Global miner Rio Tinto Ltd (ASX: RIO) is planning to invest another $US790 million ($A866 million) to grow its iron ore operations in Western Australia's Pilbara region.

The company aims to expand the annual capacity of iron operations in the Pilbara to 330 million tonnes, according to London-based Rio Tinto in a statement on Wednesday.

Rio Tinto's Pilbara expansion is focused on lifting capacity at its Cape Lambert port from the 80 million tonnes at present to 180 million tonnes by 2016.

The miner will build a new 1.8km jetty and a four berth wharf that will run parallel to the present jetty.

Among its latest investment are $US375 million on marine works related to the construction of the new wharf and $US415 million for the procurement of long lead items such as pile and marine structure, on shore earth works and machines.

Rio Tinto had already declared $US200 million worth of dredging works for the Cape Lambert expansion last month.