It was most likely a tournament chip but its possible that it was a promotional chip (someone with knowledge of the Treasure Bay might now). Its a nice looking chip regardless.

Example, the left is a promotional chip from the Sunset Station. This was given away as part of a drawing and used on the tables similar to cash value chips (however only exchanged for the cash equiv if you win the bet). The chip on the right was used in the poker room for tournaments (when they had a poker room, and when they could pull together enough people for a tournament).

In this case they were kind enough to write "Tournament Chip" on the chip but there are other examples as well between the promotional and the tournament.

(images linked from the chipguide)