The La Encantada silver mine is located in Coahuila, Mexico. The underground mine has been in operation since the early 1970s. In 2010, the mine produced 3.8 million ounces (oz) of silver at a cash cost of $7.4/oz.
The mine was originally operated by Minera La Encantada, a wholly-owned company of Minas Penoles and Industrias Penoles. In December 2006, First Majestic Silver Corporation purchased Minera La Encantada.
La Encantada is now operated by Minera La Encantada, a wholly-owned subsidiary of First Majestic.
Geology and reserves at La Encantada
La Encantada mine is part of the Sierra Madre Oriental mountain range. It consists of skarn type deposits surrounded by calcareous sedimentary formations of the lower cretaceous age.
Proven and probable reserves at the mine are estimated at 5.1mt grading at 208g/t. Measured and indicated reserves are 5.3mt grading at 176g/t. Inferred reserves are estimated at 2.5mt grading at 220g/t.
Mineralisation of the region
Mineral reserves occur in metasomatic deposits which extend vertically to a depth of 435m. The deposits have been subjected to prolonged oxidation and secondary enrichment.
Most of the mineral reserves consist of oxidised concentrations of silver, lead, hematite and other minerals such as cerussite and anglesite.
Silver mineralisation occurs in the form of argentite and native silver. High grade silver is concentrated in the Chicotón breccia zone.
Lead mineralisation is found in the form of carbonates and sulphides.
Mining was initially carried out through shafts with rail haulage levels. Rail-bound loading and hauling equipment was used to handle recovered ore.
After acquisition, First Majestic converted the mining operations to a trackless operation. Rail haulage, however, is still used at deeper levels of the mine.
The mine is currently accessed through a trackless adit which can be expanded to fit 20t capacity highway-type dump trucks. The interior of the mine features a ramp system which provides access to diesel powered mobile equipment.
Loading is carried out by diesel powered front-end loaders and haulage is done by highway-type diesel rear dump trucks or battery powered rail haulage trains.
In April 2010, a new 3,750 ton-per-day (tpd) cyanidation plant commenced operations at the mine. Construction of the plant started in July 2008 and was completed in November 2009.
Built at a cost of $30m, the new plant features state-of-the-art induction furnaces to produce higher quality silver doré bars. The plant helps in achieving higher recovery rates and enables First Majestic to concentrate on silver doré production. It is expected to increase 2011 production levels to 4.5 million oz.
Ore processing at La Encantada silver mine
Recovered ore includes a combination of oxide and sulphide minerals. Its soft and fragile nature makes it easy to crush and grind.
The ore is first crushed and ground and then passed through a two-stage 1,000tpd floatation circuit. The first stage includes sulphide mineral floatation and the second is oxide mineral floatation.
The floatation plant is nearly 30 years old and was the primary method of silver recovery. With the completion of the cyanidation mill, First Majestic decided to suspend production from the floatation plant.
The move reduced smelting and refining costs for First Majestic. It enabled the company to produce silver doré bars instead of concentrates. The floatation plant is maintained by First Majestic.
The mine has been subject to several exploration programmes since it commenced operations. Following the mine’s acquisition, First Majestic embarked upon a continuous exploration programme aimed at increasing the reserves of the mine.
In 2010, 7,257m of diamond drilling was completed to confirm additional reserves at the mine.
First Majestic is planning to invest $1.9m in 2011 to complete 12,150m of diamond drilling at the mine to increase reserve estimates. Another $6.5m will be spent on constructing ramps and mine infrastructure to accommodate current and future production.