Monday, June 2, 2014

Prospecting for Placer Gold with Geomagnetism (Part 1)

(Magnetite being removed from heavy black sands using a strong magnet.)

I suspect the old timers would shake their heads in amazement at most of the technology based gold prospecting methods in use these days. One of these new-fangled approaches is geomagnetic prospecting and it's something those of you interested in finding your own placer gold deposit should consider.

Most of you who've been around the gold prospecting and mining arena for some time are fully aware of the connection between heavy black sands and placer gold. One of the main constituents of those black sands is, of course, magnetite. Magnetite (like its geological cousin hematite) is essentially a form of iron oxide and, as its name suggests, is a type of natural magnet.

Magnetic Surveys

The magnetic intensities of the black sands in various placer gravels can vary widely and are often difficult to measure due to various geological factors. One way to detect and measure the magnetite in potential gold-bearing gravels is by using a magnetometer. A "mag" is an electronic instrument for measuring the strength (and often the direction) of a magnetic field. Mag's are capable of detecting irregularities in natural magnetic fields and more importantly, can indicate the location of magnetic ore/black sand deposits that may contain gold (and sometimes lots of it). Large gold corporations and even many smaller commercial operations frequently use geomagnetic surveys to lock onto potential gold locations of economic value.

Most geomagnetic surveys are conducted on the ground but in the last 30 years or so airborne surveys have become increasingly common, especially with larger corporations who have the cash to bankroll this sort of endeavor. Although the mags used in these aerial surveys are too large and too expensive for small fish like us, don't despair. Harness-mounted mag units weighing as little ten pounds (like the Geometrics model shown below) can be used effectively out in the field in all sorts of conditions.

 (Geometrics Model 856 Proton Precession Magnetometer.)

Using a portable mag to effectively locate potential gold-bearing deposits is contingent on three main factors:

1) How much magnetite is in suspected gold-bearing gravels.

2) Gravel depth.

3) What sort of bedrock underlays those gravels.

Remember, heavy black sands and magnetite are found in many locations throughout the world where NO gold mineralization exists at all. Just because you find black sands in a stream bed or wash doesn't necessarily mean gold will follow. To assume all blacks sands contain gold is a very common mistake made by the inexperienced, the uniformed, or by newbies or novices to gold mining and prospecting.

On the other hand, recent geomagnetic surveys conducted in the western United States and in Canada have shown that placer gold values ARE found associated with black sands and magnetite more often than not. I guess the upshot is this: if you're going to go to the expense of buying a good mag and then using it for prospecting, it'd make better sense if you're in an area with recorded gold production or verifiable gold mineralization.

We'll talk more about all this in subsequent posts. Until then be safe and keep smiling.

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

Questions? E-mail me at


  1. Good morning Jim, are black sands and the formation of gold related or are they found together more because of weight? Some places I've been to, there is so much black sand that it is a problem. It chokes your sluice riffles and is hard to separate any gold from. I have been told it has some value of it's own, is that true? If so, what do they do with it and where can you sell it? Thanks, Gary

  2. Geologically speaking, their similar specific gravities are what link placer gold and heavy black sands. Yes, some people/companies do purchase black sands concentrates but they typically do that in very large amounts. Typically they recover any metal or mineral constituents of economic value from those black sands. Best, J.R.

  3. The old timers considered iron the mother of gold. They would look for quartz veins with rusty iron in it or in contact with the quartz. It seems that as the veins of quartz with gold are formed, iron is a catalyst and causes the gold to form in contact or close contact to the iron. A lot of pocket gold is found in a pocket of snuff of sorts, which is nothing more than rusty iron.
    Unfortunately there is a lot of iron around with no gold at all associated with it.

  4. Thanks for the info and for sharing your perspective. Best, J.R.