if Teck excercises its back in right for 75% do they have to spend the full ~$360M? Or could they, for instance, spend $50M and then back out a year later?

Most of the terms are in the Salazar agreement here:

https://docs.google.com/file/d/0B1opssTdj4BuY2FkMmQzMDctNzdiZC00Njg1LTg3YzUtNTY2MzhjODRlMGFl/edit?hl=en_US&authkey=CPLqmtQO

The langauge is generally interpreted that Teck does not earn its backin percentage until the specified multiple of costs has been invested in the mine, 4X for 75%

There is another agreement that specifies a 4 year period to production in order to earn their interest

This entire agreement however, according to Paragraph 15.1, muct be superceded by another which will allow for renegotiating all of the included terms, within 60 days of the BFS delivery