This question opens the door to several issues: indirect
detection when direct detection is not possible, and unanticipated down-sides
(such as death) to some mineral exploration projects.
Q: Hello Sir,
Could you please tell which is the best method for
locating asbestos, pyrrhotite and manganese? Whether airborne magnetic survey
or ground magnetic survey?
Regards, Ahtisham ul-H.
A: Asbestos (an aggregate of six
different but related silicate minerals) and manganese are not magnetic.
Pyrrhotite (FeS) is sometimes weakly ferromagnetic if there are iron deficits
in the ideal FeS lattice. However, these minerals are all usually associated
with serpentinization, a hydration and metamorphic transformation of ultramafic
(dark, iron and magnesium-rich) rock. Serpentinization usually has significant
magnetite associated with it, and THIS is strongly magnetic. I've worked in
ultramafic rocks where I have personally encountered 3-cm-thick veins of pure
magnetite. It doesn’t take much more than a percent or two of this to make a
rock really magnetic.
The usual method for mapping these sorts of deposits is
ground magnetics and geochemistry, but keep in mind that you are only
indirectly imaging the minerals you are interested in. Airborne magnetics are
often used for regional surveying, to outline target areas for later follow-up
with ground magnetics.
As an aside, please keep in mind that mining asbestos, or
fabrication of asbestos products, is dangerous. My father died of mesothelioma-related
lung cancer. In his 80's he was an avid bicyclist in San Francisco. The pipes
in the basement ceiling of his apartment building, where he stored his bike,
were insulated with blown-in asbestos. When he developed a persistent cough, a
biopsy showed his lungs to be poisoned with asbestosis.
