Heavy black smoke soars from a fire in a secondary crusher unit at the Freeport McMoRan Sierra Mine Complex on Duval Mine Road in Green Valley. Firefighters from various agencies battled the blaze. There were no injuries.

Rick Wiley / Arizona Daily Star

A fire that broke out Monday morning at Freeport McMoRan’s Sierrita Mine in Green Valley burned for about five hours, sending a column of thick black smoke into the sky before it was extinguished about 4 p.m.

No injuries were reported.

The fire started at around 11 a.m. in a secondary ore crusher unit at Sierrita, Freeport’s Eric Kinneberg said.

Units from the Green Valley, Rural/Metro and Drexel Heights fire districts, as well as the Tucson Fire Department, battled the two-alarm blaze.

Damage to the facilities was being assessed and the cause of the fire is under investigation, Kinneberg said, declining to elaborate.

L.T. Pratt, public information officer for the Green Valley Fire District, said the fire mainly burned rubber insulation on ducts used to gather dust from the crushing process, making it hard to reach.

“It was a very difficult fire,” Pratt said. “It was pretty much contained in the ductwork.”

Firefighters ended up extinguishing the fire by chasing it down both ends of the ductwork, he said, adding that initial indications were that the fire was caused by a mechanical malfunction.

About 700 workers are employed at Sierrita, where Freeport mines and processes copper as well as molybdenum, a metal used to strengthen steel; and rhenium, a rare metal used in jet engines.

Freeport cut about 500 jobs at Sierrita in 2015, citing low metals prices, and announced it would close the entire operation before reversing that decision in mid-2016.

Sierrita was first worked as an underground mine beginning in 1907 and open-pit development began in 1957, according to Freeport, which became owner when it acquired Phelps Dodge Corp. in 2007.

Contact senior reporter David Wichner at dwichner@tucson.com or 573-4181. On Twitter: @dwichner