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GB Numbers = Mineralization?


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I often see posts on various forums where people use high GB phase numbers as examples of hot, mineralized ground. I thought that GB phase numbers are only indicative of the TYPE of ground(rock,soil,clay,salt). Rock, soil, and clay can actually be pretty benign or very mineralized. Isn't the determining factor for hot ground that affects metal detectors  the amount of Fe3 in the soil? A phase reading of 89 may be mild soil if the amount of IRON in the soil is low. Conversely a phase reading of 65 may be very hot ground if the ground consists of clays with high iron content. Some VLF detectors now have Fe3 meters on them and the higher the reading is on that meter the more the ground will affect your detection depth and the accuracy of the VDI number(if supplied).

It seems like many people are confused by this. I think it is important info that can affect your coil selection(size and type), the amount of discrimination you may choose to use, and the mode(all-metal or discriminate) that we run in.

We need somebody that knows their stuff to give us a definitive answer!

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I would say you have it spot on Merton.

It is a common misperception that the ground balance (ground phase) setting on a metal detector tells you how bad the ground is. It is only vaguely related to that. The ground balance setting is determined by the type of ground mineralization you are dealing with, but it does not directly report the amount of ground mineralization or magnetic susceptibility. In other words, you may be setting the ground balance to reject magnetite, the most common iron mineral in a lot of locations. What you do not know is whether you are tuning out a lot of magnetite or a little magnetite. Several Dave Johnson detectors like the White's GMT and the Fisher F75 and Gold Bug Pro plus Teknetics T2/G2 have Fe3O4 amount readings to complement the ground balance reading. It is this combination of the TYPE (ground phase) of ground mineral as indicated by the ground balance number and the AMOUNT (magnetic susceptibility) of ground mineral as indicated by the Fe3O4 meter that matters. Now for the technical details to back that up.

From the Gold Bug Pro manual:

Understanding ground conditions assists the user in setting up the machine, knowing when to readjust ground balance, and in understanding the responses of the machine while searching.

This detector displays two kinds of ground data:

1. The type of mineralization (which affects where the ground phase should be set). This is GND PHASE

2. The amount of mineralization (the greater the amount of mineralization, the greater the loss of detection depth & ID accuracy; this loss is more pronounced in Discrimination Mode). This is Fe3O4.

The goal of ground balancing is to equate the GND BAL number to the PHASE number. PHASE is the measurement of the ground. GND BAL is the detector’s internal setting which calibrates the detector to the ground’s phase. Notice that the GND BAL number is three digits, with a decimal point. PHASE has only two digits. GND BAL is a higher resolution number, so may differ a bit from PHASE in a perfectly balanced scenario. After pumping and releasing GG, the exact measurement of the ground will be transferred to the GND BAL setting. The two-digit PHASE number displayed on the screen indicates the type of ground mineralization.

Some typical ground mineralization types are:

0 – 10 Wet salt and alkali
5 – 25 Metallic iron. Very few soils in this range. You are probably over metal.
26–39 Very few soils in this range -- occasionally some saltwater beaches
40–75 Red, yellow and brown iron-bearing clay minerals
75–95 Magnetite and other black iron minerals

Fe3O4 BARGRAPH

The Fe3O4 7-segment bargraph indicates the amount of ground mineralization, independent of type, expressed as an equivalent volume concentration of magnetite (Fe3O4). It updates every second. It is sensitive to motion and will give the most accurate readings if you pump the searchcoil up and down several times over the ground. The presence of metal or “hot rocks” will cause the readings to be inaccurate. If you stop moving the searchcoil, the bargraph will go blank.

INDICATION RELATIVE % Fe3O4 SUSCEPTIBILITY MINERALIZATION

7 Bars -------- High over 1 over 2500
2 to 6 Bars -- Medium .026 - 1.0 61 - 2,500
1 Bar --------- Very Low 0.006 - .025 15 - 60
none -- less than .006 less than 15

Magnetic susceptibility is expressed in micro-cgs units. In a salt water environment in the absence of iron minerals, the bargraph indicates relative electrical conductivity. In soils with greater than 10,000 micro-cgs units magnetic susceptibility, the signal from the soil may saturate, or overload, the circuitry. This will not harm the detector but the machine will not be usable in that condition. The solution is to hold the searchcoil several inches above the soil surface so it is not “seeing as much dirt.” By listening and watching you will know how high you need to hold the searchcoil in order to avoid overload. The highest magnetic susceptibilities are usually found in soils developed over igneous rocks, in alluvial black sand streaks on beaches, and in red clay soils of humid climates. The lowest magnetic susceptibilities are usually found in white beach sands of tropical and subtropical regions, and soils developed over limestone.

The Fisher F75 and Teknetics T2 have a better defined Fe3O4 meter as explained in the T2 manual:

BAR GRAPHS Fe3O4 (magnetite)

This bar graph displays the magnetic mineralization factor, or magnetic susceptibility, of the soil. Magnetic susceptibility is expressed in terms of the percent volume of the iron mineral magnetite, which most black sand is made of. The depth to which objects can be accurately identified is strongly influenced by the magnetic susceptibility of the soil. High Fe3O4 values have a greater effect on detection depth in the Discrimination mode than in the All Metal mode. For the most accurate Fe3O4 reading, pump the searchcoil as though you were ground canceling.

Fe3O4 approx. Range micro-cgs Description

3 --- 7,500 --- uncommon but not rare, heavy mineralization
1 --- 2,500 --- heavy mineralization, not uncommon in goldfields
0.3 --- 750 --- heavy mineralization, but not uncommon in some regions
0.1 --- 250 --- medium mineralization, typical
0.03 --- 75 --- light mineralization, but common
0.01 --- 25 --- light mineralization, often low G.C. setting
blank <14 --- quartz & coral white beach sands

From Bruce Candy at https://www.detectorprospector.com/files/file/52-metal-detector-basics-and-theory/:

"In geologically new soils, the degree of mineralisation is usually weak, except for some volcanic soils. These relatively new soils are commonly found in North America and Europe (from glacier scrapings during the last ice age and mountain erosion etc). In contrast, surface soils which have remained surface soils for a long time often have high mineralisation, because the action of water, over a long period, causes iron compounds to migrate to the surface. For example, Australia has old soils, having had no glaciers recently or significant mountains to be eroded. Some volcanic rocks or sands, known as black sands, may be highly mineralised and are found, for example, in a few USA mainland and Hawaii areas. These black sands (or rocks) are made of mostly magnetite, an iron oxide called ferrite. These typically produce almost entirely X signals, and almost no R. They are heavy, that is they have a high density, and can be identified because they are strongly attracted to a magnet. Small roundish magnetite/maghemite pebbles (a few mm in diameter) are also attracted to a magnet. These, for example, may be found in many Australian goldfields, but do produce significant R signals. Thus, USA goldfields are typically different from Australian goldfields:

  • The USA soils are mostly mildly mineralised but in some areas may contain either nearly pure magnetite black sands or rocks, which  are problematic for metal detectors as they have very high X components (strongly attracted to magnets).
  • Australian goldfields have highly mineralised soils, but very few black sands or rocks that contain nearly pure X magnetite. The magnetic materials are in the forms of magnetite-rich small pebbles and rock coatings, clays and general “sandy” soils. These all contain magnetic materials that produce high levels of X signals as well as R. The ratio of X and R is random, and the R component arises from extremely small magnetic particles called superparamagnetic materials, which are discussed below."

