* To idle Northshore mine from Dec. 1 through end Q1
* United Taconite mine to also remain idle through Q1
* Shares fall as much as 8.2 percent (Adds job cut details, updates shares)
Nov 17 (Reuters) - Coal and iron ore producer Cliffs Natural Resources Inc said it would temporarily halt operations and cut 450 jobs at its Northshore iron ore mine in Minnesota by Dec. 1, aiming to lower costs at a time when miners battle weak prices and demand.
Cliffs has already temporarily idled operations at its United Taconite mine in Minnesota in August and said on Tuesday it expected both mines to remain idle through the first quarter of 2016.
Northshore employs about 540 people. Cliffs said it would retain 90 employees through January and only 70 employees after that for as long as the mine remained idle.
Cliffs, whose shares fell as much as 8.2 percent on Tuesday, said it expects that idling the two mines will cost it $9 million per month.
Cliffs and other U.S. miners have been hit by a drop in demand from steel mills and weak iron ore prices due to excess supply from big miners such as Vale SA, Rio Tinto Plc and BHP Billiton Plc.
"The historic high tonnage of foreign steel dumped into the U.S. continues to negatively impact the steel production levels of our domestic customers," Cliffs Chief Executive Lourenco Goncalves said on Tuesday.
U.S. steel companies in June had filed a complaint with the U.S. government over cheaper imports of corrosion-resistant steel from China, India, Italy, South Korea and Taiwan.
Goncalves said Cliffs will immediately ramp up production by restarting idled facilities "as soon as the unfairly traded steel problem subsides and domestic steel production recovers to normal levels".
The company said inventory at the Northshore and United Taconite mines were adequate to meet current demand.
Cliffs will continue to operate the Hibbing Taconite mine in Minnesota and the Tilden and Empire mines in Michigan.
The company's shares were down 8.2 percent at $2.47 in late morning trading. The stock had fallen 62.3 percent so far this year through Monday. (Reporting by Shubhankar Chakravorty in Bengaluru; Editing by Anil D'Silva and by Savio D'Souza)