A Few Facts About Gold
Most gold machines will find gold nuggets weighing less than 1 grain (.064 grams). There are several models which can detect tiny 1/10 grain pieces under good conditions—so small as to be almost invisible. The overwhelming majority of gold in the ground is small stuff. Large nuggets are very rare: they are found more by luck than by skill. You have to be finding the small stuff (especially the 1 to 10 grain size range) for gold prospecting with a metal detector to pay off.
A metal detector will detect the tiniest gold only within about 2 inches (5 cm) of the search coil. A pennyweight (24 grains, 1.56 grams) nugget can usually be detected 4-8 inches (10-20 cm) deep depending on the machine, ground conditions and user skill. High iron mineralization content will greatly reduce depth.
A one Troy ounce (31 grams) nugget can usually be detected at more than 8 inches (20 cm) depth, but again this depends on many variables.
“Flour gold”, such as is recovered by panning, cannot be detected by a metal detector unless there is a fairly substantial mass of it. The reason is that the electrical resistance of loose powdered metal is much higher than that of solid metal.
The distribution of gold size varies by location, by previous prospecting history, and by your own detecting capability. In the Western U.S. and Western Canada, the distribution tends to be medium size although it varies greatly by location. In the Eastern U.S. and Canada the gold tends to be small, although large nuggets are not unheard of. Alaska has a reputation for larger gold than both the Western U.S. and Canada. Australia is famous for producing large gold, but most areas there have already been worked over with metal detectors. The size distribution is now smaller overall than it used to be because so much of the large gold has already been found. In gold producing areas around the world where there is little or no prior history of use of metal detectors, the size distribution of gold tends toward large.
On a particular site, gold will not be scattered uniformly. It will tend to be clustered in “pockets” or “hot spots”. You may be searching in a productive area and yet go the entire day without finding any gold at all, and the next day you may hit a “hot spot” and dig gold totaling half an ounce or more. When you find a “hot spot” make sure you get it all: gold is where you find it. …Some prospectors haul the dirt from a hot spot to a place where it can be panned or sluiced out. This way you get a lot of gold that can’t be found with the metal detector by itself.