Three ideas for detecting mineralized beach sands?

[dragline]

I am new to the forum and detecting technologies, so please take it easy on me given my relatively high ignorance quotient.

I do have some gold nugget detecting experience (more than 20 years ago with a White’s 6000-D), but I am interested in figuring out a practical method and technology for detecting black mineralized sands in marine beach placers.

Some beach sands contain gold, enough to mine recreationally or commercially, but the gold is usually extremely fine and it is almost always found mixed within relatively thin layers of mineralized sand. These mineralized sand layers (sometime black and sometimes ruby red in color due to and abundance of garnet) usually consist of magnetite and/or hematite and are therefore more conductive and more magnetic than the surrounding blond sands that are mostly quartz (silica).

Since the gold that I am after is always mixed in with mineralized sand layers (called pay streaks), and since these mineralized sand layers comprise a relatively small percentage (usually 2% or less but sometimes more) of the overall volume of sands on the beach, I was hoping to find a way to detect these mineralized sand layers from among the predominantly blond sands of the beach.

Depending on the beach these mineralized sand layers can be less than half an inch thick to as much as 12 inches or more thick (very unusual however). The thickness of mineralized sand layers that I would like to detect would be 1/2 inch thick or thicker. Typically these black sand layers will run horizontally (parallel to the shore) along the beach for hundreds of feet and be a few feet wide. So, dimensionally these mineralized sand layers will typically be 1/2 inch thick, 1 to 4 feet wide, and then 20 feet or more long running parallel with the beach (sometimes patchy, however). Typically there will be a minimum of 6 inches of blond sands on top of the mineralized layer but there it could certainly be up to several feet deep and obviously beyond the range of conventional detection technologies.

Beach sands will typically be wet (and salty with seawater) and so there will be increased conductivity of the blond sands when they are wet versus dry. I have asked some metal detecting friends about the possibility of using metal detectors to detect these black sand layers and have received a universal answer... No! It can't be done! The beach miners I've asked have also stated that magnetometers also won't work to detect black sand layers either because sensitivity just isn’t practical without a megabucks investment in equipment.

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Idea #1: So I was wondering if one might be able to use an impedance meter and something like the coil of a metal detector to detect these black sand layers?

Here is an example of a handheld impedance meter costing about $150 USD...



If I made a big enough coil that I set on and then dragged across the beach sand surface I was wondering if the presence of the black sand under the coil would increase the impedance of the coil and register on the meter as a higher resistive reading?

Here are the features and specifications for this impedance meter...

Features: Measuring a transformer with a standard multi-meter will not give accurate results because it is measuring DC resistance, not AC. This invaluable unit has a whopping 30mm high, backlit display and is fantastic for checking transformers and speaker impedances. Applies a 30 second test tone signal upon activation of the “test” button. May also be locked in test mode for continuous tone. Requires 6 x AA batteries (included).

Specifications:
Measurement freq: 1kHz
Range: 20 ohm, 200 ohm, 2k ohm
Display: 3.5 digit LCD
Accuracy: ±2% reading, ±2 digits
Dimensions: 110W x 168L x 65H mm

I am not sure what the "test tone signal" is all about with this unit since it measures impedance on the digital display. I also don't know that much about how impedance meters actually work, but I assume that it measures the resistance of a give circuit to the 1 kilohertz signal that it puts out and then compares that resistance to a DC for an indication of impedance in ohms?

If I had an impedance meter I could test it and see if it could detect black sand in saltwater, but at $150 USD I would first appreciate some feedback from any of you as to whether there is a better way or if this idea wouldn't likely work for some reason. If there is a possibility of it working, then the next problem would be to figure out a coil design that would optimally work with it.

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Idea #2: I have another completely different idea that I will throw out here if the impedance meter idea falls flat. Since beach sands are usually only loosely compacted, wet, and reasonably soft, I am thinking that I could drive two probes up to a few feet deep into the beach, and a few inches apart, and I could measure the resistance of the sand. If there is a shared black sand layer contacting both probes in that location the resistance should go way down since the black mineralized sand layer will conduct electricity far better than the blond sands. This effect would be much greater if the sands were dry, but given that areas of the beach with the gold are usually close to the surf, and below the high tide line, and saltwater wet, then I am pretty much stuck with having to work in saltwater.

If measuring sand electrical resistance actually worked, one might be able to determine the depth and extent of the mineralized layer with successive probing measurements. One could also employ probes that are mostly insulated except for the tips. As the probes are inserted they might contact multiple mineralized sand layers at various depths. By measuring the depth of the mineralization one could not only determine the depth of the various layers but the thickness of individual layers as well given the right problem electrode design.

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Idea #3: Given that the mineralized sands (magnetite and hematite) are far more conductive than the surrounding blond (silica) sands, I am wondering if it would be possible to insert a probe deeply into the beach and then design a detector to measure the capacitance created between the mineralized (conductive) sand layer in contact with the probe and a large insulated plate resting on top of the sand. Handheld multi-meters reading capacitance, inductance, and resistance are a lot less expensive than an impedance meter (Idea #1 above), and it might be possible to design a capacitive detector.



