Image source: Getty Images.What: Shares of DRDGOLD (NYSE: DRD), a South African gold miner, tumbled as much as 16% during Tuesday's trading session, following a downgrade from JPMorgan Chase.So what: The downgrade from JPMorgan may not come as a huge shock, considering that DRDGOLD shares had rallied by more than 70% since the Brexit vote was announced a little more than two weeks ago.