These myths came about as Europeans were given various gifts by the native population. Gold was amongst these gifts. When asked if they had any more gold the native populations gave it away freely as they did not value it the same way as the Europeans did. For the natives it was just a shiny metal with no real use except as a shiny metal. For the Europeans gold was a founding part of the entire economy and they assumed, rather stupidly, it would be integral to the economy of other societies too. This led to the idea that because the native populations were just giving away gold it must be plentiful.

"Where is more?"

"Try such and such city in the jungle?"

The idea that gold had value stemmed from The Mediterranean region