How to say a professional thank you?

A director at my organisation has recommended me for a promotion and is organising a significant pay rise for me. I have been working very hard but it's still unexpected and very flattering. This sort of thing just doesn't happen (well at least not to me)! He's leaving the company at the end of the month and I want to tell him how appreciative I am that he recognised my potential. I only found out about his recommendations through my manager in my recent performance review though and the director hasn't brought it up with me. If anyone has any suggestions on the proper way to say thank you in this sort of situation I'd be very grateful. We work in an open plan office and no one other than my manager knows about the promotion yet so I'm finding it hard to figure out how I can bring this up with the director without everyone hearing that I'm going to be promoted. 

Thanks everyone!