Bob,
I've been working in a gravel pit in Ohio for 35 years, and there are specific places where gold accumulates.
If you have access to the crushing and washing equipment, be very careful. Even a simple conveyor belt can rip your arm out of its socket in a split second. Be sure the plant is locked out before you begin.
The best places to look in the machinery, are where water and material are held as part of the washing process. Sand screws are probably the best place, because most glacialy deposited, and most gold in general is sand size and finer.
A sand screw is a giant screw, in a long inclined tub of water, where sand is added to the low end, agitated and washed by the screws action, then dumped on a conveyor belt to be stacked. The screw has to fit loosely in the tub so the heaviest material works its way past the screw and is trapped in the low end. I made a sluice box that hooks onto the drain, then I use a water hose to flush the material through.
It works fairly well, but it is hard to regulate the ammount of material and the riffles are full of sand too often.
Even sampling from around the sand screw drains can show alot of gold. In freezing temperatures, the screws are drained daily to protect from freezing, and the heavies will build up on the ground close to the drains.....just look for the black sand.
I've got several places i pick up gold at the pit, let me know if you are interested in hearing more.