Shares in BHP Billiton and Rio Tinto have dropped as the mining giants enter the next stage of merging their Pilbara iron ore assets, expected to deliver more than $US10 billion in savings.

Shares in BHP Billiton and Rio Tinto have dropped as the mining giants enter the next stage of merging their Pilbara iron ore assets, expected to deliver more than $US10 billion in savings.