Commodity trading giant Glencore and Mercuria, one of the world's biggest independent energy traders, have signed a deal on the sale of Mercuria's 15 percent stake in South Africa's Optimum Coal Holdings (OCH) , sources close to the deal said on Thursday.

The deal includes Mercuria's marketing agreement for coal from the Koornfontein mine and future output from the Optimum mine.

Glencore will begin marketing OCH's coal in June next year, the sources said.

Glencore will shortly make a formal offer to all remaining OCH shareholders, they said.

Glencore was not immediately available for comment.

(Reporting by Jacqueline Cowhig; Editing by Anthony Barker)