I don't know where you heard that but it doesn't sound right. There's nothing that would imply that from the mythology. The ram named Chrysomallos, was the offspring of Poseidon raping a woman whom he had turned into a sheep. The ram was born golden and later sacrificed. This is according to Ovid and maybe there are other stories, but I don't know any that would imply the origin is from the use of fleece to mine gold.