The road there is a wild and long one and their account is excellent- it starts as the leave Creel and wind down through San Rafael, leaving the pavement:
We left the pavement behind in San Rafael, not long after leaving Creel. Many of the simple dirt roads that eventually took us all the way to El Fuerte were built to give access to the mines in the region and most were not on the detailed maps in our Gia Roji road atlas. The locals and the mine employees all know exactly where they’re going so no one ever bothered putting up any signs either.
See the whole story here as well as many other adventures in Mexico…and all over.