SANTIAGO, May 29 (Reuters) - Three unions of Chilean state-run miner Codelco's giant Chuquicamata mine rejected the company's final offer for a new contract on Wednesday and approved a strike, according to a union leader.

After a two-day voting period, 80% of members of unions 1, 2 and 3 declined Codelco's final offer, the official told Reuters. The three unions represent nearly 3,000 workers and make up over 80% of the mine's workforce.

However, the unions or the company can still request government mediation which could potentially avert a strike at the mine.

Codelco, the world's largest copper producer, had presented its final offer in contract negotiations with the unions on May 24. However, the proposed bonuses were about $5,500 less per worker compared with an earlier offer which had been rejected by the unions.

After the unions voted to strike, Codelco said in a statement that it aims to reach an agreement that "reconciles the business conditions, the future sustainability of the division and the legitimate expectations of the workers."

Codelco is seeking to transform the 100-year-old open-pit deposit at Chuquicamata into an underground mine. The plan is part of a 10-year, $39 billion overhaul of Codelco's key operations as it seeks to maintain output despite rapidly falling ore grades at its deposits. (Reporting by Fabian Cambero; writing by Cassandra Garrison; Editing by Lisa Shumaker)