Americas owns the San Rafael and nearby Nuestra Señora and El Cajón deposits, which are located in the Cosalá mining district. Silver, lead, zinc, copper, and gold occur in a variety of deposit types within the district, including skarns, carbonate replacement deposits, breccia deposits, and massive sulfides.

The Nuestra Señora property lies within a sub-circular inlier of Cretaceous limestone approximately 10km in diameter situated in the eastern part of the 139 to 45 Ma-old Sinaloa Batholith. Contact metamorphism of the limestones created re-crystallized limestone, marble, and skarn. Initial skarn development in the area was contemporaneous with emplacement of the batholith; however, there were several pulses of magmatic and hydrothermal activity. Carbonate replacement-style mantos, veins, chimneys, chimney breccias, and mineralized exoskarn and endoskarn occur within limestone and felsic and lesser mafic intrusions. Pyrite, sphalerite, chalcopyrite, galena, and lesser tetrahedrite are the principal minerals.

In the San Rafael-El Cajón area, Cretaceous limestone, commonly recrystallized and marbleized but only locally skarn-altered, is exposed within windows in Tertiary volcanic rocks. Massive sulfide mineralization at San Rafael occurs primarily along the contact of dacite tuff with Cretaceous limestone, with additional mineralization within the dacite in the Upper Zone and within skarn-altered limestone in the 120 Zone. The protolith at El Cajón is altered limestone, thought to be of Cretaceous age. San Rafael contains silver, lead, and zinc mineralization with minor gold and copper. The main minerals are pyrite, pyrrhotite, sphalerite, and galena with minor marcasite, chalcopyrite, and magnetite. The El Cajón-type of mineralization, also seen within the San Rafael deposit’s 120 Zone, is related to skarn alteration of calcareous sediments and occurs as both mantos and chimneys. It consists of silver-coppergold mineralization in the form of chalcopyrite and tetrahedrite with minor pyrite, galena, sphalerite, arsenopyrite, chalcocite, jalpaite, native silver, copper, and bismuth.

Nuestra Señora underground mine development commenced in 2005, and prior to the commissioning of the process plant in May 2008, approximately 87,000 tonnes of ore had been stockpiled on surface. Since attaining commercial production of the process plant in January 2009, the Nuestra Señora mine has been the sole source of ore feed for the plant. Mining at Nuestra Señora uses mechanized mobile mining equipment, principally twin-boom jumbos, 6yd3 scoop trams, production drill rigs, and rigidbody highway trucks.

Complex geometries of the mineralized bodies make it imperative to employ high-density definition drilling techniques to determine the extents of the mineralization. During development, ore-control definition is performed by drilling 12m long holes on approximately 4m centers. Samples are taken from the cuttings of each 1.5m of drill steel and are assayed in the operation’s laboratory. This detailed sampling control allows for more appropriate development, improving mining recovery and minimizing dilution.

As successive levels are developed, each mineralized body is tested for continuity and size. The mining method employed for each mineralized body is determined by its geometry. Predominantly long-hole or cut-and-fill stoping is designed for production; however, shrinkage stoping via hand-held air-leg drill mining is available for implementation where warranted.

Waste rock from access development is used as fill material in successive cut-and-fill lifts and exhausted long-hole stopes. Tailings are recovered from the Nuestra Señora process plant and delivered by truck to exhausted long-hole stopes for use as fill along with development waste rock.

A hybrid underground cut-and-fill/room and pillar mining method has been selected to extract material from the Main and Upper Zones at San Rafael due to the depth, the shallow-dipping angle of the mineralization and the variable thickness of the mineralization.

For the purpose of mine planning, the mining zones were grouped in 16m high x 50m long mining panels. Access to the panels will be provided by attack ramps located approximately 65m away from the panels to allow for the appropriate maximum ramp gradient. Attack ramps will provide access to adjacent panels. A main production drift and a series of cross-cuts will be developed at the sill of each panel. Then the panels will be mined sequentially in 4m lifts by driving 4m x 4m drifts; the drifts will have a maximum length of 50m leaving vertical pillars as required for ground control and stability.

The Los Braceros process plant is a conventional polymetallic concentrator currently producing zinc, lead, and copper concentrates. It is currently treating ore from the Nuestra Señora mine at a nominal throughput of 1,500 tonnes per day. For the San Rafael ores, only lead and zinc concentrates will be produced with a design nominal daily throughput of 1,800 tonnes per day. The increased throughput will be possible following installation of a different primary ball mill (see below) and conversion of the two existing ball mills to secondary mills. Adequate capacity already exists in the flotation, dewatering and tailings handling circuits. The revised process flow diagram is illustrated below in Figure 17.1.