BHP Billiton and Rio Tinto have signed a binding joint venture agreement covering the entirety of both companies' Western Australian iron ore assets at Pilbara.The deal, which was flagged in June, still has to be approved by the European Commission and the Australian Competition and Consumer Commission.The combined operations will control about 350m managed tonnes a year of production capacity, which would make it the World's leading supplier, though the two companies have to market any output separately."Signing binding agreements brings us one step closer to unlocking the full production potential of our Pilbara iron ore assets and achieving substantial benefits for all our stakeholders," Rio chief executive Tom Albanese said.