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What Metal Detector System To Use For Auriferous Rocks?

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Auriferous: In my case contains 20388 grams/ton Au (20.4kg/ton)


I found a single extremely high grade gold rock specimen and I am unable to locate more. Lode and Placer mining equipment would not find these other than a lode process crushing everything in the area. Specimens at these grades are worth more than the gold they contain. I do not own a metal detector but have spent a good hundred hours learning what I can about the technology and systems. 

Cold Rock:
I believe I need a metal detector system that is optimized to locate cold rocks (non-ferrous negative hot rocks, opposite to ferrous hot rocks). I cannot obtain a resistance reading with a multimeter that is capable of reading millions of ohms even though the gold concentration is extremely high in both the assay results and with the naked eye. To me this means it cannot be metal detected as a solid metal object (nugget) of any size. However it might hopefully be detected as a cold rock of some size.

Ground Conditions:

  • Dry and no salt.
  • Size: Shale (loose rock) slope with a little vegetation on top of it. The size of the rocks are mostly 2 inches to 12 inches with a few larger rocks going up to about 4 feet. There is very little "gravel" size rocks (under 3/4in). This specimen is about the size of a baseball (3in/75mm diameter) that I cut in half. The other half went to the assay lab (had I known I would have sent a MUCH smaller sample size). This half of the specimen weighs 146grams. The other half that got fire assayed weighed 180grams. So the original specimen before I cut it in half weighed at least 326grams.
  • Mineralization: The location contains very little mineralization (as in very little iron). The only sign of "iron" is decaying/oxidizing pyrite. Some of the rocks have a lot of pyrite in them. I suspect some of the "pyrite" is arsenic based (Arsenopyrite) but I have not had an assay on those to confirm. Even some of the Au found in this specimen may be rare gold pyrite (Aurostibite).
  • Trash: There is very little "trash" (as in manmade metal objects). I would even go so far as to say no trash whatsoever but I'm sure there is something in there. Certainly no aluminum wrappers that have been shredded thru a lawn mower.


Au = 20388.2 gm/t

Ag = >100 ppm

Mo = 23 ppm

Cu = 435 ppm

Pb = 1471 ppm

Zn = 1032 ppm

Mn = 1446 ppm

As = 34 ppm

Sr = 89 ppm

Cd = 9.2 ppm

Sb = 39 ppm

Bi = 891 ppm

Fe = 5.57%

S = 5.79%

What Metal Detector system to use for Auriferous rocks (cold rocks)?

If I had not made this post to get feedback, my research/learning points me to purchasing a IB/VLF system like the White's GMT with a few different search coils for the first 12 inches of depth. However I do not feel confident in that decision because maybe it should be a Fisher Gold Bug Pro instead, or something else.

Can a deep large object metal detector (relics) work for detecting "large" cold rocks like this beyond a few feet of depth in the ground conditions described above?


High rez pictures of this specimen that I need to find more of are attached! 






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Me thinks a Falcon MD20 would easily pick that stuff up. Course you're not gonna get any significant depth with it, but from all the research I've done, it's pretty much designed for detecting exactly that type of ore/specimens.

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Yes a Falcon MD20 would easily detect the gold in your specimen, but unfortunately only at 1 inch or so, if you can see free milling gold you wouldn't need the Falcon to known there's gold in the rock, it would only help you detect gold hidden within a rock that's not visible on the outside and then still only within a 1 inch or so of the surface of the rock.

I also think a Fisher Gold Bug 2 or a Whites GMT would detect the gold at more depth, but you may have problems with the iron content I see within your specimen, e.g. the black areas of your specimen, both are VLF technology which sometimes has trouble with highly mineralized areas.

If I were you I would take a piece of your specimen to a Minelab dealer and test some of their detectors on it, I think the new SDC 2300 maybe your best bet, if the same dealer carries Fisher and Whites products, even better you could test the best of the best, VLF and PI detectors on your specimen.

