Current estimates of silver reserves are about 530,000 tonnes, while gold is about 55,000 tonnes.
Wikipedia suggests the total is about triple your estimate, but who cares?
The coins are WAY too big (measuring with the heaviest in both current and historic circulation), and are undervalued by a lot.
As I said before, there are comparable coins: https://en.wikipedia.org/wiki/1933_Double_Eagle
That would come in at about the size of your typical dragon hoard coin (if there is such a thing) and was 90% pure gold at about 13 coins to the pound. That's pretty freaking big.
Ironically, taken as 1 gold piece, that coin wouldn't have been enough to cover half the price of a pickax during the California gold rush 85 years before it was minted. A pair of boots would have cost 5gp. http://michaellamarr.com/grprices.html
You are trying to align a fantasy world's currency system to mining production and historical prices on Earth. What's the point? It's fantasy. I'm pretty sure the number of owlbears in D&D is unrealistically high compared to medieval europe too...
