Authorities in Mexico said Monday they have closed part of a huge copper mine at the heart of a major chemical spill last month which prompted officials to turn off the water supply in several towns and forced dozens of schools to shut.

"Various irregularities" found at the Buenavista mine in the northwest of the country pose an "imminent risk" of more problems, said the prosecutor's office for environmental issues, known as PROFEPA.

On August 6 some 40,000 cubic meters (10.6 million gallons) of sulfuric acid used to dissolve copper from ore for processing leaked out of a holding tank at the Buenavista mine, which is one of the largest in the world.

The chemical turned a 60-kilometer (40-mile) stretch of the Sonora River orange, causing authorities to shut off the water supply to 20,000 people in seven towns. Around 80 schools were closed for a week.

The mine is owned by Latin American mining giant Grupo Mexico, which on Monday released a statement blaming the leak on heavy rains, although for the first time it also said another factor was a construction flaw in a pipe.

Monday's partial closure of the mine was actually the second. On Saturday another part of the mine had already been shut down.

On Friday authorities raided the mine looking for evidence that the leak stemmed from a problem with the pipe.

Grupo Mexico also operates a coal mine that was closed in 2006 after an explosion that left 65 miners trapped. Their bodies have never been recovered.

The Buenavista mine produces 200,000 tonnes of copper a year, and is seeking to increase annual output to 510,000 tonnes by 2016 with a $3.2 billion investment.

In the acid spill case, Mexican law limits the maximum fine to the equivalent of $224,000. But PROFEPA is presenting arguments to try to increase the fine to at least $3 million.