The Cananea mine in Sonora is Mexico’s largest open pit copper mine, one of the largest in the world and – having opened in 1899 – one of the oldest on the North American continent.
No actual mining has taken place since June 2007, when miners went on strike citing unsafe working conditions and poor worker health. The ongoing legal disputes have cost the mine’s owner, Grupo Mexico, more than $3.2bn.
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A breakthrough was made in February 2010 when Mexico’s federal court ruled in the company’s favour, giving Grupo Mexico permission to terminate the contracts of the striking miners and hire new staff to resume production.
In April 2010, several striking miners responded that they would destroy the mine site if the company tried to end their employment. In June 2010, the Mexican police removed all striking miners from the site, allowing Grupo Mexico to begin diagnostics and repair works.
In September 2010, fresh clashes erupted at the site between the dismissed union workers and contract workers. Two contract workers were seriously injured.
Geology
Copper and gold ores can be extracted from a porphyry copper deposit, which lies within the southern Cordilleran region.
In the Cananea district, the basement rocks are made up of pinal schist and associated granite intrusions that date back to the Precambrian age. A layer of Palaeozoic era sedimentary rocks sits above, representing an extension of the Cordilleran miogeosyncline belt and platform sequences. This is topped by Elenita formation (Triassic–Jurassic) and Henrietta formation (Jurassic) rocks, with intrusions from Jurrasic plutons.
Volcanic and plutonic rocks of the Mesa formation were formed during the Laramide orogeny, a period “basin and range” crust extensions that began in the Cretaceous period. This event is associated with most of the porphyry copper deposits in southwestern North America.
Mineralisation
Copper mineralisation in the Cananea region includes intruded stocks of rhyolite and quartz monzonite porphyry. The porphyry stocks include 2mm to 5mm quartz, feldspar and biotite phenocrysts in quartz and orthoclase, overprinted by intense sericitic alteration.
Hypogene copper mineralisation is present throughout the region as sulphides spread in breccias and stockworks. High-grade and low-tonnage ore bodies have also formed in regions with pre-Larimide rocks.
Mining and Processing
Ore from Cananea mine is extracted using conventional open pit mining techniques, and then refined further at an on-site concentrator. Ore with more than the mill cut-off grade (0.34%) is reduced to approximately half an inch in large rotating crushers. It is then ground to a fine powder in the ball and bar mills.
This leaves a finely ground powder, which is agitated in a mixture of water and reagents before it is delivered to the flotation cells. Air is pumped into this mix, causing it to froth and the copper mineral to float. Copper concentrates with an average 26.26% copper value are produced once the recovered copper is filtered and dried. It is then transported to the smelter at La Caridad by rail.
Advanced computer monitoring systems have been installed in the concentrator, crushing plant and flotation circuit to optimise their operation.
Copper ore graded between 0.34% and 0.15% is processed in an on-site leaching facility and two solvent extraction and electrowinning (SX/EW) plants. To achieve a recovery rate of 56%, a five-year cycle of leaching and resting is carried out. The SX/EW plants can produce up to 54,750t of copper cathodes per year.
