Grupo Mexico declared force majeure at its La Caridad copper smelter in northern Mexico after a labour dispute with the National Miners Union.
In a statement, Grupo Mexico's subsidiary, Southern Copper, said supporters of the union have been blocking the transport of workers to the refinery complex at the mine since 2 September, forcing the smelter to operate below capacity.
Southern Copper said the company does not expect the disruption to have an economic impact, reports Reuters.
The La Caridad mine, the biggest copper producer in Mexico, produced around 139,600t of smelter copper and 117,100t of refined copper in 2009.
La Caridad's open pit operations are, however, running normally, the company said.