Looking at the exceptions to the basic rules is sort of an advanced prospecting discussion, but the readers of ICMJs Prospecting and Mining Journal are plenty able to handle it.

I have found gold more than 200 feet above the river. It was not unusual for nuggets of an ounce or more to be found in these deposits, but fine gold and flakes are much more common.

Fortunately, there are products on the market that can address these problems and make a small-scale mining operation feasible.