Induced Polarization
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The induced polarization (IP) method was originally developed for field use in porphyry copper exploration and has now advanced to detection of other deposit styles with sulphide content. It is the only geophysical technique capable of directly detecting scattered primary sulfide mineralization. When current flows across the surface of a mineral exhibiting metallic-like conductivity, the behavior is like a metal electrode in an electrolyte. An over-voltage or additional voltage, over that required to pass current through the normal ohmic resistance of the ground, within the electrolyte in the rock pour spaces is required to force the current across the electrolyte-metal particle interface, which behaves as a charged capacitor.
This additional voltage represents the energy stored in creating the capacitor-like double electrical layer of charge at the interface, and can be measured either as a decay voltage when a direct charging current is shut off or as the change of impedance (resistivity) across the leaky capacitor interfaces with change of A.C. current frequency.Most metal sulfides produce IP effects, e.g. chalcopyrite, bornite, chalcocite, pyrite, pyrrhotite, arsenopyrite, molybdenite, but not sphalerite. Graphite is another metallic property
mineral that presents strong polarisation and causes “geologic noise”. Some clay minerals (e.g. montmorillonites and vermiculites) give moderate IP effects whereas oxides are not responsive, with the exception of some iron and magnesium oxides (pyrolusite and magnetite).
All rocks are responsive to some degree, which can be called background. This is due to the unsatisfied charges at lattice imperfections, boundaries, fractures, etc. In general, older rocks have a higher background than younger rocks, due to greater alteration, and plutonic rocks have a higher response than sedimentary rocks, due to greater crystal irregularity and less continuous current pathways (lower permeability).
In planning an induced polarization and resistivity survey to detect zones of sulphide mineralization that may contain economic values, it is necessary to understand the critical importance of the choice that must be made regarding the electrode configuration and the electrode interval. (i.e. array geometry) These decisions must be made before the survey can be properly planned. In some cases a test over a known occurrence is very useful to further refine the geophysical approach. Further, it is very important to employ an electrode interval that has been scaled to the possible width of the target zones of metallic mineralization.
From an IP surveying perspective, Quantec Geoscience offers both Conventional (standard IP) and Deep Imaging IP (Titan 24). Where projects do not require the deep penetration of Titan 24 (i.e. up to 750 m), Conventional surveys are recommended. These can consist of any type of array, such as gradient, pole-pole, pole-dipole, and dipole-dipole (etc.) according to the depth of penetration and resolution required. The company has many examples of successes (i.e. discoveries) including Spense (porphyry), San Nicholas (volcanogenic massive sulphide) and Collahuasi (porphyry) which are detailed on this page.
For more information:
- Comparison of Induced Polarization Arrays
- Electrical Field Diagram (see below)
- Titan 24 versus Conventional IP
- Titan 24 Chargeability Calculations- Quantec Technical Note
Discoveries / Selected Case Histories:
