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Rumoured bid target Xstrata said the resource estimate for the Collahuasi copper mine in Chile has been raised 28%.

The upgraded resource, to 5.19 billion tonnes at an average grade of 0.83% copper, is a result of last year's drilling programme. It comes amid weekend press reports that Brazil's Vale was considering a bid.

Shares rose 146p, or 4.2% in early trading to 3,646 pence. Xstrata and Anglo American hold 44% each of the mine, and a group of Japanese companies headed by Mitsui & Co own the remaining 12%.

The results 'confirm Collahuasi's position as one of the world's great copper deposits, with enormous potential to expand its resource base further,' said Xstrata Copper chief executive Charlie Sartain.