New search techniques are neededto discover buried mineraldeposits in urbanised and highly

developed countries with a long historyof mining such as Britain. The BGS hasbeen addressing this problem throughthe development and application of newmethods and technologies.

Study of gold grainsAn exciting new tool for gold explo-ration has been developed at the BGS inrecent years. This technique involvesthe microchemical characterisation ofalluvial (river sediment) gold grainsusing an electron microprobe. Alluvialgold grains frequently show internalchemical heterogeneity, and they alsocontain minute inclusions of otherminerals. These features are usuallyinherited from the bedrock source andare preserved throughout the processesof weathering and transport.Examination of alluvial grains can thusprovide important information whichcan be used to deduce the geologicaland structural setting of the source fromwhich the gold was derived. Multiplesources for alluvial gold can be recog-nised, and the most appropriatepathfinder elements for use in explo-ration can be identified. This methodhas attracted considerable interest frommining companies worldwide and hasbeen applied to the study of alluvialgold in various geological environmentsin South America, southern Africa,south-east Asia and Europe.

Alteration around mineraldepositsMineral deposits are commonly associ-ated with hydrothermal alteration of thesurrounding rocks, the style and extent

of the alteration reflecting the type ofmineral deposit. Such alterationcommonly forms a halo around the min-eralisation, providing an explorationtarget considerably larger than thedeposit itself. The delineation and char-acterisation of hydrothermal alterationcan therefore be of great value inmineral exploration, for the identifica-tion and assessment of new targets.

Until recently, assessment of alterationassemblages was often very difficult,because of the fine grain-size of theminerals, and could only be accom-plished by expensive and time-consuming laboratory techniques suchas X-ray diffraction. However, thisproblem can now be addressed in thefield using an instrument known as aPortable Infrared Mineral Analyser(PIMA). The PIMA is a compact,portable, hand-held spectrometer whichcan provide the field geologist withimportant mineralogical information onrocks, minerals and soils. The instru-ment is capable of detecting many ofthe minerals commonly found inhydrothermal alteration systems, such asclays, carbonates and sulphates.

The PIMA, originally developed inAustralia, is increasingly being usedworldwide in mineral exploration and inthe evaluation of mineral resources. TheBGS has used it in studies of oredeposits in the UK and internationally.

New exploration techniquesAdvanced

instrumentsand informationtechnology

Gus Gunn

Keyworth

BritishGeologicalSurvey

22Microchemical maps showing the distribution of gold (Au), silver (Ag) and copper (Cu) in a single gold grain.

50 µm Au Ag Cu

A recent project, supported by theDepartment of Trade and Industry, theBGS and commercial companies,examined the nature and distribution ofalteration in the volcanogenic massivesulphide base-metal deposit at ParysMountain in Anglesey, North Wales.Spectral features derived from PIMAanalysis of drillcore were incorporatedin a three-dimensional model of thedeposit using the VULCAN softwarepackage and provided important newinsights into the relationship betweenalteration patterns, geology and mineral-isation.

Advanced IT

The increased availability of geoscienceinformation in digital form and majorimprovements in information technol-ogy have had a significant impact onmineral exploration in the last decade.

Geographical Information Systems(GIS) are being used to examinedatasets of many different types(geology, geochemistry, etc.) in order toidentify combinations of key featureswhich are consistently associated withmineralisation. Recognition of theseassociations, or deposit signatures,allows new targets to be identified andprioritised. This approach of multi-

dataset analysis has recently been takena step further by the BGS, through thedevelopment of a prospectivity mappingsystem in which the datasets aresearched for the various components ofthe deposit signature to produce aprospectivity map showing the potentialfor a mineral deposit of a particular typewithin the search area. The system isflexible and allows various models to be

tested, by changing the weightingattached to each feature relevant to thetargetted deposit type.

The BGS prototype system has been usedto map the prospectivity of the Dalradianterrane of the Scottish Highlands for lodegold mineralisation. Regional datasets,including geology, structure, geophysicsand geochemistry, were integrated toproduce a multi-dataset map for the southLoch Tay area of Perthshire.

The application of advanced informa-tion technology using a GIS platformlinked to a knowledge-based prospectiv-ity mapping system offers considerablepotential for assessing the likelihood ofthe discovery of new mineral deposits inan area. The use of these methodologiescan also assist government departmentsin land-use planning worldwide.

23

Above: Multi-dataset map for the southLoch Tay area of the GrampianHighlands.

Left: Derived gold prospectivity map ofthe same area. Zones of highestprospectivity are coloured red.