At least 20 miners were still trapped in Freeport-McMoRan’s Big Gossan Mine as of May 15 and it was confirmed that four workers had died as a result of a tunnel collapse at the mine in the highlands of Papua in eastern Indonesia.

The Ministry of Energy and Mining Resources said the company had suspended underground mining operations as rescuers try to reach 23 miners trapped in a small training room hundreds of meters below the surface. It was also reported that 13 miners had been rescued.

The collapse occurred at a deep underground training facility at the Big Gossan Mine, more than a kilometer away from Freeport's giant Grasberg Mine, the world’s third-largest openpit copper mine and the largest gold mine. Grasberg lies at more than 4,000 m (13,000 ft) above sea level, not far from snow-covered peaks in Papua province on the Indonesian half of New Guinea island.

Thamrin Sihite, the director general of minerals and coal at the energy ministry, said workers had been gathered in the underground training room when the roof collapsed, caving in most of the room. He said the room was built 15 years ago and has been in use since 2000, The Wall Street Journal reported.

Mining fatalities at major companies are relatively rare in Indonesia, although each year dozens of illegal and independent miners are killed in accidents, often in landslides.

Copper prices have so far failed to be affected by the news, and instead are taking their lead from macroeconomic cues from China, Europe and the U.S.

While a protracted outage at Grasberg would tighten the fundamentals of the global copper market, Singapore-based traders said they are so far unconcerned by the incident at the mine site.

Freeport Indonesia, a subsidiary of the Arizona-based Freeport-McMoRan Copper & Gold Inc. said before the suspension of activity that it didn't expect the accident “to have a material effect on mining and milling operations or development activities.”