Molycorp Inc said the U.S. Securities and Exchange Commission (SEC) has completed an investigation against the rare earths producer and has not recommended any enforcement action.
Molycorp shares were up 10.5 percent at $6.20 in after-market trading on Thursday.
The SEC had started investigating the accuracy of Molycorp's public disclosures last August.
Rare earths, produced primarily in China, are essential for making high-tech products such as smartphones, tablets, hybrid vehicles and automotive catalysts.
Molycorp's acquisition of Canada's Neo Materials last year gave the miner access to processing capabilities and patents, making it the only major North American player with the ability to both mine and process rare earths.
Rare earths prices skyrocketed in 2010 and early 2011 as China clamped down on exports, prompting a global rush to secure supplies. China has eased export controls since then and prices have declined.
Molycorp shares closed at $5.61 on the New York Stock Exchange on Thursday.
(Reporting by Shounak Dasgupta in Bangalore; Editing by Joyjeet Das)