It would NOT be an annual outlay of 900 million troy ounces per year, because metal coins are used year after year after year. More, there would still be other coinage in circulation and in fact paper money would still be widely used (but would be payable in gold).

I've long thought that the monetary standard for any country is actually based in the labor used to produce whatever is arbitrarily decided upon as the basis of the country's currency. Gold was chosen because it is reasonably scarce and yet also easily recognizable and (within limits) easily recoverable. This means that someone has to take the trouble to find it, to recognize it for what it is, and to recover it for use. Panning the gold out of the dirt in my backyard (yes, there is gold in your backyard too) isn't worth doing, but if there were a vein of it or placer pockets then yes, it might be economic.

Personally, I suggest that the entire world economy be based upon something more convenient and useful, like potatoes or sugar beets.