In fact, you have to look for the mouth hole of the studs. The only clue is a gold mine symbol on a single tree. There you have to climb the slope on the right, come to a small level and stand in front of the tunnels that meet in the mouth hole. You can follow the one on the left for a good 30 meters into the mountain, the one on the right is wall-high with water, so that you need rubber boots. In a good 20 meters the water seems to end and the tunnel continues.
Incidentally, the tunnel opens up the same copper ore layer "Yellow Gang" as the opposite copper mine "Hosenberg".