Well, I have been told that you can't do it and in most cases that is true, but there is an exception that has worked for me.
What makes it possible is that fine gold 'bunches' up on or under rocks in dry washes during the spring runoff from mountain deposits.
I have used a Surfmaster PI detector to locate find gold.
Method: Detect in the washes. When you get a faint signal, carefully pick up the rock and wipe it off IN A ZIPLOCK BAG.
Also scoop a small amount of material from the spot where the rock was. Now recheck with the detector.
The signal will be gone and the rock will no longer give a reading.
I work the contents of the ziplock thru my Gold Magic Wheel.
Yea, it's a lot of work, but it pays off in gold.
Give it a try. It worked for me at the base of the Chocolate Mts.