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It hard to say as there is no exact scale, but all metal detectors have a size and distance limit. The tiniest stuff can only be seen at the shallowest depths. I would guess that a falcon would detect that, but its not a detector you walk around with and scan the ground - its a hand held probe used to test rocks one at a time. The Gold Bug 2 is as sensitive as anything on the market in the way of ground scanning detectors, and I don't think a Gold Bug 2 would detect that tiny stuff, but without a scale I am just guessing. I would take it in to a detector dealer and try sliding the rock over a GB2 coil and see if it reacts. At least a GB2 is set up for scanning the ground. Even if a GB2 does react, it wont see it very deep.

It pretty rock, so good luck in your search.

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The Falcon MD20 will easily pick out tiny gold like that, but as already mentioned it is more a probe/pinpointer than a metal detector and so has a very limited range of about an inch and therefor limited applications. I am sure my Gold Bug 2 or White's GMT with small coils would hit on those samples, but again we are talking only a few inches at best.

Depth/distance is directly related to target size, and unfortunately the effect is only cumulative to a small degree. The problem is with dispersed small gold. If a detector cannot pick up one tiny piece of gold, it also cannot pick up 1000 of those same tiny pieces sitting next to each other. Even in a vial there is barely enough contact between pieces to enhance the signal, which is why you see people complaining all the time that a detector will not pick up a vial full of very small gold. The same issue arises with small gold chains with tiny links.

You mention "cold rocks". A hot rock is simply a rock that does not match the current ground balance setting in one direction, and which therefore signals as if it is a metallic target. A "beep-beep" if you will.  A cold rock is also just a rock which does not match the current ground balance setting, but in the opposite direction, producing a distinctive null followed by overshoot effect that can best be described as "boing-boing".

If a detector has sufficient ground balance range both hot rocks and cold rocks can be ground balanced out, but then the main ground mass will be out of balance, causing it to signal. Hot rocks and cold rocks only exist as out of place items. If all the surrounding ground were the same composition as the cold rock or hot rock, you would simply ground balance the detector and they would cease to exist.

Because hot rocks and cold rocks are simply an accidental artifact of the ground balancing system, manipulating the control can change many hot rocks into cold rocks and vice versa. They only exist relative to the current ground balance setting. If a detector could have two ground balance settings, like the old Tesoro Diablo uMax, you could ground balance out both the overall ground plus the hot rocks or cold rocks.

Hot rock and cold rock effects are rapidly masked by the overlying soils. So a large vein of material that is markedly different than the surrounding ground can be detected, but generally not at great depths. Like everything else metal detecting, it would depend on the difference between the main ground mass and the target material, and the size of the target material.

For example, a vein of magnetite in relatively moderate ground would produce a large cold rock type signal. the depth it could be detected at would depend on how pure the magnetite is, and how much actual surface area is presented towards the coil of the detector. A vein a foot across might be detected to a foot or two with most detectors.

The reverse happens and does catch some people. If you have iron mineralized bedrock that is cut by a vein of bull quartz (pure white quartz) the detector is ground balanced to the mineralized bedrock and so will produce a positive signal over the quartz. It sounds like there is gold in the quartz. But unfortunately it is the lack of iron mineralization in the quartz producing a classic hot rock sound from barren quartz!

Typically these types of things are the realm of induction balance (VLF) detectors and much less so something that cam happen with pulse induction detectors. Your question is interesting because in theory old style two box detectors like the Fisher Gemini 3 White's TM808 http://www.whiteselectronics.com/tm-808 can be used to locate large veins buried under topsoil if the are large enough and distinct enough from the ground mass. I would however hold out very little hope of this being applicable to your situation as you would need either a huge quartz vein buried in mineralized soil or lots of magnetite mixed in with that gold and quartz, and then the overall vein buried in relatively mild soil.

