Investec values South32, a collection of alumina, manganese, nickel and silver mining assets being discarded by the world's biggest mining company, at $12bn, according to a note from analysts Hunter Hillcoat and Marc Elliott. That is 35 percent lower than Investec's September estimate of $18.6bn and compares with a March valuation of $14.3bn. South32 is due to start trading next month.