Geophysics v’s Geochemistry

Geophysics and geochemistry are earth science disciplines that have a vast application in mineral deposit exploration and discovery. Mineral deposits are not discovered without either or both of these disciplines being used.

Unless an explorer trips over a deposit [because it’s sticking out the ground] geophysics and geochemical surveys will be required.

Geophysical exploration methods test for variations in rock properties such as density, conductivity, resistivity, chargeability, and gamma radiation.

Geochemical exploration methods test for variations in elemental (metal) content of rock.

Geophysical surveys test rock properties.

Geochemical surveys test rock element (metal) levels.

Geophysical Surveys

A geophysical program comprises two main components: data collection (the survey, which can either be on conducted on the ground, or in the air – hence – airborne) and data modelling (the interpretation). Both are usually outsourced because they require specialised skills not usually available in-house for junior explorers.

Geophysical exploration methods include, but are not limited to, ground or airborne gravity, ground or airborne magnetics, radiometrics, ground and airborne electromagnetics, induced polarisation, seismic, and ground penetration radar.

Mineral (metal) deposits typically have properties that are different to the surrounding rock. Deposits can be heavier or lighter (gravity anomaly), more conductive (IP anomaly), and so on… Geophysics works by identifying these types of differences.

Geophysics is an indirect measure of mineralisation.

There’s a “sting in the tail” for the unaware investor. Geophysical surveys almost always produce anomalies. This is simply because geophysical surveys measure the difference of rock properties and in a survey area, there will always be variations in rock properties.

All metal deposits have a geophysical signature but not all geophysical signatures are metal deposits.

For this reason, geophysical surveys are often combined with geochemical surveys. Note that when exploration targets are deep below the surface, geochemical surveys may not be used.

Geochemical Surveys

A geochemical program comprises three main components: data [sample] collection (the survey), data [assay] collection (the laboratory work), and data modelling (the interpretation). Sample collection is usually carried out by the explorer. Assays are always carried out by certified laboratories. Data modelling is usually carried out by the explorer and/or by an geochemical expert/consultant.

Geochemical exploration methods include, but are not limited to, stream sampling, grid soil sampling, rock chip sampling, trench or channel sampling, drill chip/core sampling.

Mineral (metal) deposits will have different geochemistry to the surrounding rock. By definition, they will have elevated levels of an element (metal), for example gold, copper or iron. They may also have other non-economic elements associated with them. These elements are called pathfinder elements.

Geochemical targeting works by identifying concentrations of economic and/or pathfinder elements (metals) that are above background levels.

Geochemical surveys tests for metals/elements.

By this it is direct measure of mineralisation.

When are geophysical and geochemical surveys used?

Geophysical programs are used by companies when:

  • The test area is very large because the cost per unit area is relatively low.
  • The test area has no or limited rock exposure.
  • The test area is prospective for hidden, or deep targets.
  • Direct land access has not been achieved but the company wishes to, or obligated, to conduct exploration.

Geochemical programs are used by companies when:

  • A geophysical survey has been used and geophysical targets are generated but need further definition.
  • The test area is relatively small so the costs for doing such programs is manageable.
  • The test area has good to excellent rock exposure.
  • The test area is prospective for surface targets.

Exploration companies tend to begin exploration activities at a new project with a geophysical survey. The “go to” is a combined AMAGRAD survey.

If geophysical surveys are in the news – the project is typically at an early stage of development.

Exploration companies may also combine initial geophysical surveys with first phase geochemical surveys, such as rock chip sampling and/or stream sampling.

If rock chip surveys are in the news – the project is typically at an early stage of development.