Wed, 10/21/2015 - 09:38

Located in the state of Sonora in the municipality of Santa Cruz, the mine of Milpillas saw the light in 1998. In that year, Industrias Peñoles acquired Rancho Milpillas to start a joint exploration project with CODELCO the following year. More than 100,000m of test drilling established that underground mining would be economically feasible in combination with the on-site operation of a large copper solvent extraction (SX) plant. The Milpillas Copper Project was launched in 2001, but it did not reach commercial production of high-purity copper cathodes until 2006 after an investment of US$242.2 million. 2009 marked a bumper year for Milpillas, as a new overflow pool was built to increase water retention capacity while the expansion of the mine’s leach pads was begun, and a radial stacking conveyor was installed on one of them.

Today, the Milpillas mining unit consists of some of the biggest heap leaching pads in Mexico and a specially designed electrolyte plant to produce copper cathodes. The operation has an installed capacity of 45,000 tonnes of copper cathodes per year, although only 51% of its capacity is currently being used. In 2013, Milpillas milled over 22.3 million tonnes of minerals in order to produce 22,814 tonnes of copper cathodes. The mine currently has an expected lifespan of 15 years, while ore reserves are estimated at 388,000 tonnes.

The Milpillas deposit, located in an extensional zone called Cuitaca Graben, which is covered mostly by Tertiary gravels and Quaternary alluviums. Only very scarce outcrops, which show altered and oxidized volcanic rocks, give evidence of the deeply buried deposits. The host bodies are volcanoclastic rocks from the Jurassic Henrietta formation as well as from the Laramide Mesa formation. The Milpillas mine exploits a partially oxidized porphyry copper deposit with a series of alternating copper carbonate-oxide and chalcocite enrichment blanket developed on top of low grade primary chalcopyrite-bornite mineralization.