There is no hard and fast answer though, it just depends on the specific situation. The TM808 is probably the easier machine to use, but the Gemini 3 has the ability to be separated into transmitter and receiver components, and operated held far apart by two operators, basically creating a huge search coil that could be, for instance, about 20 feet in diameter. The Gemini 3 manual at http://www.fisherlab.com/hobby/manuals/Gemini-3-07.24.14-Printer.pdf details this:

"Two operators line up at least 20 feet apart,one with the receiver and one with the transmitter, parallel to the assumed direction of the buried conductor. The transmitter and receiver should be in line with each other, facing the same direction.

1.  Keeping the receiver and transmitter sections in line, the two operators can now walk their predetermined search pattern. The SENSITIVITY control should be checked periodically to ensure that the receiver is tuned just below the “air coupling” threshold.

2.  If both operators cross the same conductive body (pipe, cable, ore vein, etc.) at approximately the same time, the receiver tone and meter will rise to indicate its presence. The receiver operator should alert the transmitter operator that they have detected a conductive object.

3. The receiver operator should then hold his position while the transmitter operator moves back and forth for the strongest receiver response. At this point, the transmitter operator should stop and place the instrument on the ground with the handle grip on top.

4. The receiver operator can then pinpoint the buried object by moving the receiver back and forth in line with the transmitter. The object should be directly beneath the point of maximum response.

A dated but excellent reference on this and other geophysical prospecting methods is the Handbook of Geophysical Prospecting Methods for the Alaskan Prospector downloadable at See page 46 "Electromagnetic Methods".

None of this is a sales pitch. Frankly, we all want a magic box to make the hard work easier, but in reality you need some very favorable conditions for these methods to be easy and clear cut. More often results are fuzzy at best, requiring a lot of operator expertise and interpretation.

Metal detectors as we know them respond to both conductive and magnetic effects. Magnetometers are often more effective, employing just magnetic differences in materials. So here are a couple references that expand on all this. Many of the prospecting concepts are identical to those you would see with electromagnetic devices.

Applications Manual for Portable Magnetometers

Applications of Geophysics to Gold Exploration In South Africa

Exploration of Placer Gold Deposits by Geomagnetic Surveys

Hope this helps.

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Steve I made a point of posting to your forum versus others hoping that you might reply and reply you did!!! Thank you and to the others that replied as well.

The more a learn about metal detector technology and trying to use that technology to solve a problem the more I am amazed out how technical it is. When I knew nothing about metal detectors I thought all you had to do was swing the thing side to side until it beeped alerting you to a metallic item of interest. Was I ever ignorant.

Before I started this thread I had never heard of a Falcon MD20 despite having put a good 100+ hours of research into this over the past few months. I have now bought a Falcon MD20.

The Falcon MD20 comes with a business card that has been laminated with three things to it for you to play with. Pyrite, Gold (yes, a tiny spec of real gold) and little pile of black sand. This is what I experienced:

Pyrite - silence no matter what you do.
Gold - only makes sounds approaching it, never leaving it.
Black sand - only makes sounds leaving it, never approaching it.

The Falcon MD20 definitely makes noise with my auriferous rock. I'm calling it "noise" because my ears are virgin to metal detectors. I can't hear the difference yet between a zip, beep or a boing, I just hear noise or silence. The probe only makes noise approaching my auriferous rock, never leaving the rock (no matter what the acceleration or velocity of it leaving the rock). No matter what setting or anything I do, I cannot not get the MD20 to make any sound whatsoever as the probe moves away from my rock. The faster I approach the rock the louder it screams.

Ground scanning: Just like everyone has said it is not a ground scanner. This needs to be almost touching the rocks for it to work and therefore you can't hover it over the ground. Hovering an inch off the ground makes this useless but it is not useless because it wasn't designed for that. I bought the pole mount for it (just some white PVC pipe) so I can walk around and poke/touch rocks with it. You often can't swing the probe over your target rock because there is no space to do so (i.e. branches, other rocks, crevice, etc.). I found a poking/stabbing motion worked best because you can always do that. From a distance I must look like a blind person lost in a rock field with my white walking cane. I have never used a metal detector but this no doubt is a much slower process than swinging a "normal" metal detector designed to scan the ground quickly and to some depth. However I can poke it into tight areas a search coil can't fit.

