A Mystery Metal, The Whites’ GMT and the Falcon MD-20

Yesterday I went out to Lathrop Wells in Nye County looking for placer gold in featureless alluvium, possibly 240 deep where I was. I had the audio on my metal detector turned way down to save on battery life. I wasn’t listening for gold nuggets or gold in quartz that day, instead, I was reading the display on my Whites’ black sand tracker feature. It gives a numerical reading on any ground the coil is moving over. I sample when I see it go above 45, unfortunately, it only hit 42 once.

But one sandstone rock with black streaks blasted out despite the volume on the speaker being turned down. Well, I thought, that’s interesting. Aside from the possibility of gold, I wasn’t aware of any other metallic ore in this ocean of alluvium. No mining district here or any past mining activity. The signal sounded like the hot rocks I encountered everywhere in the the canyons of the forks of the American River. You get what sounds like a good signal and then you listen as the GMT cancels the signal out. I should explain.

A decent gold signal is a strong, low, mellow tone that happens each time the search coil passes over a rock. The signal remains no matter how many times the search coil goes over the target. What usually happens with a bad rock, often with an iron component, is that a signal is heard initially but that signal disappears after several passes. With each sweep the GMT is logging information to determine if a rock is ferrous or not. If so, the signal weakens and then goes away. This is called cancelling out. This rock did not cancel out. That’s what you look for with gold. Most non-metallic rocks are neutral and the GMT also passes them by without raising any signal.

The signal, however, was weak and wavy. Not good. You want to hear a steady signal, nice and low. You’ll rarely hear this. The Whites also has a probability of iron meter. It was reading at nearly 100%. Okay, I thought, this is coming home with me to figure out later. When I broke it open at home it revealed a dark material like that which it showed from the outside, hardness about three. Not attracted to a magnet, not even my super magnet. No U, no UV. Dull, off-white streak on a black streak plate. Metallic luster when freshly broken, annealed appearance in some spots with fresh breaks. Crushed and panned a piece, nothing unusual, no free gold at the macro or micro level.

To get the ID process going I mailed off a chip today by regular US mail to Kerry Day. He charges less than ten dollars for a seasoned opinion based his experience and the results produced by his electron scanning microscope and its supporting software.  I’ve written about his service many times before. I taped a small chip to my business card, put it in a normal envelope and the charge for mailing was less than $2.50.  Make sure your sample fits into a No 10 first class envelope, anything bigger might be considered a package and then mailing rates go up enormously.

My Falcon MD-20 handheld metal detector indicated that the material was non-ferrous, despite what my Whites said. Ferrous material produces a noise when an object goes away from the Falcon’s probe. Non-ferrous produces a signal as you move material toward the probe. This material only produced a signal going toward the probe, however, there are cases where a material is so strong that it may overwhelm the device and make it impossible to determine which is which. There were also some tiny gold colored spots arranged in lines here and there in the black material.  On seeing gold coloring, always assume mica or pyrite. The nice thing about the Falcon handheld and the GMT is they do not react to pyrite. Never. I have lumps of pure pyrite and both devices stay silent when moved over that nonsensical material. Pyrite is an iron associate but too weakly so. Yet the rock continued sounding off. I suspect that it was is not those gold colored specks making the noise but the black material.

Unable to resolve the difference between the two metal detectors, I gathered up the rock pieces and went back to my truck and the GMT.  I ran the coil over the rocks again. Sure enough, I hadn’t read the meter right in the field. This time the meter did not go above the halfway line, indicating something less than ferrous. The signal remained weak but still would not cancel out. Now, I just wanted to figure out what the material was, never mind that it wouldn’t be worth anything.

After further research I discovered that there are three long abandoned copper mines in the hills that drain to the wash I had been in. I still don’t know what that black material is but I am now convinced that I have a rock with copper. Not worth anything but a nice find. Those exploratory diggings are now unclaimed and the ground is on open BLM managed land. A little close to the Nevada Test Site boundary but at least a mile away. I may go exploring there to look for copper mineral specimens. Here’s a link to one mine listed at the the MRDS if you want to go:

Lucky Claim:


Materials Type of material
Malachite Ore
Hematite Gangue

Fractured quartzsite with slight banding

Lathrop Wells Collecting from Thomas Farley on Vimeo.

Black Sand Gets Stranger

Classified a half bucket of sand and gravel and soil to 100 mesh. Got rid of the blond sand by panning. Black sand left. _Two_ levels of black sand. My musings in the video . . .

If anything is as mysterious as gold, it is black sand. Black sand may not indicate that placer gold is present, but a lack of black sand almost certainly rules it out. But little is settled.

There is a spot at the east end of Folsom Lake where black sand covers hundreds of yards of raggedy beach, all deposited by the north fork of the American River. All that sand is completely sterile, although, to be fair, I have never had a pound or two of it fire assayed. Let’s say visible gold isn’t present.

These heavy occurrences causes my White’s GMT to howl or grunt in protest. “Lift coil. Bad ground.” The best black sand happenings that I find are small streaks, not big ones.

My White’s has a black sand tracker feature which gives a numerical readout. I start sampling when it hits 45. Above 54 or so it will register bad ground. A reading of 5 to 20 or indicates well developed or poorly developed soil and not the sand and gravel I am looking for. Time to move on.

With this feature I can assess a large gravel bar or a desert wash fairly efficiently, instead of guessing at where to begin, or concentrating solely on bends.

Update: Tony Wirtanen from Facebook provides a good explanation, that it is simply particle size at work that makes the difference. “Magnetite I think is what it is called and the more ferrous is probably just a micron or two bigger giving it more mass for the magnet to pull on. There are many different types of black sands from granite to graphite and everything in between. But the ferrous material I am almost positive is all magnetite.”

Granite sand would result from the black color of biotite, a common rock building mineral that gives a granitic rock its sometimes salt and pepper look.

But my friend the mineral dealer, he with fifty years of collecting experience, disagrees with the mass solution.

“The magnetism in the sand is probably a mix of the magnetite and either hematite or goethite and that is my thought on why some is magnetic and some not. The particle size is not something I agree with, small would be even more attracted to the magnet. Then a second is that some magnetite is very magnetic, the extreme example is “lode stone” a magnet in itself, some is barely magnetic so that can vary also.

You did find a good crystal and that is often the key to which one you have. In our local black sand, the stuff we actually find right on our property, it is still quite sharp in the crystals. Not that far from the original source.

The photo you sent looks like the material was a bit farther from the original source so rounded by the rolling in the flooding that took it along. It is funny stuff and can have platinum in the black sand but finding that is something I have never attempted. I know by reading it is in there too but to differentiate it may be something I can’t figure out.

In your photo I can see that same tetragonal shape, even in the more rounded nodules too.”

Here’s a picture of some of the highly magnetic black sand under my microscope. Magnetite’s crystal form is an octahedron. Can you spot the lone well developed crystal?

This graphic is from Wikipedia, showing an octahedron’s form.

Crystallography is a dark art for beginners and I am keeping clear of it where I can. I just got a reference specimen of magnetite from the Marble Mountains in San Bernardino County, California. Could anyone possibly relate these well defined crystal faces to the form above? And I have seen a picture of magnetite which shows a cubic form. Good grief.


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