* Chile port strike had slammed copper exports * Planned Codelco strike would have "marginal" effect-CEO * Codelco seeking to boost 2013 output to around 1.7 mln T SANTIAGO, April 8 (Reuters) - Chilean state miner Codelco has restarted its copper shipments following the end of an extended port strike, though it will take roughly a month for normal rhythm to resume, chief executive Thomas Keller said on Monday. Most ports in Chile returned to normal operations on Saturday after an extended strike that slammed the No. 1 copper producer's exports. Key ports in the mineral-rich North are operational. Codelco had said nearly 60,000 tonnes of its copper had been blocked by the stoppage, equivalent to around $500 million in revenue. Keller told reporters ahead of the CESCO/CRU copper conference in Santiago this week that he does not expect a relevant impact on copper production due to potential labor strife during an electoral year. Unions representing workers at Codelco, the world's biggest copper producer, and mining companies including BHP Billiton said they will announce the date of a planned nationwide strike later on Monday. The impact of the planned strike will be "marginal," Keller said. Chile's hotly contested presidential election in November is seen galvanizing labor unrest as unions seek to make their issues heard. Angamos port workers walked off the job about three weeks ago to seek a 30-minute lunch break and other benefits. That dispute over relatively minor issues escalated into a crisis for export-dependent Chile. UPBEAT ON 2013 OUTPUT Codelco aims to propel this year's output above the disappointing 1.647 million tonnes it produced in 2012, as its new Ministro Hales mine starts operations in December, Keller added. Increased production at Codelco, which produces roughly 11 percent of the world's red metal, and other mines could lead the global copper market to tilt into a "moderate" surplus this year, he said. Copper prices will remain volatile this year, Keller forecast. Copper demand from top metals consumer China should grow between 5 and 7 percent this year, he added. Keller said there has been some relevant destocking in bonded warehouses, putting the figure at around 150,000 tonnes.