In 518 AD, when the emperor Anastasius died, wikipedia says the imperial treasury had 160 tons of the stuff.
To compare, China supposedly just built the world's largest gold vault, 2200 ton capacity, according to current-day gold ads on AM talk radio.
Was it left over from Alexander?
That’s a lot of dough. The largest single score was when Alexander seized the Persian Empire treasury, which was filled with the loot of the Middle East, Egypt, and a good bit of Asia. By the time The Great got up into what is now the ‘stans and needed more troops, the call for (mostly) Greek mercenaries had resulted in a couple hundred thousand arriving just in time for the spring campaign. Alexander had wintered in Balkh and healed up from serious wounds. Prior to that, his enemies couldn’t attack the city and he couldn’t venture out; he split the new forces into four armies of 50,000 each, and sent them up four river basins to corner and conquer any and all comers. That nickname wasn’t a joke. :’)
Because of the relatively limited amount of gold, its form changed repeatedly. A bunch of gold (like the hundreds of various shapes and forms kept in Solomon’s temple) would get hammered out and/or recast into other uses, including “books”, ritual collars, rings, scarabs (in Egypt), other jewelry, chains, etc.
When coin became commonplace (the invention of coin is generally attributed to the Kingdom of Lydia) it served the purpose of the medium of exchange, and also advertised the power of the name and image on the face of the coin.
Currency would get recycled by a successor, or conqueror, or rival, or neighbor, so many if not most of the ancient coins ceased to exist (and were probably minted in small numbers to begin with).
So, to make a short story long, Alexander’s big payday wound up recirculated throughout his empire, and that of his successors, and their successors, etc, but it’s probably impossible to know which gold was and wasn’t at any one place and again in another, because any large ancient hoards were sacked, carried off, and sacked again.
The biggest haul during the Roman Empire was Trajan’s conquest of Dacia. The Dacians had gold mines and piles of the stuff, and the oldest known examples of worked gold appears to be from that area.