Induced Polarization Surveys: A Copper Exploration Technique

How does induced polarization surveying work?

Induced polarization was discovered during the early days of resistivity surveying. When geologists were conducting resistivity surveys for sub-surface mapping, they found out that certain bodies were polarizable; in other words, when an electric current was passed through them and then shut off, the charge did not disperse immediately, but wore off over time. This effect is called “induced polarization.”

Today, resistivity and IP surveys are for the most part conducted as a single survey. An electrical current is sent through the ground, and the surfaces of metallic minerals get charged. An overvoltage is applied to the ground, and then switched off. Once the current is switched off, the overvoltage decays, but the ground is left with a small storage of energy. This amount of stored energy gives clues into what minerals are laying below the surface.

While all rocks provide a certain IP effect, many produce only a slight effect known as “background noise.” However, most metal sulfides and some clay minerals give a significant IP signature. Areas that show an IP signature that suggests there are minerals of interest below can be good drilling targets.

What is induced polarization surveying used for?

The IP effect is particularly useful when detecting disseminated sulfide deposits, which include copper porphyry and strataform deposits. In fact, IP surveying is the most effective geophysical technique in discovering these deposits, as they are non-conducting and therefore are not detectable by electromagnetic exploration methods. Many world-class copper deposits are disseminated sulfide deposits.

Some minerals associated with gold, including pyrite and arsenopyrite, can also create an IP effect, meaning that gold mineralization can be indirectly picked up by IP surveys. The main drawback of IP surveying is that some sources of significant IP anomalies are not of economic importance.

One notable example of successful IP survey targeting involves the Collahuasi District in Chile. In 1990, the Collahuasi joint venture, which at that time consisted of Falconbridge (now Glencore (LSE:GLEN)), Chevron (NYSE:CVX) and Shell (NYSE:RDS.A), refocused its exploration efforts from a small high-grade vein resource in the Rosario porphyry area to a more regional evaluation for primary and secondary copper mineralization.

The ensuing 200 kilometer IP/resistivity program not only defined the Rosario/La Grande porphyry system (710 MT at 0.93 percent copper) in its entirety, but also led to the discovery of a totally new resource, the Ujina deposit (1,200 MT at 0.8 percent copper, including 126 MT of secondary enrichment), under 170 meters of cover. It is still recognized as a significant achievement today.