b'Minerals geophysics Minerals geophysicsBefore going further, we might considerusing a range of di\x1eerent geophysical whether we even need to undertaketechniques over all anomalies of interest, geophysical follow-up. If the geophysicalbut this could be an expensive and time-survey results con\x1drm other indications,consuming process. A staged approach, such as the presence of gossans orstarting with an appropriate technique anomalous rock or soil geochemistry,(or techniques) speci\x1dcally designed we might have su\x1ccient reasonto highlight anomalies of interest and/and information to drill-test withoutor eliminate spurious anomalies before further geophysical work. In the blind\x1dne-tuning targets makes more sense. mineralisation scenario relying solelySo, some thought on what petrophysical on geophysics, if our initial surveyproperties could be used to discriminate technique was uniquely appropriate tobetween our target style and that of other the target style sought, and su\x1ccientlyspurious anomalies is required. Using Terry Harveydetailed to accurately discriminate thethe electromagnetics example, if we are Associate Editor resulting anomalies, we might reasonablyafter conductive sulphides, magnetics for Minerals geophysics dispense with geophysical follow-upmight help identify those sulphides terry.v.harvey@glencore.com.au surveys. However, while the accuracy andcarrying pyrrhotite, gravity might help to capabilities of modern geophysical surveydiscriminate graphite from more massive techniques and processing regimes havesulphides, IP might separate sulphides dramatically improved, this ideal scenariofrom non-sulphidic source material such Geophysical follow-up is unlikely in most circumstances. as porous crush zones, etc. Using the In mineral exploration the prime aimTypically then we face the prospect ofgravity example, if we are after massive is to locate economic mineralisation,designing and implementing follow- sulphides, passive seismics might help to that is, orebodies. Geophysicalup geophysical surveys to furtheridentify those gravity anomalies due to components of green \x1delds mineralinvestigate our geophysical responses.basement highs, electromagnetics might exploration programmes typicallyBut, to what end? Two inter-related aimshighlight those anomalies more likely to comprise an initial literature researchare relevant: to deduce the nature ofbe massive sulphides, magnetics might for and appraisal of pre-existingthe source material and to su\x1ccientlyhelp to identify those anomalies due to data, followed by commission anddelineate its disposition. These two aimsdenser basic rocks, etc.execution of appropriate broad-scaleshould be considered in tandem, but IAnomaly source dispositiongeophysical surveying, and analysis andwill treat them sequentially below for interpretation of these results. Then, ifease of expression. Having hopefully eliminated the spurious things have gone to plan, there will beanomalies, we need to give some geophysical follow-up surveys designedAnomaly source material consideration to the design and extent to better discriminate geophysicalof detailed geophysical ground follow-anomalies of interest prior to drill-testing.Before we embark on detailed anomalyup. The speci\x1dc aim here is to delineate It is aspects of this \x1dnal follow-up stagedelineation, we might consider whetherthe disposition of the anomaly source that Id like to address here. To maximisethe source material for the anomalymaterial for optimal siting of drill-holes. our chances of success we need to siteis what were after. ConventionalIn modern geophysical exploration, this drill-holes in the best possible position togeophysical follow-up surveys typicallydelineation is typically achieved with test anomaly sources. employ a ground-based version of the3D inversion, so the parameters and broad-scale survey techniqueif weextent of the follow-up survey should be So, what sort of targets might we beused airborne electromagnetics initially, investigating in the \x1dnal follow-upappropriate. For potential \x1deld methods, then ground electromagnetics is thestation density to provide su\x1ccient stage? Ideally these would manifestde facto choice for follow-up, detailed as discrete geophysical anomaliesdetail and survey extent to establish ground gravity would follow airbornebackground would be considerations. For re\x1becting direct detection of the targetedor semi-regional ground gravity, etc. mineralisation. In some situations,electrical and electromagnetic methods, e.g., electrical geophysical surveysBut this doesnt necessarily advancesurvey parameters would be an additional the understanding of the nature of theconsideration. 3D inversion of IP-resistivity targeting zinc sulphides, we mightanomaly source materialthe follow-up be relying on indirect detection viasurveys, for example, will greatly bene\x1dt is still targeting the same petrophysicalfrom a tailored survey designinverting responses from other directly associatedproperty. Improved knowledge of source conductive metallic sulphides. In otherthe results from a series of parallel dipole-disposition from a detailed grounddipole lines can be fraught where the scenarios, it could be environmentssurvey may provide clues on the nature favourable to mineralisation that aregeology is more complex than simple 2D of the anomaly source material, butat right angles to the lines.being sought, such as fold closures,where this could reasonably be one of structural intersections, facies changesseveral options, we are going to needFinally, having possibly used more than within favourable horizons, etc. And inmore information to minimise drill- one ground geophysical technique over most cases the broad scale geophysicaltesting of unwanted target material. our target, co-inversion of the di\x1eerent survey should also contribute to theresults may improve the \x1dnal model.understanding of the geology of the areaOne approach would be to undertake under investigation. blanket follow-up ground geophysicsGood hunting!DECEMBER 2023 PREVIEW 38'