UPDATE 1-Colombia coal workers to go on strike at Cerrejon-union

* A strike had to be called before Friday cut-off date

* Union and Cerrejon discussed compensation and benefits

* Coal sector faces other problems in major producers

BOGOTA, Feb 7 (Reuters) - Workers at Colombia's largest coal exporting company Cerrejon will go on strike on Thursday, a union leader said, after the two sides failed to agree during last minute talks on wages and benefits.

"At 3 p.m. (2000 GMT), zero hour begins," Orlando Cuello of the Sintracarbon union said by telephone.

The company was not immediately available to comment.

A strike at Cerrejon will add to problems in the Andean country's coal industry, which is facing an environmental inquiry into Drummond, the second largest exporter, and a payment dispute at a Goldman Sachs Group Inc affiliate.

In total, about 85 percent of Colombia's daily coal production would be shut off.

During the last wage talks in 2011, Cerrejon and the union were able to reach agreement. The last time Cerrejon had a strike was in the 1990s.

Laborers at Cerrejon, a joint venture between BHP Billiton , Anglo American and Xstrata, voted on Jan. 29 to strike. By law, however, they cannot walk out on the first or last of a 10-day window, which ends on Friday.

Ten years of U.S.-backed military operations against leftist rebel groups have opened up large swathes of territory to new investment and helped secure areas where exploration and production were already happening.

Like other commodity-exporting countries, Colombia has faced increased environmental and social demands, which some analysts say have replaced guerrilla violence as the main risk for mining investors in the world's fourth-largest coal exporter.