Try Sloans lake in Denver we were still finding silver in 2006. Get a map and try the older parks in the Englewood area. There's a park off Santa Fe just a little north of Evans. Don't forget to work into the ponds in the parks. It was a bonanza when the drought hit and the water levels were low. Most all the schools aren't fenced off and a lot have parks adjoining. Where are you located? I can probably dig up my notes if you need better directions.
South of Denver is Sedalia where you can take the road into the Rampart mountains. Tons of campsites both old and new. In Golden there is a MD shop Gold N Detectors if memory serves me right. Real nice helpful folks there.
Not real positive about most, only been to a couple out in Pike Nat forest, and some that were stumbled upon. Best I can advise you there is to check with the owner of the property, or the forest rangers. Give a call to Gold N Detectors, they can give you better up to date info. Been awhile since I moved out of state.