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 property lies within a possible caldera that contains a prominent rhyolite body interpreted as a resurgent dome. The rhyolite dome trends northwest-southeast with an exposure of roughly 7 km by 3 km and hosts mineralization. It is bounded (cut) on the east side by strands of the NWtrending Cusi fault and on the west by the Border fault. The Cusi fault has both normal and rightlateral strike-slip senses of shear. Strands of the Cusi fault are intersected by NE-trending faults, some of which indicate left-lateral strike-slip shear. NE-trending veins associated with ........

Current production at Cusi comes from the Promontorio and Santa Rosa de Lima mineralized zones.

Each mine area is accessed from a spiral ramp (4 meters square), as well as a single shaft in each area. Minimal development is needed to exploit mineralized zones, and contract miners are developing ramps at both mines to exploit ores at depth.

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.

Bench and Fill
In veins less than 3 m wide, sublevels are vertically spaced 16 m, and the access drift is 4 m x 4 m. Blastholes are vertically drilled 8.5 m thus leaving a sill pillar of 3.5 m under the overlying stope.

In veins wider than 3 m but less than 5, sublevels are vertically spaced 16 m, and the access drift is 4 m x 4 m. Blastholes are vertically drilled 8.5 m thus leaving a sill pillar of 3.5 m under the overlying stope.

Room and Pillars
Where veins are wider than 5 m, levels are spaced every 16 m and ........

- subscription is required.

The Cusi processing facilities include two interconnected process plants, which are the Mal Paso mill purchased from Rio Tinto, and the El Triunfo (Triungo) mill. Both mills are conventional ball mill and flotation plants fed from a single crushing circuit. The flotation circuit has the ability to produce lead concentrate and zinc concentrate, although the Pb circuit represents a comparably higher percentage of concentrate production.

El Triunfo includes a cyanide leach plant that has been used to process legacy tailings and, at times, fresh tails from Mal Paso. The leach plant was idled in mid 2012 with no indication that it is scheduled to restart.

Cusi’s Mal Paso processing facility consists of a conventional concentration plant including crushing, grinding, flotation, dewatering of final concentrate, and a tailings disposal facility.

Mineralized material produced from the Cusi mine is hauled to Mal Paso Mill using dump trucks. Trucks are weighed ........