UPDATE 1-Union at Doe Run Peru says to strike April 20

(Adds background on production halt, comment from union leader, paragraphs 2-7

LIMA, April 20 (Reuters) - Workers at Doe Run Peru's sprawling La Oroya smelter will go on strike April 20 to demand the company restart all operations, a union official said on Monday.

Production at the plant was halted in March, after banks, worried about low metals prices, cut the company's credit lines and strangled its ability to buy concentrates.

The unit of U.S.-based Renco Group was pulled back from the brink of collapse some three weeks ago, when it received two new credit lines totaling $175 million from a group of miners that it buys concentrates from.

But operations at Doe Run Peru, the country's No. 4 minerals exporter, have barely resumed. Just 25 percent of workers are back on the job, the union said.

"We have decided that starting at midnight (0500 GMT), there will be no activity at La Oroya," union leader Luis Castillo told reporters.

Job losses have started to pile up in the mining sector in Latin America as companies scale back production or delay new projects. Countries like Peru, a major metals exporter, are among the hardest hit.

La Oroya is located in the mountains, about 108 miles (174 km) east of Lima, Peru's capital. (Reporting by Jean Luis Arce; Writing by Dana Ford)