Just because they conquered the planet does not mean all the gold is automatically going to come out and go through the process of mining, smelting and purification automatically. It is very interesting because the discovery of gold is not even known, they were used by most ancient civilizations.

It's like this. If we run low on water on planet earth, we know that oxygen and hydrogen creates water. The process is pretty complicated but it can be done. If we are able to genetically create a plant that can convert oxygen and hydrogen to water directly then it's the same kind of principle that the aliens could have used in order to create a system so that valuable gold is funneled into the system. They may not have mated with the apes maybe simple genetic transformation from not so intelligent primates into a very intelligent human being with the introduction of some alien genes.