Any pack will do. If you are in good shape and can carry the weight, a strongly made, heavier backpack would be optimal, make a GoRuck, Hazard 4, or even some of the Maxpedition packs. But if you are not travelling far on foot or don't want to deal with all that weight, a lighter pack would be the way to go, just remember they are liable to wear out faster.
A lot will depend on what you plan to carry in it. Standard gold pans of 14" will have a problem fitting inside and will have to be strapped or otherwise attached to the outside. If you want a pan that fits in the pack, you are probably looking at a 10" pan.
Don't forget room for your tools and lunch!
Hope that helped a little.