The first mines were veins of copper and other minerals. These were often rich in Copper, but they did not continue very far, so the small discoveries didn’t lead to big mines. They are very interesting to find because they are usually nice crystalline minerals. Veins form when the rock cracks and hot water solutions flow through the rock depositing minerals on the crack surfaces. Eventually the crack completely fills with deposited minerals.  Veins are common in the Keweenaw, and can be several feet thick.