As a non-geologist, it seems plausible to me that gold that was somehow subsumed into the mantle in New Zealand could migrate to Iceland in the span of thousands of years. As Sméagol pointed out, though, gold coming up in the magma would be more diffuse and not create an identifiable layer on the surface of the lava rock. You're right, though, he's hardly a reliable narrator when it comes to His Precious, so that could be a misdirect. And perhaps there's some attribute of magically endowed gold that causes it to behave in unexpected ways. I searched Google Scholar, but couldn't find any peer-reviewed studies on the physical properties of magic rings.