Mineral Deposit
Mineralization at the Cusi Mine has been variously described as a) low-sulfidation epithermal (Ciesielski, 2007), b) high-sulfidation epithermal (SGS, 2008) and linked epithermal-base metal system (Meinhert, 2006). Meinhert (2006) notes that although shallow (<100 m) historic mining is reported to have encountered grades exceeding 1,000 oz/ton Ag, the veins currently exposed are more base-metal rich than would be expected in an epithermal system. However, Sierra Metals geologists consider the abundance of base metals on the property to be primarily a function of depth of exposure and SRK agrees with this interpretation. Mineralization occurs along narrow fractures containing quartz, sphalerite and galena, and wall rock alteration consists primarily of silicification and the development of clays and iron oxides. The veins contain quartz with crustiform and banded textures typical of epithermal systems.

The current geologic model for the Cusi property is described as follows:

The country rock on the property consists primarily of felsic volcanics interpreted to represent a caldera with a resurgent dome. Magma is interpreted to have intruded along the Cusi fault, a regional NW-trending, right-lateral strike-slip fault, and a subsequent eruption produced the collapsed caldera and Upper Volcanic Series felsic tuffs. A resurgent dome then arose within the caldera on the western side of the Cusi fault. This dome was then dissected by numer ........

Bench and fill mining method is currently used in the main areas of the mine and to a lesser extent, room and pillar mining is also used. The mining method used varies depending on geotechnical constraints, mineralization trends, dimensions, and mine production targets.

Using the updated Mineral Resource estimate, Sierra Metals performed an expansion analysis to determine how the Cusi mine could achieve higher sustainable production rates. The analysis indicated that higher production rates are achievable through the massification of the bench and fill mining method in the new production areas, which will allow the sustainability of the operation. A new configuration of the mining method will allow obtaining a greater recovery of mining resources and increasing productivity.

Current production at Cusi comes from the Promontorio and Santa Rosa de Lima mineralized zones. Mineralized material is currently hauled to the surface using one of several adits or declines accessing the mineralized zones, and is then dumped onto small pads outside of the portals.

The mining sequence through this method is of a descending type, that is, the upper levels are mined, while in the lower ones the necessary preparations are made to start mining once the mineralized material has been extracted from the upper stopes. Within a sublevel, mining is carried out in retreat, starting at the ends of the stope and retreating towards the entrance.

In Cusi, min ........

The concentrator's processing facilities include a crushing circuit consisting of a 50 t bulk hopper, a Metso TK9-32-22V vibratory feeder, a C96 Metso jaw crusher, an HP300 secondary cone crusher, a tertiary cone crusher HP300, a 6’x 20’ Trio double-bed primary vibrating screen, a 6’x20’ Trio double-bed secondary vibrating screen and a 5’x14’ double-bed secondary vibrating screen. The material is crushed to 90% passing -5/16" and is then deposited into three fine hoppers with an aggregate storage capacity of 1000 t.

The grinding circuit consists of a 7’x 10’ ball mill with a 250 HP motor, an 8’x 7’ ball mill with a 250HP motor and an 8’x14’ ball mill with a 600HP motor, and the classification system consists of D20 hydrocyclones.

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