Why do you think the Roman government stopped coining in silver? And which metal did it begin to use for the production of coinage?
The Roman government stopped coining in silver because the Spanish silver mines were exhausted. As a result, the government ran out of sufficient stocks of the metal to support a stable coinage in silver. This caused the monetary system of the late empire to break with the silver-based currencies of the first three centuries.
In place of silver, the empire began using gold for the production of coinage. Constantine founded the new monetary system on gold, and there were vast amounts of this metal in circulation throughout late antiquity.
Explanation
The answer is based directly on the textbook context which states that Spanish silver mines were exhausted and the government lacked sufficient silver stocks. Constantine then established a gold-based monetary system.