Just confirming what bravekind stated - DeBeers' mandate is diamonds only, and if they find something else on their properties they usually pass it on to another company.

As a diamond company, Debeers is looking for kimberlite formations, which are the host rock for primary source diamond deposits 99% of the time. Kimberlites will show up as magnetic anomalies (cicular if its a pipe) in geophysics due to the metals/minerals included in the kimberlite formation, but not the kimeberlite rock itself (kimberlite is volcanic, and carries many things with up to the Earth's surface, including diamonds (not all imberlites are diamondiferous), from depths such as 80Kms.

Magnetic geophysical anomalies, are just that, magnetic anomalies. However, they are often the first step in identifying new exploration targets, since metal minerlization will obvioussly register as a magnetic signature once the geophysics data is transcribed onto maps (by computer programs). Surface sampling and drilling is employed to test those anomalies to to determine what they are.

For Debeers, if the anomaly proves to be something other than kimberlites, they move-on. What is interesting is that because kimerlite and VMS deposits are volcanic (types of lava being squirted, pushed, or forced through the Earth's surface in weak areas or faults in the crust (imagine bursting a deep pimple), they often register as circular anomalies. This is not always the case, as the lava can be forced into linear area of weakness creating formation often referred to as dikes (this is a simplied explanation).

Anyway, the great thing about DeBeers is that they have tonnes of cash to explore for diamonds. As they cross the glode drill-testing  geophysical targets, they kick-up potential mineral deposits.

 Regards,

B.