UPDATE 1-Mexico copper workers hold strike despite threat

(Adds Grupo Mexico to meet union paragraph 12)

MEXICO CITY, Dec 4 (Reuters) - Miners at a giant Mexican copper pit will not go back to work despite company threats to close the mine which has been crippled by a four-month strike, the union boss said on Tuesday.

Grupo Mexico's (GMEXICOB.MX: Quote, Profile, Research, Stock Buzz) Cananea copper mine, close to the U.S. border, has been closed since workers walked off the job on July 30 in a dispute the company says costs around $2.8 million a day in lost sales.

Union leader Napoleon Gomez, who has lived in Canada for nearly two years to avoid corruption charges, said via a blurry video conference on Tuesday the strike would go on until union demands to improve health and safety standards are met.

"We won't rest until there is justice," said Gomez, who left Mexico in 2006 when warrants for his arrest were issued amid accusations of corruption.

In a full page advertisement published in Mexican newspapers on Monday, Grupo Mexico threatened to close Cananea and three smaller affected mines if a solution was not found.

Grupo Mexico, one of the world's top copper producers, says it has met worker demands. It considers the strike an illegal union tactic to clear Gomez's name.

The company and the union are locked in a court battle to determine the legality of the stoppage.

A tired-looking Gomez said the company cannot legally close a mine while a strike is still on. However, company lawyer Cristina Rocha said a shutdown would be legal if it is necessary to shut it down.