I'm probably half way along in my search for old Spanish townsites East of the Mississippi. There are not a lot of them but they are there so somebody was doing something (I and others suspect they looked for and found gold).
Up in Canada some of the old Spanish mill sites still have stuff ~ they'd set up a mill and a run from a lake. Then they'd cut segments for segmented grinding wheels from local stone. They'd set those in a frame.
You could grind grain (provided by the Indians) and ore in the same mill (though at different times).
Some of the grain would go back to the Indians, some would go in the mash, and then you'd use a rather primitive retort (looks like a beaker on its side) to do your distillation. That would end up in the bellies of the Indians as well, and the Spaniard would get his supplies for his mining operation that way.
There are signs of comparable operations in the Eastern US as well.
One thing about a well thought out enough hunk of civilization, it will find a way to fit in ~
New Jersey's shoreline is paved over with these town sites. I suspect those were the Catholic Dutch getting a headstart on everybody else ~ they've just disappeared in our history, but the Jamestown records indicate a population of 20,000 people North of the Potomac by the early 1620s, and I don't think that's incidental. At the same time 60,000 English (et al) settlers went on to die early in Jamestown ~ usually of malaria, etc. Even Jamestown's original site disappeared into the James River ~ it was relocated just a few decades ago!
I suspect they weren’t deliberately set loose on the barrier islands from Assateague and Chincoteague on down to Ocracoke Island. They’re still there, known as Banker Ponies in NC. Their origin has long been attributed to shipwrecks, there are shipwrecks including Spanish Galleons going back to the earliest records kept.