US investment bank Goldman Sachs has turned bullish on mining companies Kazakhmys and First Quantum Minerals.Both have been upgraded from "neutral" to '"buy", largely because of Goldman's belief in a positive outlook on copper prices.In regards to First Quantum, the Goldman analysts assert that the company has a series of superior growth projects in its pipeline and has a history of project delivery on less capital-intense expenditures than most rivals.In regards to Kazakhmys, the stock now has a much more attractive valuation in Goldman's view, although elevated costs and lack of growth have affected its industrial position.In contrast, the broker has downgraded Chilean copper miner Antofagasta from "buy" to "neutral" because it sees greater upside potential for other companies in the sector.Shares of Kazakhmys are now down 1.49% while Antofagasta is down 2.02%.S.B.