Cusi Project, Chihuahua

The Cusi Project, owned by Sierra Metal, is located in Chihuahua, Mexico, and features three of the top 10 biggest silver mines in the world. The underground mine has seven mineralised zones containing huge deposits of silver, gold, lead and zinc.

Mining for the Cusi Project began in the Promontorio and Santa Eduwiges underground mines in 2008 and commercial production was achieved on 1 January 2013. The mine processed 106,853t of ore and produced 423,089oz of silver and 902,000lb of lead in 2013.

The mine is being developed in two stages. The first stage aims to achieve a production rate of 500 tonnes per day (tpd) by October, and the second stage, expected to commence this year, is expected to increase the productivity to 2,000tpd by 2015 with the addition of a new mill.

Location, geology and reserves

The Cusi multi-metal mine is located approximately 135km from the city of Chihuahua and extends approximately 11,657ha. The San Miguel and La Candelaria deposits at the mine were discovered in 1687 and produced 100Moz to 200Moz of silver between 1687 and 1940. Sierra Metals, previously known as Dia Bras, acquired the mine in April 2008.

The mine is located at an elevation of 1,950 metres above sea level (masl) to 2,460masl in the Sierra Madre Occidental Mountain Range. The un-metamorphosed andesite and dacite accumulations present at the Sierra Madre Occidental stand among the biggest accumulations in the world.

The Cusi Project is comprised of seven mineralised zones: Promontorio, Santa Eduwiges, La India, Milagro, San Nicolas, La Gloria / Minerva and San Juan. The Promontorio and Santa Eduwiges zones are structurally complex in shape and are made up of discrete and disseminated to highly fractured / stockworked mineralisation. The rest are classified as simple, typical epithermal vein deposits.

Silver mineralisation occurs in the north, north-east, east-north-east and east trending, and moderately to steeply in east to south-east dipping sulphide-rich veins. The mineralised veins extend 100m to 200m along the strike and dip to the south-east, south-west and north.

As of 27 March, the mine was estimated to contain indicated resources of 1.66Mt of ore averaging 232 grams per tonne (g/t) of Ag. It was estimated to contain 12.42Moz of silver, 4,375oz of gold, 16.76M lb of lead and 16.85M lb of zinc.

Mining and processing of ore

The cut-and-fill method of mining is used at the Cusi mine. Most of the mining at the property is currently done at the Promontorio and Santa Eduwiges mines, while the La Gloria and La India mines are still under development.

The underground mines are accessed via ramps and the mined ore is trucked to Malpaso Mill, located 37km from the project, for processing. Approximately 97,000t of ore was mined from the Promontorio and Santa Eduwiges Mines in 2013.

The ore is crushed in a jaw crusher before being processed in the El Triunfo circuit of the Malpaso Mill. The circuit has a processing capacity of 500tpd and produces copper and zinc concentrates through floatation. The tailings are cyanide-leached for silver recovery by the Merrill Crowe process and are stored at two tailings dams constructed near the mill.

The Malpaso mill was acquired by Sierra Metals in 2004 and commenced production in March 2005.

Infrastructure facilities

The mine can be accessed through a 22km paved road from the village of Cusihuiriachic, Chihuahua. The power supply is provided through a 33,000V power line from the Mexican Electricity Federal Commission (Comisión Federal de Electricidad). Electricity is supplied for the Malpaso Mill through a 1,290kW power line.

Process water and water for the mining operations is sourced from the underground workings at the mine property, while potable water is trucked in.

Key players involved

Gustavson Associates was engaged to prepare the mineral resource estimate for the Cusi Project.