Let’s set up a thought experiment: Suppose you had some material that ran one ounce per ton gold, which is generally considered high grade to fantastic grade depending on the circumstances.

When I teach people about finding gold, I often explain that it is helpful to think of any river or stream that carries gold as being something like a sluice box.

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.