South32 Ltd (LSE:S32, ASX:S32, OTC:SHTLF) is Citi's preferred mining stock as the bank becomes more bullish on copper and aluminium prices, while Glencore PLC (LSE:GLEN) is its favoured way to play the copper theme among the major diversified miners. The US bank has raised its long-term copper forecasts and now expects prices to reach $15,000 a tonne within the next year, versus a current LME price below $13,800.