Fire crews responded to a structure fire at early Thursday morning on Spyglass Drive in Ione. While enroute to assist other units on a separate incident, a fire crew from Mule Creek Fire Department observed a working structure fire in a new construction area of the Castle Oaks golf course. Upon arrival, fire crews found a 500 square foot maintenance building approximately 75% involved in fire. 18 fire fighters from the city of Ione Fire Department and Mule Creek Fire crews were able to quickly extinguish the fire. Damage was isolated to the maintenance building, which is used as the main water pumping station for the golf course irrigation system, and one portable toilet. The cause of the fire is currently under investigation by the City of Ione. There were no injuries. Damage costs are estimated at $100,000.