BAD GROUND ANALYSED

[Eric Foster]

I have recently had discussions on an Australian forum with Prospector Al about some bad ground on a gold detecting site, which is part of a claim being worked by Sawmill, a professional prospector. This ground was reported as being so severe that

 

[Eric Foster]

Today I separated the magnetic grains from the other material in dried sample of Sawmill's ground. The easiest way I found to do this was to hold a ferrite magnet with its end in the material. This would result in a big clump on that particular pole. Tapping the magnet would cause most of the the non-magnetic rock particles to fall off, so a couple of cycles of this did a pretty good separation. In fact I was surprised how little was non-magnetic. I reckon over 60% of the sample volume was magnetite particles.

A 10gm sample was weighed out as before and tested on the Bartington susceptibility meter. The low frequency susceptibility was 11,622 and the high frequency reading 11,551, which is nearly 5x that of the unseparated material. Also noted was the fact that for 10gm weight the sample pot was only half full. The specific gravity of magnetite is about 5 and many rocks are much less. the host rock here seems to be quite light, so this emphasises the fact that it will collect in pockets and gullies where there will be natural concentrations of the material. Not surprising therefore that it can cause difficulties for metal detectors.

I got a reading on 10 of the MVM, although this did not correlate with the frequency difference for some reason I have yet to investigate, but the last two digits in susceptibilities of this order are very variable. Shake the container and there could be a plus/minus difference of 20. The MVM consistently gave 10, which indicates a low level of viscosity signal for a PI unit.

For magnetite grains to become superparamagnetic, and give a viscous decay, they have to be smaller than 0.03um at room temperature, which is pretty darn small. This would likely explain the small value of the viscosity signal. Heating a sample should cause this to increase, which is another experiment worth doing.

I haven't found the figure I want yet for firing magnetite to get maghemite. The only thing found was that at 600degC maghemite and magnetite revert to hematite, which is only weakly magnetic. So it has to be less than that.

That temperature experiment in the previous but one paragraph wasn't worth doing, at least it would be better done a different way. For quickness I put the sample pot in the microwave. After only a few seconds it went off like a cannon. Now I have to clean magnetite off all the surfaces in the microwave and check out the dent in the top panel. Maybe its turned into maghemite.

Eric.

 

[Tony]

Hello Eric,

I remember years ago when prospecting that certain areas of the Western Australian goldfields had numerous areas of incredibly "hot" ground. The areas were darker and appeared "glazed". Do you think this could be the result of lightning strikes. Severe thunderstorms hit this area very hard during the summer. I always was curious to know the end result of a 25000C bolt of lightning. People have reported areas of heavy ironstone being "welded" together, possibly by lightning strikes.

Not sure if this ties in with your microwave work but it got me thinking.

Best regards,
Tony.

 

[Barbarossa]

Apologies if this question appears stupid (there's no such thing anyway) or is off-topic - what is the consensus on how large gold nuggets form...?

Something of 5 or 6oz seems rounded to a degree and must have experienced heat of some sort...so why are they available within a foot of the planet surface but seemingly alone...?

Thanks from an amateur geologist

 

[Prospector Al]

Hi Eric,

Here's an update on what I have been doing. I really would like to know what the upper limit of the viscosity signal is so I have been trying to convert magnetite to maghemite. The dental furnace I bought on Ebay broke after a while and I had to trace the circuit of the controller PCB to fix it. That took some time. In the mean time, I kept the ball mill with the magnetite going and the particles are very fine now. The way I check for the presence of a viscous signal is as follows: I have a setup where I can balance out the quadrature and in-phase signals from a sinusoidally driven coil system. An unbalance caused by the introduction of a sample of ferrite causes a quadrature signal only, whereas a ferrous metallic target causes both quadrature and in-phase signals.

A black sand sample causes a quadrature signal and I expect to see an in-phase signal, when a conversion to maghemite has occurred. I noticed the appearance of a small in-phase signal in the milled magnetite, even before firing. I was not aware of the fact that a conversion to hematite occurs at a higher temperature. Now I have to start firing my samples at small temperature increments--perhaps 50 C? Will keep you posted, and when I notice the appearance of a significant in-phase signal, I'll send you a sample for verification.

Congrats on your marriage,

Allan

 

[mikebg]

[size=medium]Hi Allan
At which frequency you measure your Re and Im signals?[/size]

 

[lazy boy]

hey we Glad we got back on here a lot of good reading here we use the TDI PRO silver coin hunting and love it over 10 in the last two weeks one was a 1825 cape bust 4 seated half dime and the rest was seated Q all in place that they say was hunted out we love it LITTLE MAN

 

[dbreese]

Eric, Would like to discuss project with you using your High Power AP, Do you use Skype?
Been a long time, hope you and your well. Warm regards Denny Breese