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Diamond output at Rio Tinto's Zimbabwe unit more than doubled to 324,000 carats last year, the local minority shareholder in the venture said on Friday.

RioZim Limited, which holds a 22 percent stake in the Murowa mine in southern Zimbabwe, said in a statement production rose to 324,000 carats from 141,000 in 2010. It did not give further details.

Murowa is planning a $300 million expansion to lift output to 1.8 million carats but has delayed the project due to a law that seeks to force foreign-owned firms, including mines, to cede 51 percent shares to locals. Rio Tinto controls 78 percent.

Rio Tinto told Zimbabwe's government last October that it had agreed to give up 51 percent stake in Murowa to comply with the economic empowerment law.

The heavily criticised law is aimed mainly at mining firms and banks operating in the resource-rich country, which is in a fragile recovery after a decade of contraction.

Rio Tinto, the world's third-largest miner, effectively invited bids on Tuesday for its diamond business, including the Zimbabwean operation, and joined BHP Billiton in a retreat from an industry that has lost its sparkle for mining majors. - Reuters