The Plumas-Eureka State Park consist of the Plumas-Eureka (Mohawk) Stamp Mill, the museum built in the old miners barracks, the Eureka Tunnell, nearby Jamisin City and mine, and the town of Johnsville. Tours are offered but the state website does not mention if it includes tours of the stamp mill.
Mining in the area began in 1851 and 36 miners formed the Eureka Mining Co. This is supposedly where snow skiing began in North America. In the winter miners would ride the ore tram up to the higher tunnels and ski down. Several tunnels appear on USGS Topo in the hills above the mill.
The adit of the Eureka Tunnel and the stamp mill both appear to be sealed off to visitors. Check out the gallery below.