From an trip learning how to silver mine in South America:
We saw the how they would take the ore and refine and separate the minerals so to get to the real silver. Later we saw how silver in the mountain runs in veins along the sides of the walls.
At the smelter the rock taken from the mines needs to be weighed and analyzed for quantities of silver, tin, zinc and lead.
Of course thats how its found in one area of our real world. As you can see multiple metals can be found together. Who's to say how the world of Eaxia is laid out, especially since magic is a part we have as well.
Good catch though - I've seen silver veins in Trolls but haven't done a ton of mining. I assume metal levels are based on monster levels where they are found to some extent. Maybe that silver is really gold, hard to tell in that poor light in the pit.