The offer had been set to expire Monday night, but was extended as First Quantum said some 61.45 per cent of Inmet's shares had been tendered to its bid.
The Inmet board has rejected the $72 per share offer — half in cash and half in stock — as inadequate.
However, even though the Inmet board urged shareholders to reject the deal, it waived application of a shareholder rights plan that would have thwarted the unsolicited bid after it failed to come up with a rival offer.
Philip Pascall, CEO and chairman of Vancouver, based First Quantum, declared himself "delighted" with the response to the offer.
"We have varied our offer such that the minimum tender condition will now be satisfied if more than 50 per cent of the Inmet shares have been tendered at the revised expiry time of the offer," Pascall added.
"Accordingly, with all regulatory approvals already received, it is our expectation that we will be in a position to complete the offer and begin taking up and paying for shares shortly following the expiry of the offer on March 21, 2013."
If more than two thirds of outstanding shares have been deposited by the new deadline, First Quantum will implement a subsequent transaction to acquire the balance of the Inmet shares, something that could take several months, the company said Tuesday.
On the Toronto Stock Exchange, Inmet shares were up $1.27 at $69.90 in early trading Tuesday, while First Quantum was up a penny at $20.65.
Inmet, with mining operations in Turkey, Spain and Finland, also owns 80 per cent of the Cobre Panama project through Minera Panama SA and the remainder is owned by Korea Panama Mining Corp.
The total cost of Cobre Panama is estimated at $6.2 billion, including $1.4 billion to be funded by Korea Panama Mining and $4.8 billion through Inmet.
Last year, Inmet signed a deal with a joint venture led by SNC-Lavalin Inc. (TSX:SNC) for the first phase of the planning and construction of a process plant at the project.
First Quantum has said the combination of the two companies would create one of the world's fastest growing copper miners with the potential to produce more than 1.3 million tonnes of copper annually by 2018.
The bid by First Quantum follows the sale of its operations in the Democratic Republic of Congo to Kazakhstani miner Eurasian Natural Resources Corp. for $1.25 billion.
First Quantum had written off the value of the assets in 2010 after its operations in Congo were nationalized by the government of the mineral-rich central African country.