OVERVIEWS: Geophysical signatures of Western Australian mineral deposits: an overview

Abstract

Geophysical exploration in Western Australia is hindered by a mantle of conductive and magnetic weathered rocks that covers much of the State. This has required the adaptation of most geophysical methods for successful application in Western Australian conditions, and has led to the development and widespread use of, for instance, high-resolution aeromagnetics and time-domain electromagnetic methods. However, these difficulties have not prevented geophysics from being an integral part of exploration for base metal, diamond, gold, iron ore, manganese, nickel and uranium deposits in Western Australia. Mississippi Valley-type base-metal deposits are difficult geophysical targets and direct detection of the ore is not usually possible. However, gravity and magnetic data can be used to locate basement highs associated with such deposits and, on a semi-regional scale, induced polarisation surveys have been used to locate marcasite halos associated with the orebodies. Volcanic-hosted massive sulphide base-metal deposits have variable geophysical responses. Physical property contrasts with their host are highly variable and thus methods such as magnetics, induced polarisation and electromagnetics may fail to generate recognisable responses. Mise-a-lamasse surveys have proved highly successful for mapping such mineralisation on a prospect scale, once it has been intersected by drilling. The only example of a sedimentary exhalative deposit in the State for which data are available has distinct gravity, magnetic and time-domain electromagnetic anomalies. Diamonds in Western Australia mainly occur in lamproite pipes. These pipes have variable magnetisations but can usually be detected using high-resolution aeromagnetic surveys. The pipes can also be conductive and mapped using electromagnetic techniques if the host rocks are suitably resistive. The major geophysical method utilised in gold exploration is high-resolution aeromagnetics which is used to map favourable structures and rock types. Electrical and electromagnetic methods can also be used where gold is associated with sulphides. Geophysics has been comparatively little used in exploration for iron ore. Exploration for supergene-enriched deposits mainly uses aeromagnetics, to map favourable structures and to detect magnetite destruction and replacement associated with mineralisation, and gamma-ray logging for stratigraphic correlation purposes. The major technique used in manganese exploration is the gravity method, taking advantage of the positive density contrast between ore and host rocks. The mineral sands industry uses aeromagnetic data to map placer deposits containing ilmenite, but the relatively low cost of drilling limits the use of geophysical exploration methods. Nickel sulphide mineralisation can be directly detected using induced polarisation and electromagnetic techniques. Gravity and magnetic surveys are also used, but mainly in a mapping role. Carbonatitic intrusions associated with rare-earth-element mineralisation give rise to large magnetic anomalies. Radiometric and gravity anomalies can also occur. Uranium mineralisation has been directly detected using radiometric data, but some deposits are concealed below cover. Magnetic, electromagnetic, electrical and gravity surveys can be used to locate the rocks and structures which host mineralisation.