Keep in mind that most gold provinces in EU3 actually produced silver.
I have also been wondering about those gold provinces though. I've been taught that in the late 1300's - 1400s the only reliable sources of silver in all of Europe were the silver mines at Srebrenica and Novo Brdo (in Bosnia and Serbia). This was a very big thing back then as many European silver mines had to close down or reduce production during the 1300's-1500's (Schwaz in the EU3 province of Tyrol is one of them, the silver mine at Kreminica, Slovakia another and Rammelsberg, Saxony another) leading to a large shortage of bullion (and giving Venice a virtual monopoly on making silver coins until the Ottomans conquered Bosnia and Serbia).
I'm by no means well read on the subject but from the overview's that I've been given in general I've never heard of the Slovenian gold/silver mines either. I'm kind of thinking they're atleast partially there to boost Austria/Venice as a balance measure.
A quick google search on Slovenian mining history does seem to indicate that other rare metals have been mined there though (
http://www.mineprofs.org/info/industry/SOMP-03-Industry-Slovenia-Bajzelj.pdf ) which could account for some of the mines?
Another gold province that seem dubious to me (though I haven't checked up on them) is the gold province in Georgia.
On the other hand: The most important sources of gold and silver in the Ottoman empire was supposedly southern Egypt, northern Anatolia (Gümüshane ) and Macedonia (Sidrekapsi). None of those have gold as a resource in the game despite feeding a quite large empire's need for coins.