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Skookum

Do You Consider Your Soil Highly Mineralized And Where?

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For the most part all my permissions or sites that i have detected on over the years are pretty mild mineral content,i do almost all my detecting just north of roman Londinium and never really have any major issues ground mineral wise,but i/we have access to a roman trading villa sites that has been detected for i guess just over 12 years,this sites was a active roman trading site for 400 years,so the amount of coinage that has been found with detectors without exaggerating must be approaching a 5 figure amount its been that prolific.

Only trouble with this site the ground is basically just like black sand from the beach,very highly mineralised but just natural occurrence and not man made,the only machines that will really work on the site and produce finds is multi freq VLF and Pulse machines ,the current king and will be ongoing is the Equinox,and also CTX and Etrac's.

This one of unique site is possibly the hardest site that i have detected,but it still keep producing finds on a regular basis even after all these years.

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On 8/12/2019 at 10:41 AM, GB_Amateur said:

It's worth noting that the map shows iron concentrations totalled over all chemical forms, not just the insidious (to detectorists and other gold recoverers) magnetite.

Interesting... I hadn't considered that some iron compounds might not sway the detector.  I was thinking that most iron compounds had some (varying) degrees of magnetic susceptibility.  Perhaps not.  However, one particular clip from that excellent Fisher reference you cited said something that made me wonder, again, if elemental iron is a problem.  Here's it is:

Quote

Most iron-bearing minerals exhibit magnetic effects similar to that of magnetite (i.e., low loss angle), but with much lower magnetic susceptibility.

I appreciate Steve's comment that localized variations can be dramatic and multifactorial.  Now, I'm back to wondering if that map reveals any generic trend in detecting difficulty.

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I agree with Gerry in regaurds to Montana being very very hot. The best areas I hunt are impossible with a vlf, but the monster is in fact capable if you reground balance often when it gets too noisy. Some areas you can't even move your coil an inch in any direction. This isn't just for a few feet or yards but 1-5 miles. You can see the black sands on the surface and if you dig down 5 inches, 10 inches, 1.5 feet, 3 feet, it is the same thing. Not tell tale signs of black sand but the dominant regolith is black sands and  granodiorite.

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18 hours ago, Skookum said:
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Most iron-bearing minerals exhibit magnetic effects similar to that of magnetite (i.e., low loss angle), but with much lower magnetic susceptibility.

 

Pretty sure the 'low loss angle' refers to the phase shift (related to target ID).  I think that shows up as the ground reading (phase) and setting.  But the magnetic susceptibility is the property that leads to loss of depth.  Maghemite also has high magnetic susceptibility.  When your manget picks up black 'dust', that's magnetite.  When the attracted dust is brown/tan it's maghemite.  At least that's my simple minded view.  BTW, I could never remember the word 'maghemite' until I just read (while researching my above post) that its first two syllables are 'mag' (from magnetite) and 'hem' (from hematite):  mag-hem-ite.

I don't mean to downplay the presence of iron as causing problems.  Even "mildly ferromagnetic" material can affect things if enough is concentrated.  It's the ferromagnetism that is the issue.  Iron in some compounds isn't ferromagnetic.  It just depends upon the form (compound) of iron, and there are many different types in nature.

 

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