"Who Just Bought Half A Billion Dollars Of Gold Futures?"

I'm guessing it was a short cover but either way, cover or outright long it's a pretty big chunk. Gold is decisively higher at $1294.40, up $21.70. Top tick on the most active contract: $1295.90.
From ZeroHedge:

Presented with little comment aside to note the surge in gold since Yellen gave markets the all-clear yesterday. It seems someone decided the open this morning was an opportune time to take on half a billion dollars of gold exposure...

And from Kitco:
Where are the Stops? Thursday, June 19: Gold and Silver

Below are today's likely price locations of buy and sell stop orders for the active Comex gold and silver futures markets. The asterisks (**) denote the most critical stop order placement level of the day (or likely where the heaviest concentration of stop orders are placed on this day).

See below a detailed explanation of stop orders and why knowing, beforehand, where they are likely located can be beneficial to a trader.

August Gold	Buy Stops	Sell Stops
	$1,289.00	$1,276.20
	**$1,300.00	**$1,266.50
	$1,309.40	$1,258.00
	$1,315.70	$1,250.10
July Silver	Buy Stops	Sell Stops
	$20.10	$19.83
	$20.25	**$19.665
	**$20.43	$19.50
	$20.50	$19.435

Stop Orders Defined...

...MORE