Nearly all soils found in gold prospecting areas contain “hot rocks”. A hot rock is a rock which does not contain gold, but which causes the metal detector to sound off. They come in two basic types.
Negative hot rocks (also called “cold rocks”) are usually magnetite or contain magnetite. They give a negative response because their ground balance value is a higher number than the soil they are found in. They tend to be dark in color, and are often heavy because of their iron content. In some cases they will have rust stains. They are usually attracted to a magnet, and for this reason gold prospectors always carry a magnet—the ultimate ferrous/nonferrous discriminator. In motion all metals mode, negative hot rocks produce a boing sound rather than the zip sound of a metallic target such as gold. Another difference is that as you sweep back and forth over a sound to check it, a zip sound will seem to stay in the same place whereas a boing sound will seem to be in two different places and to wander around. Learn these differences and you will be able to ignore negative hot rocks.
Positive hot rocks are usually iron-bearing rocks which have been oxidized by natural weathering processes, such that their ground balance number is a number lower than the soil they are found in. They are often small, right on the surface, and sound just like a gold nugget. They are common in many gold prospecting areas. They are usually, but not always, drawn to a magnet. They are most often reddish in color but are often black, brown or even yellow.
A special category of positive hot rock is electrically conductive rocks. In gold prospecting situations the most common electrically conductive hot rocks are sulfide ore (peacock ore, pyrrhotite, etc.) crystals. These can sound just like a gold nugget, but in many geological settings the gold is not found in the same type of rock as the sulfide ores. Therefore the sulfide ore signals can be ignored once the host rock can be identified. A few geological settings include graphite or graphitic slate rock which tends to give a very broad signal which does not sound like gold and which cannot be ground balanced out. If the rock sounds like metal, is black in color, and you can write on paper with the rock as you would with a pencil, you’ve got graphitic rock. In order to use a metal detector where there’s graphitic rock you’ll probably have to reduce the sensitivity setting of the metal detector.
If you’re searching in a desert area, learn to recognize meteorites. All are valuable, and some are worth more than their weight in gold. There are many different kinds of meteorites, but most commonly they will look like a lump of magnetite. However unlike magnetite, meteorite response on a VLF metal detector will almost always be positive. A good rule of thumb is that if you encounter a positive hot rock that is black or dark gray in color except perhaps for rust stains, and it doesn’t look like other positive hot rocks you find on that site, save it. Remember where you found it, and attempt to identify it later.
The subject of how to identify the various types of meteorites is too complex to go into here. (This book is about gold, not about meteorites.) Nowadays there’s lots of information on the Internet, and recently several books have been published on the subject. Often it takes a meteorite expert to be able to say whether a particular rock is terrestrial or is a meteorite. In the USA, most of that expertise is found in Arizona where there is a lot of metal detecting in the desert, several meteorite strewn fields, and a long regional history of interest in astronomy and astrophysics within the University system. (Note: a “strewn field” is a zone of meteorite debris caused by the explosion of a large meteor high in the atmosphere.)
The iron and nickel metal in meteorites is usually what makes them detectable with a metal detector. Searching for meteorites with a VLF gold machine is pretty much like gold prospecting, except that hot rocks become a whole lot more interesting. The disadvantage of standard PI (pulse induction) gold machines for meteorite hunting is that when they’re ground balanced, their response to the iron and nickel metal and iron minerals typically found in meteorites is greatly subdued.
Use A Magnet
• Iron metal is always strongly attracted to a magnet. Steel (iron alloys) are almost always strongly attracted to a magnet.
• Magnetite hot rocks and black sand are always strongly attracted to a magnet.
• Other negative hot rocks are almost always attracted to a magnet, but not as strongly as magnetite.
• Positive hot rocks are usually attracted to a magnet, but the attraction is often weak.
• Meteorites detected by a metal detector are usually attracted to a magnet. The attraction may be strong or weak.
• Gold and other nonferrous metals are never attracted to a magnet.
Always carry a strong magnet with you when prospecting. Many prospectors use a pick that has a magnet either built in or attached.