I doubt the correlation with US natural gas prices is...

I doubt the correlation with US natural gas prices is significant, over such a short time period quite likely just co-incidence. Here's a comparison between BHP, RIO, VALE over the time since they made the acquisition. Perhaps Petrohawk caused a slight drag, on the other hand rosier outlook for Aluminium might be giving RIO a bit of a lift. Vale has under-performed them both.



The main factor in all of their share price will continue to be market sentiment towards China, and China dominated commod markets (copper seems the standout correlation). BHP may be oriented a little more towards the US and energy but this might be only of use in a pairs trade as overall direction is still going to come from China.

I'm also not keen on this idea of looking for "value" in resource stocks. There used to be a maxim that resource stocks should be bought on high PEs, and sold on low. That was negated a bit by this "stronger for longer" cycle, but the idea that it is commodity price momentum/outlook - not past earnings - that drives valuations, still holds true.