La Colorada

Overview

The La Colorada silver mine is located in the state of Zacatecas in west central México. The mine-site is accessible by road approximately 2½ hours south-east of the City of Durango. The property has been mined sporadically from colonial times with fairly consistent operations from 1925 through to the present day.

In 2011, the mine produced 4.3 million ounces of silver. Proven and probable reserves at the mine currently stand at 44.1 million ounces.

The La Colorada underground mine has 3 production areas: Candelaria; San Fermin, and Veta Dos. These 3 areas mine the two main vein zones in the mine. The Veta Dos and San Fermin production areas mine those portions of the NC2E and NCP zones respectively that are closer to the surface. 

Location

The La Colorada property is located on the eastern flanks of the Sierra Madre Occidental at the contact between the Lower Volcanic Complex and the Upper Volcanic Supergroup. It lays 16-km southeast of Chalchihuites and 30-km south-southwest of Sombrerete, two mining towns with significant silver and base metal deposits.

The mine-site is accessible by road approximately 2½ hours south-east of the City of Durango. The road consists of 120 km of a paved two-lane highway (Highway 45), and 23 km of public, all weather gravel road. The access from Zacatecas takes approximately the same time on similar types of roads. Durango and Zacatecas are serviced by daily flights from México City, other major centers in México and direct flights from some United States cities.

Geology and Mineralization

La Colorada property is comprised of six non-contiguous blocks of exploration and exploitation claims totalling 2,229.9 ha (of which approximately 1,189 ha are designated by exploration permits and 1,041 ha by exploitation claims). In addition, Plata also has control over approximately 464 ha of surface rights covering the main workings, namely the Candelaria, Campaña and Recompensa mines. An additional 110 ha area of exploration claims and 98 ha area of exploitation claims are located to the north of the main claims near El Conjuro-San Cristobal. The property area hosts several other former small scale silver underground operations and surface workings such as the La Colorada, Conjuro and San Francisco el Duro mines.

La Colorada represents a typical epithermal silver/gold deposit, with a transition in the lower reaches of the deposit to a more base metal predominant system. There are indications of what might be skarn style mineralization in the deepest holes on the property.

The deposit consists of a complex system of epithermal veins and breccias in host rocks of dacite or limestone. Most mineralized veins on the property strike east to northeast and dip moderately to steeply to the south. Mineralized widths in the veins (referred to as the Candelaria system or NCP vein and the NC2 vein) are generally less than 2 meters, but may be wider if there is a halo of replacement or brecciated material.

The mineralized veins contain both oxide and sulphide material. The depth below surface and the permeability of the mineralized zone controls the level of oxidation in the veins. The most common sulphide minerals are galena, sphalerite, tetrahedrite, argentite, and pyrite.

Mineralization in the breccia pipes generally has lower silver values and elevated base metal values. Mineralization is associated with intense silicification and occurs as disseminated galena and sphalerite with minor chalcopyrite and bornite. The current Mineral Resource and Mineral Reserve contains vein and manto hosted mineralization.

Mining & Operation

Access to all of the mining areas is via declines from the surface that are nominally 3.6 meters wide by 3.6 meters high grading at -15%. The ore and waste from the Veta Dos and San Fermin areas are hauled up these declines to the surface using a fleet of 4 - EJC417 four wheel drive mine haulage trucks. The ramp system in the Candelaria portion of the mine connects to the San Fermin system and provides a back up method of haulage to the El Aguila hoisting shaft. A smaller back up system (the Compaña ramp) provides an additional means of access to the Candelaria production area.

Recent work on the El Aguila shaft deepened it to allow installation of new loading pockets, development and installation of new truck dump grizzlies, ore and waste pass raises, and slusher operated measuring bins. The new loading pockets are at the 343 level. The ore and waste is hoisted in rock skips to separate bins located just below the surface. Ore and waste are trammed out of the mine on the 20 level in rail cars to dump points. The ore is taken either directly to the crusher feed bin or to a storage area adjacent to the feed bin using a front end loader. Waste is hauled away and is typically returned to the mine via a backfill raise and placed in the stopes as backfill. The mine is currently in the process of upgrading the existing hoist with a reconditioned unit in order to provide more reliable operations and greater hoisting capacity.

The mining method is mechanized cut and fill with waste rock being utilized for backfill. In the lower part of the mine, where dewatering is conducted as mining progresses, the initial cut is developed at +3% in order to promote drainage from the stope. In the upper parts of the mine that have been dewatered by mining below, the initial access is driven at -20% to the orebody. After the initial cut has been mined and filled the stope is reaccessed either by slashing the back of the initial crosscut, if ground conditions permit, or by developing a new crosscut.

The cuts are mined and backfilled progressively until the upper crosscut is at the maximum positive gradient for the scooptrams which is typically +22.5%. At this point either a new access is started further up the access ramp or, the crown pillar between the cuts is recovered. In the newer parts of the mine, the backfill in the first cut has been cemented and the pillar can be recovered completely. In the older parts of the mine, it has been found that the backfill compacts extremely well and has some cohesion which allows for blasting out of the ore up to the backfill and mucking out with a scooptram. 

The mine's mechanized equipment includes 3.75 cubic metre, 2 cubic metre  and 1.5 cubic metre capacity scoop trams, low profile 9 to 12 tonne capacity trucks for haulage in ramps, jack-legs and 1 boom electric hydraulic jumbos for drilling. Dump trucks are used for the haulage of ore from the shaft bin to the crushers.

Processing

Run of mine ore is delivered by train from the Aguila Shaft, and surface haul trucks to the crushing plant grizzly. A common 1,000 tonne/day capacity crushing circuit provides feed for both the oxide and sulphide plants which are stand alone, each with separate grinding and processing facilities. Crushed oxide ore is stockpiled on surface in a 1,000 tonne live capacity ore bin and reclaimed to feed the grinding circuit. Crushed sulphide ore is sent to a fine ore bin (400 tonne capacity) prior to feeding the plant.

The process plant has a nominal treatment rate of 865 tonnes per day with a total availability of 92.5%, 350 days per year. This rate can be roughly broken down into 600 tonnes per day of oxide ore and 200 tonnes per day of sulphide ore.

The oxide circuit consists of a 96 hour leach cycle, followed by a five stage CCD clarification circuit and a Merrill Crowe plant for precious metal recovery. The sulphide plant is a conventional flotation plant involving rougher and cleaner flotation, concentrate thickening, and dewatering. Concentrates are shipped by truck to the Penoles smelter at Torreon, Durango, approximately 300 kilometers from the mine site.

In 2011, La Colorada milled 405,000 t ore at 369 g/t silver, 0.43 g/t gold, 2.36% zinc and 1.18% lead.

Environment & Community

Impacts on the socio-economics of the surrounding towns and villages have been positive. The mine has a bias toward hiring individuals from the region and in cases where there is no one qualified, training programs have been implemented.

The mine has also pledged to provide potable water to the town of La Colorada during the operating life of the mine as well as provided funding for infrastructure in the surrounding communities.