Cia. Minera Antamina SA, operator of the world's largest combined copper and zinc mine, said a study undertaken as part of the mine's expansion showed the site may hold 28 percent more recoverable metal than previously estimated.

    Resources increased to 1.52 billion tons of minerals with ore grades of 0.93 percent copper, 0.51 percent zinc and 10.9 grams of silver per ton, Lima-based Antamina said today in an e- mailed statement, citing a 105,000-meter drilling study conducted last year.

    A pre-feasibility study for the mine's expansion is scheduled to be completed this year, the company said.

    Melbourne-based BHP Billiton Ltd. owns Antamina with Zug, Switzerland-based Xstrata Plc, Vancouver-based Teck Cominco Ltd. and Tokyo-based Mitsubishi Corp.