Geophysical exploration for gold—A review

Abstract

It is almost impossible to get a direct geophysical response from gold because of the low grades in deposits, except when electromagnetic detectors are used for individual shallow nuggets. However, indirect geophysical indications may occur through association of gold with particular host rocks, marker beds, or structures which are, for example, of unusual magnetization, density, electric polarization, or conductivity/resistivity. Useful markers may be magnetic dolerites, banded iron formations, shales with magnetite, conductive and/or polarizable pyrites, or other sulfide (detectable by IP methods) and silicified zones more resistive than surrounding rocks. Mapping of faults and shear zones, with which gold may be associated, is also valuable (e.g., by magnetic, EM surveys, etc.). Magnetite depletion is characteristic of some deposits, resulting in zones of low anomalies; for example, ferromagnetic minerals in mafic volcanics are destroyed by carbonatization. Resistivity patterns may indicate altered rocks which contain mineralization, thus the use of VLF-EM methods in Canada and MMR in Australia. Geophysical methods have been important in the search for gold in the Witwatersrand Basin of South Africa (magnetic, gravity, and even seismic reflection methods), various areas of Canada, e.g., the Abitibi greenstone belt (magnetic, EM, and IP surveys), and to a smaller extent in Australia (e.g., Tennant Creek, NT and Water Tank Hill, WA). However, the value of geophysical methods should not be exaggerated since physical contrasts are often low.

Quartz-pebble-conglomerate gold deposits

“…Magnetic surveys can be effective when exploring for placer gold deposits (Doyle, 1986;Gunn and Dentith, 1997). Paleochannels may contain detrital magnetite or basalt flows that can give rise to magnetic anomalies if they occur at relatively shallow depths and the data are of high enough resolution.…”

“…Gravity and magnetic methods have helped researchers understand the structure of the Witwatersrand Basin, Africa (Roux, 1967;Doyle, 1986;Corner and others, 1990). The extent of the basin has been mapped in detail using both gravity and magnetic methods (Corner and others, 1990), which provides a first-order constraint for exploration.…”

“…8). Together, the magnetic highs associated with the ferruginous shale units, coupled with coincident gravity and magnetic lows over the ore-bearing Central Rand Group, helped guide early exploration in the Witwatersrand Basin (Roux, 1967;Doyle, 1986). However, the potential field methods become less effective with depth and do not provided the necessary detail needed to effectively explore for relatively deep gold resources.…”

Two-dimensional resistivity imaging in the Kestelek boron area by VLF and DC resistivity methods

“…Milligan copper gold porphyry deposit, Canada (Oldenburg et al, 1997), in shallow coal mine workings (Johnson, 2003), in the East Pennine Coalfield, UK (Chambers et al, 2007), in uranium mining sides (Legault et al, 2008;Ramalho et al, 2009) and in Landusky gold mine, Montana, USA (Rucker et al, 2009). Doyle (1990) also extensively discussed the gold exploration by electrical and VLF methods.…”

Multi-Exponential Inversion and Laboratory Study of Induced Polarization Relaxation Time Spectra of Shaly Sands

“…Induced polarization (IP) phenomena were observed more than 80 years ago, which may be caused by metallic minerals or clay content (1)(2)(3)(4)(5)(6)(7)(8) . The detection of IP in the time-domain is made by measuring both the maximum voltage during the charging period of a dc current flow, and the transient decay of the electrical potential as a function of time, after the current is turned off.…”