SANTIAGO, Aug 1 (Reuters) -

Chilean copper giant Codelco aims to reach five trapped

workers at its flagship El Teniente mine within the next 12

hours, following a collapse that killed one person, it said on

Friday.

The workers have been trapped for 18 hours without any

contact with teams outside after a 4.2 magnitude tremor on

Thursday rocked the new Andesita unit of the massive mine, which

is the world's largest underground copper deposit.

The incident triggered aftershocks that have made it

impossible for rescuers to reach the area where workers are

trapped, El Teniente's General Manager Andres Music told a press

conference.

"The tunnels are closed, they've collapsed. There's no

possibility of even radio communication," he said.

Codelco expected aftershocks to diminish within the next

12 hours, allowing the miner to send remotely operated equipment

- similar to robots or drones - to help workers clear material

blocking the tunnels, Music said.

Codelco is investigating whether the incident was due to

natural causes in the earthquake-prone country, or mining

activity, he said.

"The event we recorded yesterday is one of the largest

events - if not the largest - that the El Teniente mine has

experienced in decades," he said.

Nearly 100 people were on site to aid in the rescue,

Music said, noting that the next 48 hours would be critical. He

said neither explosives nor drilling had caused the accident.

Music did not address how the incident would affect

Codelco's output in producing areas of El Teniente. Codelco is

due to report its financial results for the first half of the

year on Friday, as previously scheduled.

Andesita is one of Codelco's newest projects at its

flagship El Teniente complex, and was due to begin production in

the second quarter of this year.

El Teniente, which opened in 1905, produced 356,000 metric tons

of copper last year. The complex spans more than 4,500

kilometers of tunnels and underground galleries in the Andes

mountains, about 75 kilometers southeast of Chile's capital

Santiago.

In addition to one worker who died in the collapse, another

nine people suffered injuries, Codelco said.