By Dan Oancea

 

Most of the world’s shallow or outcropping mineral deposits have already been discovered. The search for buried deposits can be successfully completed only by means of geophysics and geochemistry.

Geophysical surveys are useful in the first phases of mineral exploration; the resulting maps are telling us about buried ‘objects’ that are generating an anomalous electromagnetic, gravimetric or radiometric response.

We can now locate them but we don’t know what are they made of.  To find the answer without having to do any drilling, explorationists employ different geochemical methods. This way they will find out something about the nature of the mineralization (chemical elements involved and possible type of mineralization).  If the geochemical and geophysical anomalies are coincident, we get a drill target (but not before having a good look in the field at the area of interest and placing this new piece of puzzle in the larger geological picture to see if it fits).  Further on, in order to prioritize the targets sometimes we pick the strongest anomalies. Sometimes, not.

There are different geochemical techniques. A recent one is the MMI geochemical survey. The Mobile Metal Ion Geochemistry is an important worldwide mineral exploration technique; and most importantly it is a proven one.

Theory states that a buried ore body through physical (e.g. capillary rise) and electrochemical processes frees mobile metal ions which are migrating to the surface, where they are weakly attached to soil particles. These mobile metal ions are sampled by the MMI technique. Their mobility is subsequently restricted by different processes resulting in them being permanently bound to the soil particles. These soil particles are now subject to other surface physical processes (weathering, erosion) resulting in them being spread away from the original area of occurrence at the surface (i.e. they are not located anymore on top of the buried mineralized zone). This is actually a dispersion zone.

The main difference between the classical geochemical soil sampling and the MMI technique is the fact that the former samples and witnesses the dispersion zone, while the latter gives us accurate information regarding the exact location of the buried bodies. Now this is indeed a valuable piece of information.

The best site dealing with the MMI technique is the MMI Technology’s site. You can find everything there: theory, the MMI Process, technical bulletins (e.g. all you need to know about collecting MMI samples or the tools involved – garden tools would suffice), case Studies, press releases and news, a MMI Manual and other useful info.

I noticed good case studies involving different types of mineral deposits, from kimberlites to gold deposits. I also noticed that MMI proved to be able to identify real geochemical anomalies in a case when the classical geochemical soil method proved to be unable to do the same (the Botswana case). Many exploration companies are already using the method with good results and you can read some of their press releases on the same site.

Other advantages: it samples the A horizon of soils (10-25 cm below surface), produces a sharp distinctive anomaly over the ore body in contrast with the one produced by the transported material, thus being able to differentiate them. It also has a  better success rate, a better signal to noise ratio for detecting deep sources, and it targets primarily ore elements.

Everything points in one direction: a simple, high resolution and cost effective technique which could be successfully employed in the search for buried mineral deposits as an adjunct to geophysical methods.