Xstrata Plc, the world's fourth- largest producer of the metal, said workers started a strike at the Kidd Creek copper plant in Ontario after talks with a labor union over a new work contract broke down.

The Kidd facility, home to the world's deepest copper and zinc mine, produced 63,768 metric tons of copper cathode and 19,488 tons of copper in concentrate in the six months ended June 30, according to a report from Zug, Switzerland-based Xstrata. The company announced a C$121 million ($114 million) expansion of the Kidd mine on Aug. 6 to extend mining operations at the site.

"It is impossible for us to say at this stage how long the strike will last,'' Thompson Hickey, general manager of the Kidd Metallurgical site, said in a Marketwire statement today.

A collective work contract expired at midnight Sept. 30 and 96 percent of voting members of the union had supported a strike if an accord couldn't be reached. Talks broke down on concern over ''benefits, contract language including seniority and skilled trades related issues,'' the union said today.

"We have worked for days trying to negotiate a fair collective agreement with Xstrata but the company did not meet the members' expectations,'' the Canadian Auto Workers union said in a statement on its Web site. Workers at the plant should report for picket duties, the union said.

Union workers, who earlier rejected an Xstrata contract offer on Sept. 24, last went on a one-month strike in October 2005 before agreeing on the current three-year contract.

The Kidd Metallurgical facilities, which include a concentrator, a copper smelter and refinery, are 27 kilometers (17 miles) from the Kidd mine site in Timmins, Ontario. Xstrata acquired the mine when it bought Falconbridge Ltd. in 2006.