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Indonesia to announce Freeport acquisition structure - source

* Details of Freeport deal expected later on Thursday

* Heads of agreement to be signed at 0900 GMT - ministry

* Structure of deal, shareholding to be announced - source (Adds company response, context)

By Fergus Jensen

JAKARTA, July 12 (Reuters) - Indonesia is set to announce later on Thursday the details of a deal to acquire a majority stake in the local unit of U.S. copper miner Freeport McMoRan Inc, a source with knowledge of the matter said.

The announcement is expected to include the share price, share holding and structure of the deal, which would give Indonesia's state mining company, PT Inalum, a majority holding in Grasberg, the world's second biggest copper mine.

Finance Minister Sri Mulyani Indrawati is scheduled to sign a heads of agreement with PT Freeport Indonesia at 4 p.m. (0900 GMT) on Thursday, a ministry spokesman said.

Representatives from Freeport and Inalum, along with the ministers of state-owned enterprises, energy and mineral resources, and the environment are expected to attend the signing.

A U.S.-based spokesman for Freeport could not be reached for comment.

Phoenix, Arizona-based Freeport has been in negotiations with Indonesia to secure long-term operating rights at Grasberg after the government introduced new rules last year aimed at giving Jakarta greater control over the nation's resources.

As part of the proposed deal, Inalum was expected to acquire the 40 percent participating interest in Grasberg held by mining giant Rio Tinto .

Efforts to finalise a deal have been overshadowed by concerns over the environmental impact of the project in the eastern Indonesian province of Papua. (Reporting by Fergus Jensen, Maikel Jefriando and Gayatri Suroyo; additional reporting by Susan Taylor in TORONTO; editing by Richard Pullin)