By Saliou Samb CONAKRY, Aug 4 (Reuters) - Guinea's president sacked one of his closest allies on Monday for the second time in four days, state media reported, deepening a power struggle that has become key to mining giant BHP Billiton's bid for rival Rio Tinto. According to a series of decrees broadcast on state media since Friday, ageing President Lansana Conte has sacked, promoted and again sacked Sam Mamady Soumah, long seen as one of his closest and most influential allies in government. Conte has ruled Guinea since seizing power in a 1984 coup, but his administration has become increasingly unpredictable in recent years, which many analysts attribute to power struggles between rival power blocs in his entourage. The latest game of ministerial musical chairs has acquired extra significance as Soumah had a central role in questioning the legality of the huge Simandou iron ore concession which Rio Tinto <RIO.L><RIO.AX> presents as central to its future growth. Rio Tinto says the Simandou deposit is the world's biggest undeveloped iron ore province with an estimated 2.25 billion tonnes of ore in Guinea. It says its $6 billion project would produce 70 million tonnes of ore in its first year. Rio Tinto has rejected a hostile $138 billion offer from BHP Billiton <BHP.AX><BLT.L>, saying the all-share bid undervalues its growth potential. But a letter Soumah wrote in June when he was secretary general in Conte's presidency -- a role with ministerial rank -- queried the validity of Rio Tinto's concession. The company said last Friday it had received a letter from Conte purporting to rescind its permit to mine Simandou, though the company maintains its concession remained legal and valid. Hours later Soumah was sacked from his post and appointed director of social security instead. A presidential decree broadcast on state radio late on Monday announced Soumah's dismissal as head of social security "for abandoning his post". It did not mention his promotion, announced in the meantime in a previous decree broadcast on state television on Sunday, as minister of state for presidential activities, but it said he was henceforth "at the disposal" of the ministry for civil service, implying he was stripped of all other duties. Monday's decree also sacked Idrissa Thiam -- a longstanding ally of both Conte and Soumah -- from his non-ministerial post as state protocol director, for "serious fault". It gave no further details. Guinea, a former French colony, is home to around one third of the world's known reserves of bauxite, the ore used to make aluminium, and also has reserves of gold, diamonds, uranium and other minerals. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com) (Writing by Alistair Thomson)
