It was pretty common in those days to dispose of chips that were no longer in use by dumping them in wet concrete for new construction. I don't think the construction of the main tower was the only instance when Dunes chips were buried. For instance, the building for the "slots" section called The Oasis in 1979 and the second hotel tower in the 1970's.

Here's a link to a great series on the Dunes history, this section on the happenings in the 70's and later.

http://classiclasvegas.squarespace.com/a-brief-history-of-the-strip/?currentPage=26

I don't know which particular foundation this chip came from but there was a lot of concrete poured at the Dunes after 1965.