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Hi Everyone,

 

I had a call from a customer earlier this week with questions regarding hot rocks. He told me he had read a short article I had written, but couldn’t remember where he saw it. I couldn’t remember either while we were on the phone, so I started digging through my hard drives and found this one which I think dates back about 5-6 years. He is now a forum member, and I told him if I did find it I would republish it here along with a few photos. A lot of this info will be old hat for most of you, but I hope you enjoy… :)

 

 

Dealing with the Dreaded Hot Rocks

CHRIS GHOLSON

 

Persons new to the hobby of metal detecting will probably realize fairly quickly that it isn't just the ground that can cause false signals. There are other things lurking in the goldfields that can be just as noisy; namely the dreaded hot rocks! A hot rock can loosely be defined as: any rock or stone not containing a valuable mineral (gold, silver, or copper) which generates an audible signal response on a metal detector. The exact cause of this phenomenon has been debated among detectorists for some time. Numerous theories have been proposed, however those that seem most reasonable to me are the ones that focus on the iron-bearing minerals found within most hot rocks. These minerals are predominately the iron oxides: magnetite, hematite, limonite, maghemite and lepidocrocite. All of these oxides exhibit a varying degree of ferromagnetism and can be magnetized by being exposed to another magnetic field; like the one generated by a detector's search coil. If the coil is swept across a high iron content hot rock, a secondary magnetic field will be created around it. This secondary field will then be sensed by the coil's primary field and trigger an audible response via the headphones or speaker.

 

Just like gold nuggets, hot rocks seem to come in a never ending assortment of shapes, color and sizes. The typical varieties you are likely to encounter in the United States are basalt, magnetite and hematite. There are many others, but these are the most common. A variety of classification schemes and names have been proposed for the hot rocks that litter the goldfields. These names include cold rocks, hot rocks, positive rocks, negative rocks, etc. Personally, I have always tended to use the terms positive and negative when referencing hot rocks. Negative hot rocks are usually magnetite or contain magnetite. They tend to be dark in color and are often heavy due to their iron content. They are usually attracted to a magnet and in some cases, will show rust stains. Positive hot rocks are usually iron-bearing rocks which have been oxidized by natural weathering processes. They are typically small in size and found up towards the surface. They are most often reddish in color, but can also be black, brown or even yellow. Positive hot rocks are not always attracted to a magnet.

 

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Positive hot rocks are the most troublesome since the signal they produce most closely resembles a piece of metal. This variety produces a distinct “zip-zip” and can often be picked up several inches deep, or more. Many sound exactly like a nugget, and practice will be needed in order to learn how to sort them out from the real thing. Negative rocks, on the other hand, produce more of a “boing” sound. They are easier to deal with because the signal they generate differs considerably from that of a gold nugget. As the search coil is passed over them the threshold momentarily goes quiet, then the “boing” sound is heard. I have noticed that they can be difficult to pinpoint and at times, they seem to move around. You may also find that they will only sound off in one direction. In other words, a signal will be heard on the forward swing of the coil, but not the back swing. If this occurs, and the signal is not repeatable both ways, then the target is likely to be a hot rock.

 

While hot rocks can be incredibly annoying, they are actually a good indicator that gold may be nearby. As most experienced prospectors know, the yellow metal likes to hang out in highly mineralized ground. In fact, the worse the soil is and the more hot rocks there are, the better the odds of walking over a nugget. Learning to deal with them does take patience, but with a little practice you’ll have a handle on them in no time. Here are a few tips that I have found useful for dealing with them.

 

First, If your detector has an adjustable Gain (or Sensitivity control), try reducing it and elevating the coil slightly above the ground. This will cause a loss of some depth and sensitivity, but may be more preferable than listening to the constant zips and boings of the hot rocks. Second, many times hot rocks can be tuned out by ground balancing directly over them. This may eliminate the hot rock, but in turn cause the ground to become noisy, making it even harder to hear faint targets. Therefore I recommend this practice be used sparingly. Instead, I find it best to simply balance to the soil and deal with the hot rocks as they come. This usually entails kicking them aside with my boot or simply tossing them off into the bushes. The easiest way to deal with hot rocks is to simply memorize what they look like in the area you are hunting. This way you will be able to quickly recognize them when you hit them. Fortunately the hot rocks in any given area tend to be fairly homogenous, so you should only have to memorize a few different types. Keep in mind that if you ever get a signal from a piece of quartz, this is not a hot rock! I repeat, this is not a hot rock and you should check it carefully for visible gold. If nothing can be seen on the surface, you should crack it open to check for hidden gold values within. A few years ago a friend of mine detected a quartz rock about the size of a cantaloupe. We washed it off and inspected the entire surface but couldn’t see a single speck of gold. We tossed it into the Jeep and later that night decided to crack it open. Lo and behold when the sledge hammer split it, we discovered a solid lump of gold directly in the center weighing roughly ¼-ounce! As a general rule, any rock that produces a solid response on your metal detector, and is not immediately identifiable as a hot rock, should be saved for further inspection.