Advanced Nugget Hunting With the Fisher Gold Bug Metal Detector by Pieter Heydelaar & David Johnson. Part 2 of this book is titled The Effects Of Ground Minerals, Native Metals and Man-made Metals on the Fisher Gold Bug starting on page 29 https://www.detectorprospector.com/files/file/55-advanced-nugget-hunting-with-the-fisher-gold-bug-metal-detector/

Predicting Soil Influence on the Performance of Metal Detectors: Magnetic Properties of Tropical Soils http://www.jmu.edu/cisr/journal/13.1/rd/igel/igel.shtml

Influence of Soil Properties on the Performance of Metal Detectors and GPR http://www.jmu.edu/cisr/journal/17.1/RD/takahashi.shtml

Magnetic Properties of Rocks and Minerals http://wellog.com/RF003p0189.pdf

Workshop On Soil Magnetism http://www.gichd.org/fileadmin/pdf/LIMA/SoilMagn_Proceedings2008.pdf

magnetic-susceptibility-rock-types.jpg

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Can I throw a curveball into this discussion?

I get the black sand (magnetite) issues but my closest goldfield is covered in volcanic material which includes a mineral called specular hematite.

This stuff drove my local geologist's crazy trying to work out what it was and it took a full spectrum analysis to nail it down.

It is 96% FeO2 with the remaining 4% made up of rare earth minerals, the "unobtainium's"

It drives PI machines nut's, mono coils cannot be used and DD's need to be dumbed down to near useless levels to be usable.

The last thing that makes detecting very difficult is that this stuff does not stick to a magnet which means every targets needs to dug and sifted to find the target. Pieces the size of match heads sound off like nails so they need to be dug

Photo below of some bits that I have dug

IMG_0983_zpsqslypfqv.jpg

 

 

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Thanks for the great answer Steve. I hope this thread dispels the fog surrounding this murky subject.

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Great answer Steve which has illuminated some new thinking for me and given me a new focus while I'm riding out the frozen conditions here. As always, I'll be revisiting my manual again as it perpetually seems like I'm missing something, missing things I need to understand better to refine my detecting. You sure know your stuff!

Peteren, thanks for the write-up on hematite as it sure is obnoxious stuff and being non-magnetic, as you say, it's a stinker to get out of the way. Magnetics are fairly easy to deal with by using a super-magnet, but hematite has to be removed the hard way, so I understand why it's driving you crazy. 

Goldbrick, great question, thanks for posting it.

All the best,

Lanny

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Thanks Lanny but all I am doing is cutting and pasting. It is best to go to source material for this type of stuff because so much out there is based on opinion. Goldbrick is right, people constantly use ground balance numbers to compare mineralization from one site to the next. Most people would agree that ground with lower ground balance settings is generally easier to handle than ground with high ground balance settings. However, the information below will show that is not always true, it is just what we experience most often in the U.S.

I did throw that note in from Bruce Candy above because Australia ground in general is fundamentally different than most ground in the western U.S. Our big offender tends to be magnetite (Fe3O4), in Australia it is usually maghemite (Fe2O3). From Dave Johnson at https://www.detectorprospector.com/files/file/53-gold-prospecting-with-a-vlf-metal-detector/

 

Susceptibility refers to a material’s ability to attract a magnetic field. In the context of metal detecting, it corresponds to the amount of magnetically active mineralization in the soil. It is often expressed as an equivalent percent by volume concentration of magnetite.

Tangent of loss is the ratio of magnetic energy absorbed by a material and dissipated as heat, divided by the magnetic energy which is attracted to the material and not dissipated. The tangent of loss is most commonly expressed in arctangent form as the loss angle. In the context of metal detecting, it corresponds to the ground balance point of the soil. In a general way it represents the type of mineralization present rather than the amount.

Magnetite (ferrosic oxide) is a heavy black iron oxide mineral which exhibits high magnetic susceptibility and low magnetic loss angle. It is commonly found as “black sand” or as dense black rocks. It is strongly attracted to a magnet. It usually “balances” near the ferrite calibration point of the metal detector, which on most detectors is within the range of 80 to 95% of full scale. Many black colored rocks, especially igneous (volcanic & extrusive) and high-grade metamorphic rocks, contain appreciable amounts of magnetite. So do many rocks with a bluish or greenish cast, especially rocks in ultramafic greenstone belts. Magnetite in the soil is usually in the form of sand, because particles smaller than sand unprotected by rock matrix tend to oxidize to maghemite or to be dissolved by organic acids.

