I don't mean to give you a hard time, Phil (anyone who mentions Larry "Wild Man" Fischer and Nick Drake in the same post has got a permanent friend in me, no matter how crazy you might turn out to be), but I listened to both versions on YouTube, and worked through the chords,(I turn out to be fairly crazy, too, though), and, they are in C minor.

Pete was playing from an Am position, capoed up (I think to the 5th fret, because his 12 is tuned down a full step, as was once common practice) but it was/is in C minor.

That provocative descending bass line was beaten to death in pop/folk/ersatz jazz tunes for years after "16 Tons" was a big hit for Tennesee Ernie Ford. It is a descending counterpoint line, and the chords are built on those descending notes. If one was so inclined, one could create a perfectly fine chord progression for the same melody that didn't outline that descending counterpoint line.

More provocative than the descending bass line, though, was the fact that Pete stood there on a Sunday night in front of America, and sang this altogether too thinly veiled allegory about the Vietnam war, knowing that there would be nowhere to hide after he finished it, because it was pretty likely that "The Big Fool" himself was watching.

There was a lot more to performing that song than just learning the chord sequence.