When in doubt, always check with the city's Parks and Recreation Department. Most are open to metal detecting unless notice is given by a sign, city ordinance, law enforcement official, or school employee. You should always check with the school office first. And it is always best to have the permission in writing. Don't even think about it.
Permission is always the best way.
Learn proper digging techniques and teach them to others whenever you can. Pick up trash as you search and cover your holes so that the next detectorist won't find the area closed to metal detectors. Unique maps covering gold areas of Butte County are available from Big Ten Gold Maps Mines and Locations Mines in Butte County as listed in the U.S. Geological Survey GNIS Database. This is not, however, all mines in Butte County. For example, GNIS does not list the Last Chance Mine in Magalia where a 12-hour rescue took place in 1994.