Wednesday, June 4, 2014

Prospecting for Placer Gold with Geomagnetism (Part 2)

 (Operator with portable magnetometer noting an anomaly.)

Using a portable magnetometer ("mag") to search for and locate black sand concentrations (and potential placer gold) is one form of geomagnetic prospecting some of you out there may want to consider. Interested? Read on then.

Here are two key points to remember if you're considering using a portable mag in your gold prospecting endeavors:

1. The main objective of a geomagnetic survey is to locate and evaluate (measure) magnetite concentrations. However, not all magnetic anomalies (black sand concentrations) indicate the presence of gold, placer or otherwise.

2. Following hard on this last point, any magnetometer indication of black sand concentrations or other anomalies should be followed up by test panning or other gravel sampling methods. Ultimately, only properly conducted sampling will tell the tale in terms of how much gold may be present in placer gravels with high magnetite concentrations indicated via geomagnetic survey.

(The simple gold pan is still your best sampling tool after anomalies are detected.)

The depth of a given "target" gravel layer can vary widely (from a few feet to hundreds of feet), so be prepared for this out in the field. Small-scale mining guys and gals like us are not likely to test, let alone develop mag targets at great depths, but there is a sunny side to this issue. Find an economically viable geomagnetic deposit and you may be able to sell the mineral rights to a large company with the money and resources to pay you for the privilege. Remember, this potential exists in both wet and dry or desert placers.

Bedrock Characteristics

Although bedrock is usually our friend when it comes to getting the gold, some types of bedrock or bedrock formations with certain types of configurations can mask or distort mag signals. The ideal bedrock for geomagnetic surveys is described by the experts as containing one or all of the following characteristics:
  • Simple meta-sediments
  • Igneous intrusions
  • Uniform magnetic "background"
If you're not sure what these high-falutin' terms mean, let me break them down for you:

Meta-sediments are simply sedimentary rocks that have been altered or significantly changed by metamorphism. (I've discussed the importance of metamorphics in previous posts, so you may want to use the Bedrock Dreams search function to locate and read these.) Igneous intrusions are basically layers or stringers of volcanic rock that formed when lava (igneous liquid) was forced or squeezed into rock cavities or between rock layers. A uniform magnetic background means that potential magnetic interference is at a minimum in the area you're conducting your survey. That's essentially it.

 (Igneous intrusions.)

In my next post I'll give you some recommended portable mag procedural tips. Until then, stay safe.

(c) Jim Rocha (J.R.) 2014

Questions? E-mail me at

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