Tarzan and the Jewels of Opar by Edgar Rice Burroughs

Tarzan returns to Opar, the source of the gold where a lost colony of fabled Atlantis is located, in order to make good on some financial reverses he has recently suffered. While Atlantis itself sank beneath the waves thousands of years ago, the workers of Opar continued to mine all of the gold, which means there is a rather huge stockpile but which is now lost to the memory of the Oparians and only Tarzan knows its secret location.