The obsession with Gold is not natural to man, it is a habit mankind picked up from the visitors from the 12th Planet. What about Gold makes it so cherished? Platinum is as tarnish free, silver and copper as malleable, and yet Gold is accepted as a metal more precious than others, more desirable, a metal to die for. This attitude toward Gold, passed as a heritage from generation to generation, was impressed on mankind from those who had to have Gold, and did not hesitate to kill for it.
Where mankind uses Gold for adornment or as a medium of exchange, the visitors from the 12th Planet were collecting Gold for survival. Their home planet, on its long orbit out in space, is subjected to atmospheric abrasion the Earth and the other planets on more sedate orbits around the Sun do not suffer. Consider the rapid path past the Sun made by the 12th Planet, when acting like a comet. It moves from one side of the Solar System to the other in 3 months, a fast track indeed. The 12th Planet losses atmosphere here and there, on a regular basis, and where this can be rebuild from its copious oceans, being basically a water planet, certain elements become depleted. Molecules in the atmosphere, containing Gold, are necessary to retain the heat and light the planet generates, to keep the heat and light, essentially, bouncing back to the surface, as without these Gold based molecules the planet dims and cools.
Thus, the visitors from the 12th Planet came to Earth on a search, a mining mission, and were intent on raping the Earth of its relatively abundant Gold and using its primitive hominoids, humans, as slaves to do so. It was impressed upon mankind early that Gold was the ruler's due, and death fell to he who hoarded or withheld it. El Dorado, the lost City of Gold, was a staging point where the space ships used as shuttles were loaded and sent aloft at the appropriate time. Since this time only arrived once every 3,657 years, on average, a lot of Gold stacked up. Long after the visitors from the 12th Planet left the Earth, having been quarantined, the humans who had heard the stories from those who made deliveries to El Dorado searched for this City of Gold. They search still. However, the ruins, when discovered as they have been repeatedly, are not recognized as there is no gold. Why would these visitors, so desperate for what was essentially a life giving metal, leave any behind?