Workers last week voted to reject the company's final wage offer and approved a strike, but either party can call for a 5-day period of government mediation by end of day Monday to forestall the walk-off.

Union leaders told members in the memo that it had asked the company to state its position on several key aspects of the contract offer by 10:00 a.m. local time (1400 GMT) on Monday, after which workers would decide whether or not to call for government mediation.

"The company proposed that mediation ... be solicited by both parties, to which the union replied that would depend on a positive response to the points we have raised," the union said in the communication.

BHP did not immediately respond to request for comment on Monday.

Barring further negotiation between the company and the union, a strike could begin as early as Tuesday.

Last year, a 44-day walk-off at the mine jolted global copper markets and docked economic growth in the South American country, which is the world's top copper producer.

BHP's final contract offer two weeks ago included an approximately $18,000 signing bonus, and a 1.5-percent boost to salaries with increases for inflation.

But the union had asked for a signing bonus around twice that offered by the company, and had requested a salary increase of 5 percent, leaving a wide gap between the two sides.

(The story was refiled to add the dropped word "to" in the first paragraph)

(Reporting by Antonio de la Jara; Editing by Nick Zieminski)

By Antonio De la Jara