* Work stoppage not seen as imminent * Workers to decide on first actions next Friday By Alexandra Ulmer SANTIAGO, March 15 Codelco's unionized workers threatened on Friday to stage a 24-hour work stoppage at all units of the world's top copper miner within 30 days to demand greater job security and pensions. Workers will start deciding what actions to launch next Friday, with the aim of eventually building up to a nationwide stoppage, union leader Ariel Rivero told Reuters. "We've taken a very firm resolution, ... we're not backtracking on the work stoppage," Raimundo Espinoza, head of the powerful Federation of Copper Workers umbrella union group, said to the cheers of boisterous workers. The group represents some 15,000 Codelco workers. Espinoza is also a member of Codelco's board. State-owned Codelco, which owns around 11 percent of the world's copper reserves, is seeking to propel its annual copper output from around 1.7 million tonnes to over 2 million tonnes by 2021, but is battling ageing mines, soaring costs and labor strife. Union leaders say they fear restructuring of Codelco's tired mines could lead to benefit cuts, layoffs and the sale of world-class red metal deposits that are key to the state's finances. In mid-2011, thousands of Codelco workers held their first national strike in nearly two decades, downing their tools for a day in what is an ongoing feud over the future of the company. While the specter of strikes in Chile, the world's top copper producer, has spooked investors, copper prices fell on Friday after the release of mixed U.S. economic data. Codelco was not immediately available for comment. WORKER-MANAGEMENT TENSIONS Several union leaders say they fear the conservative government of Sebastian Pinera is seeking to privatize the copper miner. They're also upset about an increase in contract workers, a trend they see undermining stable employment. Tensions between workers and Codelco's management, led by CEO Thomas Keller, a former executive at retailer Cencosud, and Chairman Gerardo Jofre, seen as close to billionaire Pinera, have sometimes run high. Earlier this week, Jofre said "it's bad to launch work stoppages as sport," drawing rebuke from unions.