Geophysical Methods For Mineral Exploration June 22, 2011

Geophysical Methods for Mineral Exploration such as :
1. Magnetometric surveys useful in defining magnetic anomalies which represent ore (direct detection), or in some cases gangue minerals associated with ore deposits (indirect or inferential detection).
The most direct method of detection of ore via magnetism involves detecting iron ore mineralisation via mapping magnetic anomalies associated with banded iron formations which usually contain magnetite in some proportion. Skarn mineralisation, which often contains magnetite, can also be detected though the ore minerals themselves would be non-magnetic. Similarly, magnetite, hematite and often pyrhotite are common minerals associated with hydrothermal alteration, and this alteration can be detected to provide an inference that some mineralising hydrothermal event has affected the rocks.

2. Gravity surveys useful to detect dense bodies of rocks within host formations of less dense wall rocks. This can be used to directly detect ore deposits, iron ore deposits, skarn deposits and salt diapirs which can form oil and gas traps.
3. Electromagnetic (EM) surveys used to detect a wide variety of base metal sulphide deposits via detection of conductivity anomalies which can be generated around sulphide bodies in the subsurface. EM surveys can also be used to detect palaeochannel-hosted uranium deposits which are associated with shallow aquifers, which often respond to EM surveys in conductive overburden. This is an indirect inferential method of detecting mineralisation.
4. Regional EM surveys via Airborne Method are based mostly on Transient EM methods using surface loops with a surface receiver, or a downhole tool lowered into a borehole which transects a body of mineralisation. These methods can map out sulphide bodies within the earth in 3 dimensions, and provide information to geologists to direct further exploratory drilling on known mineralisation. Surface loop surveys are rarely used for regional exploration, however in some cases such surveys can be used with success.

5. Electric-resistance methods such as induced polarization methods can be useful for directly detecting sulfide bodies, coal and resistive rocks such as salt and carbonates.