(Adds name of Petrobras interim board chairman in second paragraph, and other resignation details)
By Stephen Eisenhammer and Rodrigo Viga Gaier
RIO DE JANEIRO, Sept 14 (Reuters) - Murilo Ferreira will take a leave of absence as chairman of state-run oil firm Petrobras, turning his full attention to his job as chief executive officer of Vale SA as the mining company grapples with a downturn in the sector.
Luiz Nelson Guedes de Carvalho, a professor of accounting at the University of São Paulo and head of the Petrobras' audit committee, was named interim head of the board, Petrobras said in a statement.
Petroleo Brasileiro SA, as the company is formally known, did not say why Ferreira is leaving. His leave is scheduled to end Nov. 30. A company source told Reuters he had requested time off to focus on Vale as it deals with falling iron ore prices and China's slowdown.
At the same time, though, Ferreira's hand-picked board alternate, the person supposed to replace him when he cannot make meetings, took a leave of absence that will coincide with Ferreira's.
Ferreira, 62, who has been CEO of Vale since 2011, was appointed chairman of Petrobras in April as the government sought to revive investor confidence after a corruption scandal resulted in billions of dollars in writedowns.
At the time some mining executives criticized the move, saying Vale was going through a difficult patch and needed the full focus of its CEO. Ferreira said the double job would only eat into his "leisure time."
But the world's largest producer of iron ore has continued to struggle. Vale shares have lost nearly 40 percent in the past 12 months and touched a decade low in August.
Analysts expect the price of iron ore .IO62-CNI=SI, the main steelmaking ingredient, to remain weak for years after falling more than half since last year.
Despite being one of the world's lowest-cost producers of the mineral, Vale finds itself in a tight spot as its investments are well above those of Australian rivals BHP Billiton Plc and Rio Tinto Plc.
Since taking the helm of the Petrobras board, Ferreira has not always agreed with other board members.
Last month the board approved the sale of the company's BR Distribuidora SA fuel-distribution unit. Ferreira was one of two members to vote against the proposal.
Another potential point of tension is fuel-price policy. Reuters reported in August, citing sources, that Petrobras had still not presented a plan to bring domestic fuel prices in line with world levels. In the past Petrobras has imported fuel to sell at a loss, a serious burden on the company. (Reporting by Stephen Eisenhammer and Jeb Blount; Editing by Frances Kerry and Lisa Shumaker)