Erm... The mint doesn't supply gold. Miners supply gold.
The coin has basically no value as currency, so it's not like, say, a Honda where the finished product is worth more than the pile of glass and steel and aluminum and such that it started as. If it were, then just putting the constituent raw materials in a big box with an H on the side would net you a sale as big as a car.
The mint just puts it in a box. The gold is just as valuable before the mint stamps it, so if the unstruck gold migrates to another part of the market, supply is not reduced.
