Detecting The Different Geological Types
Placer deposits are usually best worked by hydraulic methods if water is available. Modern “dry washers” and portable recycled water systems are much better nowadays than they used to be, and are effective for working dry placer. Placer gold tends to be smaller than hillslope gold because the process of being tumbled along a stream bed breaks larger nuggets into smaller ones, meanwhile wearing down nuggets of all sizes.
Alluvial (water flow) processes tend to concentrate what gold is present at specific spots, so if you hit a “hot spot” (paystreak) you may do very well. If you’re searching for gold in stream placer, learn to “read the stream” to figure out where heavy sediments such as gold will tend to accumulate.
Placer gold is almost always associated with magnetite black sand. VLF metal detectors tolerate black sand in moderate concentration, but in heavy black sand they lose sensitivity, become noisy, and “overload”. Under conditions of very heavy black sand the user must lift the search coil up away from the black sand in order to continue searching. Of course the ability to detect small nuggets is lost.
“Desert placer” comprises coarse sediments, often very deep, eroded from a parent mountain and deposited by flash floods and debris flows. In most cases drainage patterns are constantly changing as flow channels become clogged with debris and floodwaters find another course. On a given spot it’s often hard to say where on the mountain the material came from. “Reading” the chaotic distribution of material in desert placer is an art difficult to learn, but those few who learn it are privileged to dig where nobody else has dug.
In some cases the parent mountain no longer exists. When a desert placer land surface is no longer situated where deposition processes are active, weathering disintegrates the surface material and it is washed away by surface erosion. The gold, being heavy, is not so readily removed by erosion. It’s something like a very slow natural sluice. Over many thousands of years the gold becomes concentrated in the surface material, producing an eluvial deposit.
Metal detectors have revolutionized artisanal gold mining of hillslope gold. Apart from “booming” and “hydraulicking” which are banned everywhere because of their environmental destructiveness, there is no practical way to mine hillslope gold …..other than by using a metal detector. The old timers who had no metal detectors walked right over the top of big gold nuggets but they had no way of knowing the gold was there.
Weathered-in-place residual gold (typically in soil above a gold-bearing quartz vein) is a good situation for metal detecting, fairly similar to a hillslope setting. The difference is that if you see quartz stones and think there might be a gold-bearing quartz vein beneath, prospectors before you probably saw and thought the same thing. So, there will probably be prospect holes and possibly even nearby hard-rock mines. The early miners were looking for a mineable hard-rock vein, but without a metal detector the overlying soil was useless to them. So always check the zone around prospect holes, as well as the prospect hole itself if it’s safe to do so.
Mine tailings from hard-rock mines are another favorite place to search. The old timers threw the rocks out if they didn’t see gold with their eyes, and they often got so busy throwing out rocks that they missed seeing visible gold. A metal detector can find their mistakes.
In many placer deposits, metal detectors go together well with the non-electronic traditional artisanal mining methods of panning, sluicing, and dredging. The larger rocks in the placer gravels can’t be processed by these methods, so they have to be thrown out. Someone already dug these rocks from a suspected or known paystreak: now they’re on the surface loose, and probably even clean. Unless you happen to know that the larger rocks in this placer deposit virtually never contain gold, the rock piles can be a good place to use a metal detector.
Metal detectors are occasionally used in hard-rock (lode) mines to check mine walls to see if there is a gold vein hidden in the rock which ought to be excavated.
In humid regions the soil is usually 20 inches (1/2 meter) or more thick, protected from surface erosion by vegetation cover. Gold is heavier than soil and tends to settle to bedrock. Therefore the gold is too deep to be detected with a metal detector. With the geology hidden underground, it’s hard even to know where one ought to be searching. Therefore in humid regions the use of metal detectors is usually restricted to searching material which is not covered by soil—river gravels, rock outcrops, and rocky material excavated in mining operations. ……The use of metal detectors has historically been most profitable in arid regions. Desert soils are usually thin and rocky, with gold often lying exposed on the ground due to removal of lighter material from the land surface by erosion. And unlike in most humid regions, you can see the geology to guide you to where you should be searching.
Learn the geology of the area where you’re prospecting. Learn what geological processes created the gold, and what geological processes put the gold where it is now. Learn to identify the various rock and mineral types which are typical of that area. When you look at the landscape, try to imagine what it looked like a million years ago, or even a billion years ago if the geology is Precambrian. Then imagine what processes took place to create the landscape you see in front of you now. Being able to visualize the flow of gold through the landscape will help you make intelligent guesses as to which zones will produce gold and which will not.