The Great Monarch Mine can be found near the head of the Monarch Pass just outside the settlement of Garfield in Chaffee County, Colorado. As far as I can tell, it is now closed down but its main product was lead ore while in operation. If you happen to be skiing at Monarch Mountain, as I have been for the last few days, you’ll see the mine on the left just before the ski area entrance as you drive up from Salida.
For the technically inclined...