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Three very crazy ideas, for sure, but perhaps if I through out enough stupid ideas one of them my actually work! Perhaps some of you can think of better and/or even crazier ideas?

Thanks,

Timothy

[robby_h]

I have the same problem and just use a 2" auger. It was made from a car exhaust pipe and has a slot down the side which allows me to see the layers. It takes about a minute or so to put a 3' hole down and most of your ideas with probes or a MD wouldn't be quicker or reveal the depth of each layer. The bottom layer of black sands here is several feet thick in places but unfortunately this is only exposed when the right storm hits the beach and takes about 10-12' of the surface out to sea.
Rob. (Australia).

[dragline]

Tailpipe Auger for Sampling Beach Sands

Rob,

Great idea! I like it. Seems very simple and effective. You can auger up cores in various places on the beach and actually see the layers at any given depth and layer thickness.

You are probably right about the particular techniques that I have proposed not being practical given that probes must be inserted into the sand anyway, and if an auger can go 3 feet in less than a minute there likely wouldn't be much advantage to detecting any particular mineralization sand layer.

I suppose the only advantage that some as yet undetermined detection technology would give relative to core sampling would be if I could drag the detector coil on a sled behind my truck as I drove down the several miles of the extent of the beach. We'ver recently had some major storms hit our gold placer beaches in Southern Oregon and being able to cover several miles of beach quickly would definitely speed up the finding of the newly formed pay streaks, or the newly uncovered old deposits. Hummm... just dreaming of an easier way. I suppose it isn't practical, but I can dream.

Say, Rob, you mentioned that some of your mineralized sand layers (pay streaks) are several feet thick? Awesome! Yours is the first report I heard of such thick mineralized sand layers. I read reports of ruby red sand layers on the beaches of Cape Yakatage, Alaska, being 18 inches thick and more than a mile long after one Winter Storm of 1904-1905 revealed them, and the gold recovered from that particular pay streak was running about 4 ounces gold per cubic yard.

Do you have an occasion to auger down the 10 to 12 feet that it'd take to bring up a core sample of your really thick mineralized sand layers? Can you tell me what your typical recovery rates in ounces per ton or cubic yard when you do get to your pay streaks?

Here on our West Coast and Alaska beaches we use 3/16 inch (~5mm) expanded metal over Nomad mat for our beach sluice construction. The 3/16 inch expanded metal actually has a horizontal pattern dimension of 1/2 inch (12.7mm) but the virtical opening is closer to 3/16 inch. I have heard from some Australian beach miners that some of you use polar fleece?

Anyway, thanks for your help and your idea about the tailpipe auger.

I have another question about detecting mineralized sands levels in the tailing shoot of a sluice, but perhaps this question would best be suited for another thread. Basically what I want to do is to wind a rectangular coil around a plastic shoot attached to the end of my sluice and detect the relative amount of mineralization in the tailings. When blond sands comprise the tailings the relative mineralization would be very low, but when black sands (containing magnetite and/or hematite) the reading would be higher relative to the amount of mineralized sands being discharged as tailings. I am not looking to detect individual targets (beach gold is too small). I am only wanting to detect relative mineralization. Wouldn't an impedance meter and coil work for such an application?

Thanks

[robby_h]

I suppose I took a shortcut when answering your query and didn't explain the situation here well.
To start with, most of my fiddling was done 35 years ago and the local beaches have been closed to any form of mining for many years now so nobody is doing anything in my area at all, well not legally anyway. The beaches were worked extensively by hand in the old days by guys who dug huge trenches and worked the sands with cradles etc. I have no figures for the pay rate but some of the later large commercial bauxite and rutile dredges paid all costs with the gold recovered and also made a good profit from it.

Like I said before, the right storm will rip huge quantities of surface sand off the beach and reveal the bottom and this may stay like this until the next storm covers it but this is rare unfortunately. The bottom can be up to several feet thick but only in pockets around the bedrock but in most cases it varies and is around 6" to over 1 foot thick. The layers in the top sand are usually thin and sand crabs often reveal the location of the shallow ones. You can see their little black sand dumps all along the beach.

The idea of using a sled mounted coil to pick the layers might work but you would need a circuit which responds to the black sands "x" component. The problem though is this effect drops off rapidly with depth and you would need to do some tests to find out how well this would work. With out giving this a lot of thought I think the conductivity of the salt rich wet sands might be a problem in damp areas if you were to use vlf style detection or impedance. You could use a non-conventional pi method to detect "x" while ignoring conductivity but this would mean sampling late during the TX period. Like I say, this needs more thought.

I have extensions for the auger but have never used them on the beach because of the restrictions.
Rob.

[danielxlt]

I'm looking the same kind of answer.


Thanks, Daniel