The $US3.5 billion ($A3.19 billion) expansion of BHP Billiton's Samarco iron ore pellet joint venture in Brazil suggests a bullish long-term outlook for the steel-making commodity, analysts say.

The $US3.5 billion ($A3.19 billion) expansion of BHP Billiton's Samarco iron ore pellet joint venture in Brazil suggests a bullish long-term outlook for the steel-making commodity, analysts say.