NEW YORK ( TheStreet) -- Rio Tinto Group ( RIO) said late Wednesday it's reached terms on a bid implementation agreement to acquire Australia's Riverside Mining Ltd. for A$3.9 billion.

U.K.-based Rio Tinto said the all-cash offer values Riverside shares at A$16 each, a consideration that represents a 46% premium to the weighted average price of Riverside's stock from Nov. 3 to Dec. 3.

"The acquisition of Riversdale is in line with our growth strategy of investing in, developing and operating large, long term, cost-competitive mines and businesses driven by the quality of each opportunity," said Doug Ritchie, the CEO of Rio Tinto's Energy group, in a statement.

Rio Tinto said it's already secured pre-bid agreements representing 14.9% of Riversdale's current outstanding stock. The deal provides for a break-up fee of A$37.8 million if the deal isn't ultimately consummated.

Shares of Rio Tinto closed Wednesday's trading on the New York Stock Exchange at $70.09, up 25 cents.

-- Written by Michael Baron in New York.

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