Table Main geophysical tools in mineral exploration Basis Coverage Applicability Target Magnetics Measures spatial variations in the intensity of Earth’s natural magnetic field Very large areas covered by aerial surveys; smaller areas by ground surveys Widely used as an aid to geological mapping; also a direct detection method for some deposits: applicable wherever rock magnetic properties vary All deposits with a well defined geological setting in magnetically variable rocks; direct detection of deposits containing magnetic minerals (e.g., iron ore, chromite, diamonds, some volcanic massive sulphide deposits, some porphyry copper deposits) Radiometrics Measures spatial variations in the intensity of natural radiation from potassium, thorium, and uranium Very large areas covered by aerial surveys; smaller areas by ground surveys Widely used as an aid to geological mapping; also aids direct detection of some deposits: applicable wherever there are variations in abundance of natural radioactive elements All deposits with a well defined geological setting; direct detection of deposits containing uranium or thorium minerals, or deposits with strong potassium enrichment in alteration zones (e.g., some volcanic massive sulphide deposits, some porphyry copper deposits, some epithermal gold deposits) Gravity Measures spatial variations in the intensity of Earth’s natural gravitational attraction Very large areas covered by aerial gravity gradiometer surveys; smaller areas by ground surveys Huge areas covered by very low density ground surveys; limited coverage with high resolution aerial surveys; detailed ground surveys only in areas of low topographic relief: applicable wherever rock densities vary Direct detection of deposits with strong density contrast to surrounding rocks (e.g., iron ore, volcanic massive sulphide deposits, kimberlites) Electromagnetics Measures the induced response in the Earth from an applied electromagnetic field Very large areas covered by aerial electromagnetic surveys; smaller areas by ground surveys Can be used as an aid to geological mapping; also aids direct detection of some deposits: applicable wherever there are variations in natural rock conductivities Direct detection of deposits with high concentrations of conductive minerals (e.g., volcanic massive sulphide deposits, manganese deposits) or where there are associated anomalous conductivity zones (e.g., some porphyry copper deposits, kimberlites) Induced polarization Measures the induced electrical polarization response in minerals from an applied electrical field Ground technique mostly applied over small areas, but large surveys also possible Detects the presence of polarizable minerals (mainly pyrite, pyrrhotite, galena, chalcopyrite, etc.) Direct detection of deposits with high concentrations of polarizable minerals (e.g., volcanic massive sulphide deposits, porphyry copper deposits) Resistivity Measures spatial variations in the resistivity of the rock mass being surveyed Mostly applied over small areas Detects variations in resistivity Sulphide rich mineralization has low resistivity compared to the surrounding rock; applied to direct detection of sulphide rich masses (e.g., massive sulphide deposits) Seismic Records underground surface reflections produced by explosions or mechanical thumpers at the surface Applied over huge areas for petroleum exploration; seldom used in mineral exploration Indicates surfaces in the bedrock (e.g., faults, beds, igneous layers) In mineral exploration, mainly used for layered complexes (e.g., Bushveld) or for engineering purposes MINING GEOLOGY/Exploration 619 Tool