original rock, which is often carbonate-rich.

Mineralization at the Bolivar Property is hosted by skarn alteration in carbonate rocks adjacent to the Piedras Verde granodiorite. Orientations of the skarn vary dramatically, although the majority are gently-dipping. Thicknesses vary from 2 m to over 20 m. Skarn mineralization is strongly zoned, with proximal Cu-rich garnet skarn in the South Bolivar area, close to igneous contacts, and more distal Zn-rich garnet+pyroxene skarn in the northern Bolivar and southern skarn zones near El Val. The presence of chalcopyrite+bornite dominant skarn (lacking sphalerite) in the South Bolivar area, along with K-silicate veins in the adjacent granodiorite suggests that this zone is close to a center of hydrothermal fluid activity. In contrast, the main Bolivar Mine is characterized by Zn>Cu and more distal skarn mineralogy such as pyroxene>garnet and pale green and brown garnets. Alteration is zoned relative to fluid flow channels. From proximal to distal, the observed sequence is red-brown garnet to brown garnet with chalcopyrite ± bornite ± magnetite to green garnet ± pyroxene with chalcopyrite + sphalerite to massive sulfide (sphalerite ± chalcopyrite ± galena) to marble with stylolites and other fluid escape structures.

Mineralization exhibits strong stratigraphic control and two stratigraphic horizons host the majority: an upper calcic horizon, which predominantly hosts Zn-rich mineralization, and a lower dolomitic horizon, which predominantly hosts Cu-rich mineralization. In both cases, the highest grades are developed where fault or vein structures and associated breccia zones cross these favorable horizons near skarn-marble contacts. Zones of breccia follow faults like the Rosario, Fernandez, and Breccia Linda trends as well as nearly vertical breccia pipes such as La Increible.rly vertical breccia pipes.

Using the resources estimated by SRK Consulting through December 2019, Sierra Metals performed a growth analysis to determine how the Bolívar Mine could achieve sustainable production of 7,000 tpd - 15,000 tpd. The analysis indicates that the production objectives are achievable through the expansion of the sub-level stoping mining method in the new production areas, which will allow the sustainability of the operation. A new configuration of the mining method will allow greater recovery of resources and production rates.

The crushing stage is supplied with mineralized material hauled from the mine site using contractor operated haul trucks. A typical haul truck has approximately 20 tonnes capacity and delivers mineralized material from the mine area to the primary crusher’s mineralized material stockpiling area. Trucks can dump directly to the primary crusher or alternatively, to one of several stockpiles. Typically, a front-end loader reclaims mineralized material from the stockpiles and then feeds the jaw crusher.

The crushing plant is fed through a hopper equipped with a 20-inch x 20-inch static grizzly that discharges to a jaw crusher operating in open circuit. The nominal four-inch material discharging from the jaw crusher is classified by two double-deck vibrating screens. The top screen is two inches by one inch and the bottom screen is 3/4 inch by 3/8 inch. Material smaller than 3/8 inch becomes the final crushed product that is transferred to two silos having an individual capacity of 1,000 tonnes each. The vibrating screen’s oversize feeds a secondary crushing stage consisting of two cone crushers. The top screen is conveyed to a Sandvik HC660 cone crusher and the bottom screen oversize is conveyed to a Metso HP-300 cone crusher. The cone crusher’s discharge joins the primary crusher’s discharge and feeds the double-deck vibrating screens.

Feed to the grinding circuit is sourced from two 1,000 tonne silos that hold the final crushed product from the crushing plant. The grinding circuit consists of conventional ball mills operating in closedcircuit operation with hydrocyclones. Two 9.5 ft x 14 ft ball mills operate in parallel, each one in closed circuit with a hydrocyclone cluster. The two ball mills underwent complete overhauls during the end of 2017 to improve mechanical availability. The hydrocyclones were changed from D26 to D20 to improve plant stability. The product size in the cyclone overflow ranges between 34% and 48% passing 75 micrometers, with an average size of 43.5% passing 75 micrometers. The hydrocyclone overflow feeds the flotation circuit. The hydrocyclone underflow stream is returned to the ball mills for further size reduction.

Sierra Metals operates the Piedras Verdes conventional concentration plant consisting of crushing, grinding, flotation, thickening and concentrate filtration processes. Thickened flotation tails are placed into a conventional tailings storage facility and the mine is converting to the use of dry-stack tailings in mid-2020.

The flotation circuit operates three identical parallel flotation lines. Each flotation line includes a 12 ft x 12 ft conditioning tank and three DR 300 ft3 rougher flotation cells. From the rougher cells, the overflow feeds the cleaning cells. The cleaning flotation cells consist of two DR 300 ft3 primary cleaner flotation cells, three Sub A 100-ft3 secondary flotation cells and two Sub A 100 ft3 tertiary cleaning flotation cells. The rougher flotation tails feed the rougher-scavenger stage which consists of four DR 300 ft3 flotation cells. The final copper concentrate typically comes from the second cleaner flotation product; however, if it does not meet ........