FACTBOX: Recent major base metals mergers

(Reuters) - Rio Tinto's (RIO.AX) (RIO.L) shares surged on Wednesday on market talk that an improved offer had prompted the firm to open its books to suitor BHP Billiton (BHP.AX) (BLT.L) and media reports which said Rio had signaled a price at which it would sell out.

Rio, which has spurned an earlier offer from BHP, said it had nothing to report in response to rumors that it had opened its books on the basis of a sweetened offer.

A merger between the two companies would create the world's largest producer of aluminum and copper.

The potential combination would give total primary aluminum output of around 5.5 million tonnes based on last year's estimates. This compares with nearest rival and current top producer United Company RUSAL, which produced an actual 4.2 million tonnes of the metal in 2007.

The merged company would also surpass Chile's state-owned Codelco to gain top spot in copper mining. In 2006, Codelco mined 1.832 million tonnes of the metal. A combined BHP Billiton/Rio Tinto would have produced just over 2.0 million tonnes, according to industry consultants CRU Analysis.

Following are details of earlier merger moves.

2008

Jan 21 - Brazilian mining giant Vale said it was in talks to buy Swiss-based diversified miner Xstrata in a deal analysts said could top $100 billion.

Vale (VALE5.SA) RIO.N, the world's largest iron ore producer, said market conditions would determine whether it launched its takeover bid, the biggest by a Brazilian firm.

A deal would make it the world's top nickel producer with around one-fifth of total mined and refined output. It would also reinforce Xstrata's position as the world's fourth largest copper miner.

Jan 17 - Anglo American Plc (AAL.L) says it is in talks to buy control of two Brazilian iron ore projects owned by MMX Mineracao (MMXM3.SA)XMM.TO for around $5.5 billion to boost exposure to a commodity with buoyant prospects.

2007

Dec 21 - Russia's United Company RUSAL will buy a strategic stake in Norilsk Nickel (GMKN.MM) from Mikhail Prokhorov's Onexim Group as it seeks to build a globally competitive Russian metals and mining major. On Tuesday, one of RUSAL's shareholders told Vedomosti business daily the company was discussing a full buyout or a merger.

Dec 21 - BHP Billiton will have to decide by February 6 whether to make a formal bid for rival Rio Tinto or walk away under UK regulatory rules. BHP announced its plan on November 8 to take over third-ranked Rio and forge a mega-mining group with a market capitalization of around $350 billion.

Dec 17 - Australia's Zinifex Ltd ZFX.AX, the world No.3 zinc producer, offers up to A$775 million in cash for nickel miner Allegiance Mining AGM.AX, the latest move in a hectic round of consolidation among Australian miners.

Dec 7 - China's state-owned Sinosteel makes a $1 billion cash takeover proposal for Australian iron ore miner Midwest Corp MIS.AX, way above a rival all-share offer from Murchison Metals Ltd (MMX.AX), as Chinese companies push overseas to secure resource assets.