Mineral exploration
Electromagnetic (EM) technology for mineral exploration is widely used for decades and usually includes a combination of resistivity & induced polarization (IP) methods with audio-frequency magnetotellurics (AMT). Resistivity & IP methods are deployed in a similar manner and are therefore considered jointly here (modern multi-electrode modification of resistivtiy & IP method is often called electrical resistivity tomography or ERT). The AMT method has large depth of investigation compared with the resistivity & IP method.
EM technology allow the determination of the spatial distribution of the low-frequency resistive (electrical resistivity) and capacitive (induced polarization or chargeability) characteristics of rocks. Since both properties are affected by lithology, pore fluid chemistry, water, graphite and ore mineral content, these methods have significant potential for various geological applications.

Resistivity (left) and chargeability (middle) of typical rocks; spectral chargeability (right) of polarizable rocks
The above figures show electrical resistivity and chargeability of typical rocks and minerals, respectively. It is evident that joint analysis of both properties could increase the reliability of geological interpretation of EM data aimed for mineral prospecting due to high sensitivity of these properties to iron-copper sulfide and magnetite content.

An important property of rock chargeability is its strong dependence on frequency (Pelton et al., 1978). This property gives possibility to distinguish between different polarizable ore minerals using the spectral IP field data, especially when lab petrophysical measurements of relevant rock samples are available (right figure).

Lab data on frequency dependence of normalized chargeability for typical ore minerals: 1 – pyrrhotite, 2 – magnetite, 3 – sulfide,

4 – pyrite,5 – titanomagnetite, 6 – shungite, 7 – graphite.

Porhyry and skarn deposits
Metallic ore deposits related to porphyry systems encompass a wide variety of types and are universally typified by three main features: (1) presence of veins and veinlets forming stockworks, within which are disseminated sulphides of Fe, Cu, Mo, Pb and Zn, as well as native Au, and minerals of W, Bi and Sn; (2) the mineralization is spatially and genetically related to intrusive bodies, of which at least one has a distinct porphyritic texture (hence the name porphyry); (3) large volumes of rocks are affected by hydrothermal alteration-mineralization. The host rocks intruded by the igneous bodies may include carbonate units, and they, as a result of thermal and metasomatic exchanges with the fluids that emanate from the intrusions, will form skarn-type ore deposits. Some of the world’s best copper, gold, lead, molybdenum, tin, tungsten, and zinc deposits have been in skarn (right).

A porphyry deposit and its associated skarns. The skarns have formed within a carbonate bed near where it had been penetrated by igneous intrusions (after Sillitoe, 2010)
Porphyry and skarn deposits often are well expressed by contrasts in magnetic-, resistivity-, chargeability- and gravity responses. The more oxidized porphyry systems, which are characterized by abundant magnetite, typically indicate a central magnetic anomaly. Resistivity & IP results may indicate a chargeability (IP) halo that typically coincides with >2% pyrite and lower chalcopyrite-pyrite ratios. Elevated chargeability may also occur in porphyry centers that are characterized by abundant copper sulphide minerals. Electrical resistivity low coincides with some large porphyry deposits, which probably is a reflection of the increased conductivities associated with elevated sulphide-mineral abundance and clay-mica alteration. Resistivity is often elevated in the propylitic halo and potassic core. In contrast, resistivity lows are more common in the surrounding and overlying mica-and clay-rich, phyllic and argillic zones. Silica-rich zones of advanced argillic alteration can be highly resistive. Thus, electrical surveys of porphyry and skarns deposits need to be interpreted carefully. The most efficient technology is integration of Resistivity & IP surveying and magnetic field measurements associated with the AMT method. Geophysical data should be interpreted together with all available geological, petrophysical and logging data.
Peschanka porphyry Cu-Au-Mo deposit, Russia
The Nord-West company has been working at Peschanka porphyry copper deposit (one of the 20 largest in the world) since 2005. At the first stage magnetic, resistivity & IP profiling allowed to delineate a number of prospective zones, where the 2D resistivity & IP tomography were then carried out. One of such 2D profiles is given in the right. The near-surface anomalous layer A4 at depth of about 100 m and two deep chargeability anomalies A2 and A3 were identified at the SE part of profile at depth greater than 200 m. The near-surface layer A4 and the deep anomaly A2 coincide with low resistivity anomalies and were thus interpreted as sulfide ore bodies. This interpretation was later confirmed by drilling (shown in the figure) – the identified zone is characterized by high sulfide content. Highly resistive anomaly A1 was interpreted as a quartz reef body without sulfide mineralization. At the third stage the AMT data were collected along several profiles to estimate the bottom extension of the revealed ore bodies.

Inversion results of the 2D resitivity & IP tomography data along an exemplatory profile.

Peschanka porphyry copper deposit, Chukotka region, Russia

Benkala porphyry Cu-Mo deposit, Kazakhstan
Multimethod exploration at Benkala deposit has been carried out by Nord-West Ltd. in 2013. Again, at the first stage the zones associated with sulfide mineralization were outlined using magnetic, resistivity & IP profiling. Then 2D resistivity tomography (ERT) and AMT data were collected and inverted both separately and jointly using Zond software to image deep structure of ore bodies. The ERT data inversion (a) covers only uppermost 400 m, which corresponds to secondary mineralization zone in weathered layer. The AMT data inversion (b) resolves structures up to 800-1000 m depth, but is less detailed.
The most efficient approach is the joint inversion of ERT and AMT data, which combines high spatial resolution of the ERT data and large investigation depth of the AMT data. Joint inversion (c) makes it possible to image accurately both resistive zones H1 and H2 and conductive zone L. Deep part of the central conductive zone L was interpreted as the primary vein-disseminated sulfide mineralization, while zone H1 corresponds to the granite-diorite intrusion and zone H2 corresponds to quartz-diorite dyke complex.

2D inversion results for: a – ERT data, b – AMT data, c - ERT+AMT data jointly.

Benkala porphyry copper deposit, Kazakhstan