The Frozen river is a silver mine placed in the Silver Peak mountains. The name comes from the fact that the main silver vein looks like a river that has been frozen over. The mine also houses a rare few bits of gold and other useful minerals. This is the main reason the mining guild has decided not to choose a dedicated gold mine as most of the gold is imported from the Tellurian mines, and Tellurians are better at delving gold anyway.
The Midgardian mining guild chooses a different silver mine every few years as their main mine, when the last is exhausted. This is the current main silver mine.