U.S. firm High Power Exploration Inc (HPX), founded by mining billionaire Robert Friedland, said that it had signed a letter of intent with Liberia's government and Guma Africa Group to develop rail and road projects linking it with Guinea. The infrastructure projects, known as the Liberty Corridor, are expected to support the West African region's connection to world markets and are estimated to cost between $3 billion to $5 billion, HPX said in a statement. Guma Africa is led by South African businessman Robert Gumede. The planned corridor will have a new heavy-duty railroad connecting the Nimba district of Guinea…