Minmetals Resources, China's biggest metals trading firm, yesterday offered US$6.5 billion to buy Equinox Minerals, chasing the target company's copper assets in Zambia and Saudi Arabia.

Minmetals, which owns mining operations in Australia and Asia, said it would offer C$7 (US$7.26) per share for Equinox, a 23 percent premium to Equinox's close in Toronto last Friday of C$5.71. It would be China's fourth-biggest outbound M&A deal, according to Thomson Reuters data.

Equinox's Australian shares surged 29 percent to a record A$7.35 (US$7.63), topping the value of the Minmetals' offer on expectations a rival bid may emerge.

"It's game on now," said Ausbil Dexia Chief Executive Paul Xiradis, a shareholder in Equinox. "They'll be looking to defend their turf, and it may entice another party to come in as well, looking for quality assets such as those held by Equinox."

Minmetals' shares rose 2.4 percent to HK$6.72 (86 US cents).

Chief Executive Andrew Michelmore told Reuters the Equinox offer was Minmetals' best price, adding he was not considering increasing it.

"It fits into a strategy of building a leading international diversified base metals upstream business," he said in Hong Kong.

"It certainly fits in with the strategy in terms of growing the base metal size, particularly in terms of copper," said Michelmore, adding Minmetals would be the world's 14th-largest copper producer after the deal, from 30th now.

The offer is conditional on Equinox dropping its C$4.7 billion bid for Canada's Lundin Mining, which has been the subject of a separate takeover tussle between Equinox and Inmet Mining.

Investors said it was possible rival bidders could emerge for Equinox.