 

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The type, or configuration, of search coil you use will also have a big impact on the way the detector handles hot rocks and ground mineralization. The most popular types in use today are referred to as either Monoloop or Double D. Both have their pros and cons. Monoloops or “Monos” will penetrate deeper into the ground than a similar sized Double D. They also have excellent sensitivity to small nuggets, and larger pieces at depth. They also run “hot” all along the outside edge, which helps immensely when pinpointing a target. The biggest downside to a Mono is that they can cause your metal detector to behave erratically in highly mineralized ground. In most cases the machine will sound unusually noisy. The Threshold will become unstable and you will have difficulty achieving a perfect ground balance. You will also find them to be much more reactive to hot rocks. Therefore, Monos are better suited for areas with milder ground conditions and fewer hot rocks.

 

Double D or “DD” coils do not penetrate as deeply as Monos, but they are better equipped to handle heavy ground mineralization and hot rocks. Simply put, the DD coils are stable and run quiet. This reduction in background noise keeps the operator from having to dig a majority of the false signals generated by both soil and hot rocks. It is for this reason that they are the preferred choice for Australian detectorists who routinely tackle some of the harshest and most highly mineralized dirt on Earth. Ultimately the coil you decide upon should be dictated by the severity of the ground. Personally, I always try a Mono coil first, and then if the hot rocks or ground proves too noisy, I switch over to a DD.

 

One last point worth mentioning is that hot rocks can mask gold. By this I mean that if the hot rocks themselves are mineralized enough, the signal they generate can actually be stronger than the signal generated by a nugget beneath it. This is a common situation in many goldfields around the globe that will require the detectorist to pay close attention to the signals received from these hot rocks. When scanning over a hot rock, if the signal doesn’t sound the same as the rest, you should move it out of the way and check the ground beneath. I have found many small nuggets hiding just an inch or two beneath a hot rock. A general rule is, if the hot rock is not too big; kick it out of the way. Or, “When it doubt; kick it out!”

 

I don't enjoy listening to hot rocks any more than the next person, but as I mentioned above, I do like seeing them. Their presence is a strong clue from Mother Nature that gold may be lurking nearby. If you find yourself in an unusually noisy patch of hot rocks, slow down and work the area very carefully. These spots do require extra effort, but you never know that next signal you get could be the start of a new patch!

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Great info Chris. This was the main reason I gave up trying to use my VLF and got a PI machine. I had dug 2 different targets that I was sure were gold but after foot deep it was still getting louder so I called over a friend who was using GPX 4500 or 5000 and he confirmed that I was chasing hot rocks. Good practice digging and an even better sales pitch for the PI machine I ended up getting as you know. Thanks again.

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Chris:

" I have found many small nuggets hiding just an inch or two beneath a hot rock. A general rule is, if the hot rock is not too big; kick it out of the way. Or, “When it doubt; kick it out!”"

 

I will second that.

Found this one under the flat square negative hot rock.

Tom H.

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Hey Tom,

 

Boy that's a tight little gully there. Crawling through that brush is no easy job. I always come out looking like I've been in a cat fight & lost! :P Beautiful nugget - it was definitely worth the scratches!!!

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Chris,

Glad you posted, even though it was and old article. Always good to review. Wonder how many other articles you have stashed away that you need to re-post for people like me who needs all the help they can get.

Thanks

Brent

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LOL....yah don't you love it when you get snagged on the headphones, turn around and a piece of cats claw snags you, go the other way and snagged again...Its like CRAP! this thing is attacking me!! :)

I'm usually a bloody mess after those tribs.

That's where ya got to go though.

Tom

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Hey Tom ... Great chunk of gold out of the tangle ... "attack you"? Yup! ... Sure feels that way sometimes. Can't wait to try the GPZ in a place like that. I am really liking the wireless speaker/headphone jack. No battery cord or headphone wires to get snagged!

 

Chris ... Great old article revisited. That one is classic and should be required reading for all electronic prospectors before the start of each season. Hot rocks sure can be a pain in the neck but they are great indicators so should not be avoided.

 

Mike F

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Hey Mike,hope you do real good with the GPZ,i can,t wait to here how you guys do all i have been able to do is practice with mine in the pasture with some buried nuggets and lead every time i think i can head out it snows up here,good luck hunting.

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