Brace yourselves, shantymen:

I'm confident that the "Rio Grande" of the shanty is indeed the one between Texas and Mexico. The "golden sands" are plain yellow sand.

Why do I think so? Because there's no trace of the shanty "Away, Rio!" before the Mexican War. Because the Texas Revolution and - especially - the Mexican War made the Rio Grande newsworthy in the U.S. And because - brace yourselves - the so-called "18th Century gold rush" to Rio Grande do Sul, now entrenched in folkie folklore, may never have happened.

The reference books I've seen agree that "minor" gold deposits were discovered in Rio Grande do Sul as early as the 17th Century. However, they do not seem to have created any international "rush."

There really was a Brazilian gold rush in the 18th Century, but it wasn't in Rio Grande do Sul. It was in the state of Minais Gerais, hundreds of miles away.

So I see no evidence that "Away, Rio!" was sung earlier than the 1830s or (more likely) during or after the Mexican War of 1846-48.