NAFTA
If I understand the arrangement correctly, Molycorp (a US company) is selling their product through a subsidiary corporation, Neo Material Technologies (a Canadian company) that ultimately sells the products to Chinese manufacturers.
To my knowledge the North American Free Trade Agreement (NAFTA) would prevent tariff to be imposed on the product that is sent from the US to Canada. Additionally, the US has no ability to impose Tariffs upon goods that are exported from Canada, regardless of their destination.
So it looks like Molycorp will sell their minerals to buyers outside of the US because NAFTA and other such forward looking policies have already exported most of the manufacturing facilities and jobs. And the US will not be able to control the exportation of our natural resources or even receive compensation through tariffs because of these same trade policies.
Good thinking.