Introduction

Airborne electromagnetic surveying originally grew out of the field of mineral exploration, in particular the search for conductive metallic sulphides and oxides in resistive geology. These applications are still common for airborne geophysical surveys, but the application of EM has been extended to almost every kind of mineral deposit currently of economic interest.

Volcanogenic Massive Sulphides (VMS)

Volcanogenic massive sulphides were the original incentive for the development of airborne electromagnetic exploration. Base metals (copper, lead, and zinc) often occur in combination as highly conductive sulphides in massive lenses - ideal targets, detectable at great depths with airborne EM. Pyrrhotite is often, but not always, associated with these deposits, creating a magnetic anomaly. The coincidence of a magnetic anomaly with the conductive pyrrhotite was sometimes used as an indicator to separate sulphides from graphite (below), a mistake that resulted in some VMS deposits being overlooked, and some pyrrhotite-rich graphite beds being drilled!

Case Histories Knife Lake, ABM, Storliden, Caber, Phelps Dodge, Perserverance 

Magnetic Nickel & Copper

Nickel and copper deposits are typically very conductive, and return a strong EM response (so much so that specialised processing (B-field) may be needed to detect them with time-domain EM systems). These deposits are generally associated with ultramafic intrusives, which also produce strong magnetic anomalies, useful for targeting prospective geology within regional magnetic surveys. The presence of pyrrhotite in the deposit may create a magnetic anomaly, although it can be lost in the strong magnetic anomalies of the associated intrusives.

Case Histories Voiseys Bay DIGHEM and GEOTEM surveys

Hydrothermal Magnetite, Copper & Gold 

These deposits are characterised by their iron oxide (magnetite or hematite) association, so they create very strong magnetic anomalies. They are generally highly conductive, so are an excellent EM target, and the potassic and sericitic alteration may be detectable with a gamma-ray spectrometry survey.

Case History Copper Lake

Platinum Group Elements (PGE)

While much of the platinum group production comes from magmatic nickel and copper deposits (above), deposits with PGE alone have much less sulphide, and may create only weak EM anomalies. Processing high-resolution frequency domain EM data to correct for the magnetic susceptibility of the mineralisation and surrounding host rocks can highlight these deposits.

Case History Stillwater

Porphyry

These large intrusive structures are associated with very large economic deposits of copper and molybdenum in South America, Asia, and North America. While the main economic mineralisation may be only moderately conductive, the pyrite halo and secondary mineralisation may be very conductive - an excellent EM target. The large size of the intrusives make them excellent targets for regional mapping. Radiometric and hyperspectral surveys are very useful in arid climates (little tree cover) to aid in identifying the lithology and search for characteristic alteration minerals.

Gold

Despite being highly conductive, gold does not occur in sufficient concentration to be detected directly with airborne geophysics. However, the effects of the geological processes which result in gold deposition are often detectable, and may have an effect over much larger areas than the gold deposit itself, presenting a larger target to the geophysical survey.

The geochemical alteration commonly associated with epithermal gold deposits may create clay minerals that increase conductivity, or deposit silica that decreases it. It may also reduce magnetite to sulphide, creating a magnetic low and conductivity high; or transport and deposit sulphides, also creating a magnetic high. The potassium enrichment (sericite alteration) associated with some gold deposition is sometimes the target of radiometric surveys. This variety of anomaly types may seem confusing, but with a basic knowledge of the deposit model being sought, the proper airborne geophysical system and interpretation can be employed.

Placer gold deposits can be difficult to detect, but it is sometimes possible to map the thickness of the coarse-grained (less conductive) beds among more clay rich sediments using EM surveys.

Uranium

If uranium mineralisation is exposed at surface, either in outcrop, boulders, or soils, then gamma ray spectrometry is the obvious method, directly detecting the uranium by its radioactive decay. EM surveys are commonly used to detect the conductive graphitic sediments normally associated with unconformity associated deposits. "Roll front" deposits have been located by mapping the paleochannels in which they are hosted. 

Metasomatic / Skarn

These deposits tend to be small and scattered but often high grade. Fast coverage of an area can reduce the time and cost spent finding them. Airborne geophysical surveys can be used to detect the mineralisation directly with EM or magnetic surveys, or map the intrusives or their alteration aureoles with any or all of the geophysical methods. 

Kimberlites

Like gold, diamonds cannot be detected directly with airborne geophysics however, the geophysical signature of the kimberlite in which they occur usually presents a clear contrast with the surrounding. Electromagnetic surveys are generally most reliable, and when the kimberlites are hosted in crystalline igneous rocks, the pipes appear as conductive targets. In sedimentary rocks they are likely to be more resistive than their host rock. In non-glaciated terrain, the deep weathered cap of the kimberlite pipe is quite conductive.

Kimberlite has a higher proportion of magnetite than most rocks into which it intrudes, so kimberlite pipes often create magnetic anomalies. However, due to the rapid rate of intrusion and cooling, there is often strong remanent magnetisation that can oppose the induced magnetic field. Thus a kimberlite pipe can have a positive anomaly, a negative anomaly, or virtually no anomaly at all. 

Case Histories Ekati, Willy Nilly, Point Lake, Suzie Pipe, Fort a la Corne

Coal

Coal can be difficult to map, since the resistivity is highly variable, and in the same range as many other sedimentary rocks.Where it does contrast with the host rock, it may be possible to map the depth and thickness from EM data.  EM has been successfully employed to locate diabase and felsic dikes intruding through coal seams, to provide mine planners with guidance to avoid these troublesome intrusions.

Case History Forzando Coal Mine

Graphite

The nemesis of electromagnetic surveys is graphite, which can be highly conductive and often closely associated with VMS deposits or unconformity uranium deposits. However, it can be an economic mineral in itself, or serve as an easily detected marker horizon for exploration.