(Adds new partial closure order from Mexico's environmental prosecutor)
MEXICO CITY, Sept 1 (Reuters) - Grupo Mexico, a metals miner and railroad operator, said on Monday that operations at its Buenavista mine were not affected by a search of its offices late last week by the federal prosecutor's office, part of an investigation into a toxic spill.
But later on Monday night, Mexico's environmental prosecutor Profepa said it had imposed a new partial order of closure on the mine, saying it had decided to shut the mine's leaching storage areas due to Grupo Mexico's failure to abide by applicable rules as well as "highly risky activities".
The order "limits" the mine's copper leaching activities, a chemical process by which metals like copper are separated from ore, but it was unclear how large an impact the order would have on the mine's operations.
Calls to both Grupo Mexico and Profepa were not immediately returned.
Profepa has previously said that the copper mine, one of the world's biggest, released 40,000 cubic meters of toxic mining acid into the Bacanuchi River in the northern state of Sonora last month.
The mine's leach solution yard is where the spill originated, Profepa said, and also ordered it partially shut citing "imminent risk to the environment".
"At no point did (the search) affect the functioning of the mine complex, which continues operating normally," Grupo Mexico , said in a statement on Monday morning, adding that the search which began on Friday concluded on Saturday.
Grupo Mexico is in the midst of a $3.4 billion expansion project at the mine, which has the largest proven copper reserves in the world. The expansion aims to boost production capacity to 1.3 million tonnes by 2017. (Additional reporting by David Alire Garcia; Reporting by Alexandra Alper and Veronica Gomez; Editing by Simon Gardner, Jeffrey Benkoe and Muralikumar Anantharaman)