Due to the method of gold extraction.
According to one of the most widespread theories, described by the 1st-century BC historian and geographer Strabo, the myth of the Golden Fleece originated from an ancient method of gold extraction used in the Caucasus. Locals would lower the skins of rams into gold-bearing mountain rivers, where tiny particles of gold carried by the current would gradually settle on them. Early Greek sailors could observe such 'gilded' skins in the Caucasus, and their accounts eventually transformed into the legend of the Golden Fleece.
However, it is possible that gold in this context is a common poetic device indicating the wondrous nature of an object or being possessing this characteristic (for example, the golden apples of the Hesperides, Aeneas's golden branch, etc.).
As is well known, the mythological Golden Fleece is the skin of a magical ram sent by the goddess Nephele to save her children Phrixus and Helle.
