RIO DE JANEIRO, Sept 3 (Reuters) - Brazilian miner Mineracao Rio do Norte (MRN), backed by Glencore and Rio Tinto, said on Tuesday it had received an initial license from environmental agency Ibama to extend the useful life of its bauxite mining operations in northern Brazil. (Reporting by Marta Nogueira in Rio de Janeiro; Writing by Andre Romani; Editing by Kylie Madry)