The interpretation of magnetic anomalies has been an enjoyable part of my job as a geologist for many years now. I'm currently working in the Iron Ore industry in the Pilbara which makes this especially challenging. The Archean rocks of the Pilbara are variably magnetic, with fresh Banded Iron Formation (BIF) creating magnetic highs and remnant magnetic fields creating magnetic lows. Neutral magnetic signatures can come from enriched iron deposits where the Fe is no-longer magnetic or from overlying detrital packages. Detritals can also be enriched in magnetic pisolites which create shallow, highly variable anomalies.
Shown above is a standard geological probe with a small magnet imbedded on the top. I have run the magnet through the dust next to the car and then rinsed the result in my water bucket. As you can see, I'm left with very fine grains of magnetic material which form pseudo-crystals as they self organise in the magnetic field.
With current supplies, the soil and BIF is not deemed to be sufficiently enriched in iron (it's less than 45%) to offset the work of removing gangue material such as the silica/chert.
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