I’m fairly sure there are no canon statements about this.
I’m not aware of anything in the books, Pottermore or an interview statements which explicitly describes wizarding coins as being made of pure gold or silver.
There are statements on their size, mass and value relative to Muggle currencies, but I have nothing about their composition.
I would be happily corrected on that.
Personally, I doubt you could have currency made of pure silver or gold. They’re both very malleable (soft) metals, and I think any coin made only of those metals would be too soft to be practical. Of course, that doesn’t stop the magical folk applying charms to the coins to stop them falling apart. But I am not a chemist or a coin expert.
ETA: And Martha points out in the comments that sometimes the malleability of gold made it a very desirable substance for coins. Like I said, I am not a coin expert, so I don’t know when pure gold coins dropped out of general usage. A description of a Quidditch Match in 1269 tells us that coins called Galleons were in use in the mid-thirteenth century:
According to Madam Rabnott, Bragge brought a caged Snidget to the match and told the assembled players that he would award one hundred and fifty Galleons (equivalent to over a million Galleons today) to the player who caught it during the course of the game.
– Quidditch Through the Ages, Chapter 4 (The Arrival of the Golden Snitch)
As with some Muggle coins, this is probably a different physical form to the modern Galleon, with merely the name carried down. Perhaps (as per Martha’s comment) early Galleons were pure gold, but I still suspect this might not be the case with the modern coin. /edit
The HP Lexicon has a good essay on wizard money that attempts to pin down the size of Galleons and Sickles, which assumes different levels of purity. That suggests that gold or silver coins are possible, but obviously HPL isn’t canon and that doesn’t prove that they are.