Ruling 2133
If silver dust from a mine is sold for pure silver, or gold dust from a mine is sold for pure gold, the transaction is invalid unless it is known that, for example, the amount of silver dust is equivalent to the amount of pure silver. However, as explained previously, there is no problem in selling silver dust for gold, or gold dust for silver.
-Ayatullah Sistani, Practical Laws of Islam, Buying and Selling