Maghemite (gamma ferric oxide) is an earthy iron oxide mineral found in most soils and some rocks. Red iron rust is a form of maghemite with which everyone is familiar. Maghemite is formed by the oxidation of lower oxidation state iron minerals such as magnetite, free iron and pyroxene. The oxidation commonly happens through weathering and exposure to fire. Maghemite is usually reddish brown or red in color, and even in low concentrations its color tends to dominate the material it’s in. Like magnetite, maghemite has high susceptibility. It differs from magnetite in having a substantial loss angle, causing it to ground balance in the range of 40 to 80% of full scale on most metal detectors and under most conditions.

Now, if you read that again, magnetite will usually produce ground balance numbers in the 80 - 95 range. Maghemite, although it is much worse for detectors and in fact is what gives even PI detectors trouble in Australia, ground balances in the 40 - 80 range. In other words, ground with lower ground balance settings in Australia can be much worse than ground with high ground balance settings in the U.S.

Final tidbit. You know that really bad, bad soil that drove you nuts in the forest fire area? That is maghemite. Intense heat will bake our more normal soils and produce soils more common in Australian goldfields. Those of you that have run into it now know what the Aussies deal with in some regions.

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Steve,

Thanks for your updates and for the information, regardless of the source. I sincerely appreciate your efforts to be helpful.

As for the forest fire soil, yes I know what you're referring to, and I know how the detector responds to the dirt, though I had no idea why the coil responded to the altered soil, so thanks for that as well.

All the best,

Lanny

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  • 1 year later...

As far as I know the White's GMT is the first detector that Dave Johnson was involved in that included the Fe3O4 meter.

The White's GMT constantly displays the ground balance setting, typically in the 70-80 range. This number is telling you what type of mineralization is affecting the GMT and is very much akin to the target VDI number displayed on coin detectors except that here it is a ground reading. The GMT also displays the amount of the mineral being detected. This is called the "Follow Black Sand" reading because it can be used to trace shallow black sand deposits that in turn can be used to locate gold deposits. A reference number in the upper right portion of the display shows this value. The number grows as the coil is pumped up and down over concentrations of black sand. The readings are relative so the idea is to take readings at various locations across a dry wash for example, and then to focus on the highest readings as being the probable location of a black sand deposit.

From the GMT Field Report:

"The left number is labeled Ground Balance (Type of Mineral). This number is on a scale of 1-100. Higher numbers indicate more negative or "colder" ground, such as black sand. Lower numbers indicate positive ground, such as salt or alkali. This scale corresponds to the ground balance range, and the number shown will tell you roughly where the unit is ground balanced. The number on the right, again on a scale of 1-100, is labeled Follow Black Sand (Amount of Mineral). It will not only let you follow a stringer of black sand hidden in a wash, but it will also let you find old hard-to-see fine tailings piles from drywashers. Pumping the coil up and down may be more accurate than sweeping side to side for these chores."

whites-gmt-follow-black-sand.jpg

So, you are walking up a dry wash. You pump the coil here and you pump the coil there. You pump the coil over a spot to get an accurate amount reading. The answers are relative. One spot reads 30, another reads 50 (I am making these numbers up). The spot reading 50 generally has more magnetite in the soil. Not always as a shallow magnetite deposit might give a higher reading than a deeper deposit that actually has more. But all things being equal, the higher the amount number, the more magnitude. Ignore small differences. But twice as much in one spot versus all the others, perk up!

Field Report by Jim in Idaho here

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There's nobody in "forumdom" world who works like our Steve.

this thread will likely be the "go to" reference for mineralization basics in time to come.

thanks Steve.

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