Treasures are "auto-produced" by certain buildings: gold and silver mines, fur trade posts, tobacco and sugar plantations, etc. All of them become available with the "Resource Usage" technology. Most of them require the "Governor's House" to be built first, and also they require the corresponding resource to be within the big fat cross of the town. (Though it does not need to be hooked up with a road.)Treasures
So do treasures appear only in cities? I was under the impression they materialized in random locations, requiring units to search the fog to find and carry them to a city.