First Quantum increased its offer last month to $72 per Inmet share, half in cash, half in stock, for a total offer worth about $5.1 billion.
It was the second time First Quantum boosted its cash and stock bid for Inmet, which rejected a $4.9 billion stock and cash offer in November. At the time, Inmet said the "highly conditional offer" was not in the best interest of its shareholders.
"We are taking the significant step of making this proposal directly to Inmet shareholders and requesting their support," said First Quantum CEO and chairman Philip Pascall in a statement Wednesday.
"I believe this combination creates a uniquely attractive, large, liquid and high growth copper focused company. I see the combination of the two companies creating not only a new global leader in copper, but also one of the most exciting, 'must-own' companies in the global mining sector."
Inmet said it will carefully evaluate the offer and urged shareholders to wait until a special committee completes its work and a formal recommendation is made.
"We will carefully consider, among other things, whether or not the First Quantum offer adequately compensates our shareholders for Inmet's full value, including the untapped exploration and expansion potential of Cobre Panama and the substantial increase in valuation our shareholders will receive once Cobre Panama reaches commercial production," Inmet chairman David Beatty said.
"We will also consider the potential risks associated with receiving First Quantum shares as consideration."
First Quantum said the offer will be open for acceptance until 5 p.m. ET on Feb. 14 unless extended or withdrawn.
First Quantum said last month that despite the rejection of its earlier bid, it has been approached by a number of Inmet's shareholders expressing interest. It said combining the two companies would have the potential to produce more than 1.3 million tonnes of copper annually by 2018.
Inmet shares were up 34 cents at $72.59 on the Toronto Stock Exchange while First Quantum shares were down 37 cents at $21.17.