Now my confusion with hot and cold rocks and how that relates to my Falcon MD20 and my quest for Auriferous rocks. Bear in mind as you read this that I have zero experience with "normal" ground scanning metal detectors and about 8 hours now with my Falcon MD20. I find I get only one of three results. No sound, sound when the probe is approaching the object or sound when the probe is moving away from the object. I tried experimenting with ways to get sound while both approaching and leaving and could not. The sound seems to be mutually exclusive to either approaching or leaving the object, never both. I have experimented with all kinds of metals and they all give the exact same result: sound when approaching, never when leaving. So I am confused then as to how the black sand on their sample card causes the opposite result. To me black sand is metal and should therefore I would expect the same result I get with metal, sound when approaching and not leaving. I watched a bunch of Youtube videos about this Falcon MD20 and a few of the video mention that this detector discriminates, non-ferrous sounds when approaching and ferrous when leaving, however that is not what I experience at all with the exception of the black sand sample. I am confused. Without understanding what is really happening my current guess is that sound you get when pulling the probe away from an object is from a semi-conductive material, but then I would think pyrite would make a sound and it is silent. I am still trying to wrap my head around what cold and hot rocks are and from what Steve is trying to explain to me sounds very similar to what I am experiencing with the probe only making sound when approaching or leaving an object but not sound in both directions, ever. Does hot and cold rocks have any relation to what I am experiencing?

Steve, you mention the "old Tesoro Diablo uMax" that had two ground balance settings. If you balance out the overall ground and rocks that only make sound when leaving them, you would be left with only getting sound on approaching a rock, which therefore has solid metal in it of some kind.?

If a metal detector responds to both conductive and/or magnetics does that mean you could detect a ceramic magnet (ceramic can't conduct)? Would my Falcon MD20 make as sound approaching or leaving the ceramic magnet? I'm guessing leaving?


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Black sand is not a metal. It is a non-conductive magnetic mineral called magnetite. What you are seeing is the hot rock effect.

Most people call any rock that makes a noise a hot rock. However, some rocks make a sound as you approach them, and others make a sound as you leave them, all depending on the ground balance setting. Some people call the ones that make a sound as you approach them a hot rock, and the ones that make a sound as you leave them a cold rock. Some other people may do it the other way around - these are not defined terms just nicknames in general use.

I am one of those that think if the rock makes a sound that mimics gold it is a hot rock. They are actually rarer than the other rocks, the ones that produce a null in the threshold and overshoot when leaving them. I refer to those as cold rocks. So a magnetite rock would be a cold rock the way I look at it. I am thinking more of the sound produced.

Other people think the more iron a rock has the "hotter" it is, so pure magnetite would be hot indeed. A rock totally devoid of mineralization would be a cold rock. Again, you can look at it either way.

The best book on the subject is Advanced Nugget Detecting With The Fisher Gold Bug. See page 21 "Hot Rocks" and all of Part 2 beginning on page 29. http://www.fisherlab.com/hobby/davejohnson/AdvanceNuggetHunting.pdf

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  • 5 years later...

The topic of metal detectors is quite fascinating, although I've only had experience with more professional equipment, like ground penetrating radar. I'm in construction, and in a lot of things, it's very important to check everything before you do it. The gpr does a great job with that. I think if you are interested, you can find a lot of interesting information on this.

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I found some similar looking gold bearing rock, the gold being dispersed like yours is, with my Minelab Gold Monster.  But my rock doesn't have any other conductive metals in it. The Falcon will report on gold-bearing rock where the gold can't be seen. I found this place with the Monster originally with the gold bearing rocks down around 6" with the 5" coil, then a few deeper ones with the 10" coil. This was slide material in a wide old cut. Found a short vein by detecting above the cut. I think the old mining in this area was digging on random short veins. Host rock is shattered quartz.

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Discussion of detectors aside...lol, I'm curious about the amount of gold in the assay. If my math is correct, that's saying 655 +/- ozs/ton. That seems incredibly high to me. No offense to the OP.


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