Chilean copper miner Antofagasta Plc (ANTO.L: Quote) posted a 68 percent slide in first-quarter core earnings on Thursday after prices collapsed due to the global downturn, but costs also declined.

The firm's shares, which have gained 39 percent this year, fell 1.2 pecent to 591.5 pence by 0937 GMT, slightly better than a 1.7 percent loss in the UK mining index.

The London-listed company -- which owns three copper mines in Chile -- said earnings before interest, tax, depreciation and amortisation (EBITDA) for the first three months of the year declined to $264.6 million from $825.9 million a year ago.

First-quarter revenue fell by 52 percent to $544.5 million.

The figures were better than two analysts had forecast. Canaccord Adams had expected EBITDA of $152 million and Exane BNP Paribas forecast $211 million.

Antofagasta said benchmark copper prices fell by slightly more than half, but the decline was tempered by provisional pricing adjustments, which added $113.5 million.

Under provisional pricing, an initial price when the metal is produced is adjusted during the shipping period. Even though average prices were much lower than a year ago, they rose during the quarter.

Cash costs, excluding by-product credits, fell 13.4 percent.

On April 30, Antofagasta released first-quarter copper production figures, showing a decline of 2.4 percent to 111,900 tonnes and reiterated that full-year output was due to fall 9.4 percent to 433,000 tonnes.

Antofagasta said it had cash and equivalents of $3.08 billion at end-March and total group borrowings were $723 million.

(Reporting by Eric Onstad; Editing by Julie Crust and Simon Jessop)

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