La Guitarra silver mine is situated 130km southwest of Mexico City in the Temascaltepec Mining District. The mining district is 100% owned by Canadian mining company Silvermex Resources and covers an area of 397km2.
The operations of the La Guitarra mine first began in the early 1550s and were ceased in the 19th century due to lack of technology and investment.
The mine was reopened in 1990 by Compania Mineria Arauco with an initial production capacity of 30 tons a day (tpd). Luismin purchased the licensing rights of the mine in 1993 and expanded the production rate to 340tpd.
Genco Resources (thereafter renamed Silvermex Resources) acquired the La Guitarra mine and a 340tpd processing plant from Luismin in August 2003.
The mine was closed in 2009 due to lack of development. Its operations recommenced in April 2010 and achieved its full production capacity of 320tpd in November 2011.
Reserves of gold and silver at the Mexican mine
Proven and probable reserves of the La Guitarra mine stand at 11mt graded at 113g/t Ag and 0.93g/t Au.
Geology of the Temascaltepec mining district
The La Guitarra mine lies in the southeastern end of the Mexican belt. It is situated in the southern part of the Sierra Madre Occidental or the Mid-Eocene Ignimbrite volcanic terrain.
The region is characterised by mafic volcanics of the Trans-Mexican belt superimposing the intermediate to felsic Eocene volcanics.
The mine hosts rocks from Mesozoic schist and slates to Tertiary age volcanics and intrusives. It is understood to have been mineralised due to the late Miocene volcanic disruptions.
Hundreds of epithermal veins cross the property. The veins are classified into four main trends namely El Coloso / Nazareno, La Guitarra, Mina de Agua and El Rincon. These veins are spanned to a strike length of 15km and depth of 4km.
Mineralisation at the La Guitarra silver mine
Mineralisation is hosted in the vein type ore bodies in the form of quartz-monzonite compositions. Silver mineralisation occurs in silver sulphides, acanthite, polybasite, proustite-pyrargyrite, sulfosalts and electrum.
Gold mineralisation contains electrum particles and free gold with oxidation features.
Other minerals include white crystalline quartz, chalcedony, amethyst and adularia, sphalerite, pyrite, fluorite, calcite, chalcopyrite, barite, galena and clay.
Production rates, mining methods and ore processing at La Guitarra
Production at the mine commenced in 1993.
The mine produced 57,343t of ore containing 303,847oz of Ag and 3,284oz of Au between January 2011 and September 2011.
The La Guitarra deposit is mined using both open-pit and underground mining techniques. The conventional drilling and blasting is carried out in the open-pit operation. In the underground mining, access to cut and fill stope is provided by adits and ramps.
The development involved three operational pits (Los Angeles, La Cruz and Creston) and four main vein trends.
The stockpiled ore is processed in a concentrating mill having a rated capacity of 340tpd. It is crushed to 12.5mm in three stages by using jaw crushers.
The crushed ore is fed to two stockpiled conveyors for transmission to two grinding mills operating in parallel. The ground ore is carried to two counter current decantation (CCD) thickener circuits to undergo cyanide leaching in agitated tanks and generate pregnant solution.
The pregnant solution is transferred to the standard Merrill-Crowe Zinc Precipitation Circuit to recover silver and gold. The precipitates are fed to smelter to remove the contaminants and produce pure silver and gold dore bars for export.
The barren solution from the precipitation circuit is recirculated to the grinding circuit and CCD thickeners.