The 16:9 HD master of Prometheus is a strange beast (no pun intended). I only caught the ending and made a quick recording of it (off the channel FX in the US). Unfortunately, it not so easy to say its crop or open matte, since many shots are cropped but others are totally recomposed. For example:

Cable broadcast/BD

All these shots appear to have the sides cut (barring a little elbow room in the last one). I thought initially that it was just the CGI shots that were cropped from 2.35 to 16:9 but I found live shots that were cropped and some CGI shots opened up. So I thought the whole thing was cropped.

Nope, there are many shots that are opened up but not just a simple opening of the top and bottom from the digital captured, they are totally recomposed. Some have more on the top and bottom but lose on the sides or some variation thereof:

Cable broadcast/BD

You can see that the recomposing is all over the place. Add a little of the bottom frame here, lose the right side of the frame there. What's clear is it wasn't random, it was very specific. Its kind of like a S35 re-composition.

What I don't know was whether the Imax was like this or not. They said they opened it up for the Imax digital but that may not be that simple. I also don't know if all HD masters are like this.