Barrick Gold Corp said on Tuesday that production at its Bulyanhulu gold mine in Tanzania has been halted due to what it termed an "illegal" strike, and that it has fired about half of the mine's work force.

Barrick, the world's top gold producer, said about 1,000 of the mine's 1,971 workers walked off the job on Thursday, halting milling, and reducing mining operations to a trickle.

Some areas of the mine are still functioning, Barrick said, but the operation has produced almost no gold since the strike began.

"We have some mining, but very little mining of ore is occurring. And as a result of the mill being down primarily, we haven't produced any gold to speak of in the last four or five days," said Barrick spokesman Vince Borg.

Borg said the union representing workers had called the strike without informing management and despite having reached a mediated agreement with the company on terms for labor discussions.

Some workers did report for work. Barrick said it was not sure whether other workers who had not been scheduled to work over the past week would also join the strike.

Some operations, including blasting and support services, are continuing, but milling is not operational, Borg said.

He said it was too soon assess risks to production and cash costs targets, nor how long the interruption would last.

"The outcome is dependent on mediation and the government representatives have tried to get the union to the table," Borg said.

He said that if remaining employees do not show up, they will also be terminated, but that fired employees will have the chance to reapply for their jobs.

The union could not be reached, but the Nigerian daily newspaper This Day reported in its online version that negotiations between workers and management broke down over salaries, working conditions, medical care and other issues.

Barrick would not comment on the specifics of the negotiations.

The mine, which Barrick acquired in 1999, produced 330,000 ounces of gold last year at a total cash cost of $339 an ounce. It had proven and probable reserves of 11.2 million ounces as of the end of last year.