Service: Locate areas where magnetic signatures are overprinted
SOMETIMES the important things don't always scream the loudest. If your targeted mineralization style is associated with overprinting processes such as metasomatism or hydrothermal alteration, it can pay to look for what's not there. Zones containing rocks that are relatively magnetically quiet can speak volumes.
The alteration processes that often accompany mineralization can sometimes subdue — or even completely destroy — a lithology's usual magnetic signature.
The most important magnetic mineral in mafic to ultramafic rocks and in granites is magnetite. In metamorphosed black shales and schists, the most important magnetic mineral is pyrrhotite. 
Introducing sulfur-rich fluids into a lithological system can convert pre-existing magnetite, a strongly magnetic iron oxide, into pyrite, an iron sulfide with a magnetic susceptibility that is virtually non-existent. Minerals undergoing this kind of swapping-out to incorporate sulfur are said to be sulfidized.
Another example of demagnetization can occur when magnetite is oxidized. It's often converted to the virtually non-magnetic mineral hematite.
Pyrrhotite, a reasonably magnetically susceptible iron sulfide, can also be altered to form a non-magnetic mineral. The precise alteration product depends on the environment's pressure, temperature and pH profile, but the possibilities include ferric oxyhydroxides such as goethite , and the minerals pyrite and hexagonal pyrrhotite .
Aside from oxidation and sulfidation, other alteration processes include biotitization, carbonation, and silicification. All of these are capable of destroying or impairing a rock's pre-existing magnetic signature .
Demagnetization has been associated with the following mineralization styles:
- Porphyry copper: Significant deposits of this kind almost always involve magnetite-destructive alteration, because they'll have reached the advanced alteration stages that produce a large outer halo of disseminated pyrite , among other minerals.
- Banded iron formation-related iron ore: It's thought that structurally-controlled hydrothermal fluids contribute to upgrading banded iron formation (BIF) to hematite-rich iron-ore deposits such as Mount Tom Price in Western Australia. The process destroys the initial magnetic fabric of the BIFs.   
- Orogenic gold in greenstone belts (aka lode-gold, aka structurally-controlled gold): For instance, in Finland, hydrothermal alteration led to magnetite destruction, which in turn reduced the amplitude of magnetic anomalies seen in the Central Lapland greenstone belt. 
- High-sulfidation epithermal deposits: Zones of flattened and smoothed magnetic response were found to occur around gold deposits of this type. 
Note also that several examples of Mesoproterozoic-aged gold prospects in the central Gawler Craton region of South Australia were observed to contain demagnetized mineralized zones in which host granitoids' magnetite had been hematized, sulfidized or removed.  
Iron-ore exploration case study
If magnetic minerals are demagnetized in a dramatic enough and widespread enough way, it will show up in high-resolution airborne magnetic data.
Let's take a look at a case-study area in the African nation of Cameroon (see Figure 1).
FIGURE 1: Image showing the negative of reduced-to-the-equator magnetic data (total magnetic intensity). The lithology displays significant remanence, which is seen most clearly here as numerous dark-blue lows directly adjacent to dark-red highs.
The area is dominated by a gneiss that contains magnetite-rich zones (see the areas showing up in yellows, oranges and reds in Figure 2).
FIGURE 2: Analytic signal of the data shown in the previous figure. This reduces the influence of remanence on the data.
Generally, in a magnetic image, the partial or complete destruction of magnetite can be seen on a local scale as incisions, plateaus and other disruptions in the typical 'fresh' magnetic response of a given rock unit.
We can see these sorts of interruptions throughout the gneiss rock-unit in our case-study area. It's thought the magnetite-rich zones have been significantly hematized in places, a process that would locally demagnetize the gneiss.
What's important to note about our case-study area is that any hematization would also have locally removed gangue-mineral elements such as phosphorus from the rock and bumped up the iron grade.
In other words, places involving dramatic demagnetization would likely make good candidates to investigate further for targeted economic mineralization.
With our geophysical data-processing technique for detecting demagnetization, we were able to locate what are likely to be heavily-demagnetized zones.
Note that we were able to accomplish this on quite challenging data. The dataset was not only acquired close to the magnetic equator but also featured lithologies affected by significant remanent magnetization (see Figure 1).
Here is a general overview of how we carried out our demagnetization detection technique:
- We identified anomalous, linear highs (magnetic ridges) and their interrupting lows (demagnetized valleys) in the magnetic data (see Figure 3).
- We found where the magnetic ridges and demagnetized valleys were most significant (see Figure 4).
- We located zones where the significant magnetic ridges were cut by significant demagnetized valleys (see magenta zones in Figure 5).
FIGURE 3: Results of Fathom Geophysics' demagnetization structure detection, in monocolor. Magnetic ridges in the data were identified (these trend roughly northeast), along with the demagnetized valleys that sharply interrupt the ridges. These valleys trend roughly northwest.
FIGURE 4: Demagnetization structure detection results in monocolor, with yellow vectors identifying the location of significant magnetic ridges and cyan vectors identifying the location of significant demagnetized valleys.
FIGURE 5: Demagnetization structure detection results in monocolor, with magenta zones highlighting the locations where significant magnetic ridges (yellow lines) are interrupted by significant demagnetized valleys (cyan lines).
We can then take the detected demagnetized zones and see where they occur on the magnetic image for the case-study area.
Figure 6 shows the results superimposed on a monocolor version of the magnetic image, which helps prevent the eye from being distracted by the magnetic signal in the background.
FIGURE 6: Analytic signal of the data, but this time in monocolor, with magenta zones highlighting the locations where significant magnetic ridges (yellow lines) are interrupted by significant demagnetized valleys (cyan lines).
Figure 7 presents the same thing, except the magnetic image is shown in its original, full color range. The vectors representing the demagnetized valleys are shown this time in black, which stands out better than cyan against the brightly colored background.
FIGURE 7: Analytic signal of the data in color, with magenta zones highlighting the locations where significant magnetic ridges (yellow lines) are interrupted by significant demagnetized valleys (black lines).
Finally, Figure 8 shows just the detected demagnetized zones over the full-color magnetic image. This permits clearer viewing of where the zones are located relative to the magnetic signature of the area's rocks.
FIGURE 8: Analytic signal of the data in color, with magenta zones (ringed in white throughout to make them easier to see) highlighting the locations where significant magnetic ridges are interrupted by significant demagnetized valleys.
— Fathom Geophysics gratefully acknowledges Mark Wilson of Legend Mining for permission to discuss this Cameroon case study and to present some imaged products of our data processing on the area.
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