Gold was also the heaviest known element until the middle of the 19th century, which means that all impurities swim at the top and so it is easy to purify simply by melting it. Plus you can assay it by a touchstone. So there are a lot properties that make Gold the best metal for coins.
This only applies to impurities that don't dissolve in, or alloy with gold. For example, silver and copper cannot be removed from gold simply by melting because they form solution alloys that don't segregate in the molten state. Some solution alloys of gold include crown gold (silver, copper, gold) and white gold (palladium, nickel, gold).
