Alexander the Great's gold = 2,500 tons +

This amount of gold was discovered by Alexander after he conquered Persepolis.
Where did the Persians find their gold that Alexander inherited ?
The nearest source of gold is the Tien Shan Gold Belt that extends from Uzbekistan, through Tajikistan / Kyrgyzstan into China. This belt has the largest concentration of gold in the world after the South African Witwatersrand deposits. Some of the valleys in Tajikistan have alluvial gold, such as the rich Darvaz deposit on the north bank of the Oxus river.
The Persian Empire was extremely rich in gold reserves and thus able to finance its expansions and its dealings with other tribes. Expedition to conquer Greece ? No problem, the money was there!
Later on, the Bactrian Greeks with their prosperous "one thousand cities" had a thriving Empire that extended into the fertile Ferghana Valley and managed to reach and trade with the Chinese for the first time probably around 220 B.C. The Chinese marveled at their "heavenly horses", their manufacturing abilities and love for wine. They called the Greeks "Dayuan" or the Great Ionians. In return, the Greeks adopted methods of metal casting and the use of a new metal in coinage : Nickel, which was in use in China at the time and was called "white copper". A direct result of this, the Silk Road was developed in the 1st century B.C. and it passed through this already opened road of communications: The Ferghana Valley and the city of Marakanda or Samarkand.
The picture above is from the museum at Urumqi, China depicting a Greek soldier from a 3rd cent. B.C. burial site.