I just picked up the Dwarves' Mine and there are no copper-coated pieces in it! They are only reddish-brown.
I noticed that in the Burlington Massachusetts Lego Store's display model of the Troll's Mountain Fortress that pieces like the king's crown were not copper either (though it looks like in all the promotional material that the king's crown is copper).
Is this is a fluke, or have the dwarves delved too deeply and depleted the supply of copper?