b'CommentaryMinerals geophysics mineralisation is typically denser than Minerals geophysics the country rock host leading to the reasonable expectation of a positive gravity anomaly over the mineralisation. But local conditions may conspire to Terry Harveychange this. Sulphide mineralisation Associate Editor for Minerals geophysics preferentially weathers in the intense E terry.v.harvey@glencore.com.au and prolonged tropical climatic conditions that have prevailed over much of Australia over geological time. Bigger isnt always Better and thesignificantly impact geophysicalSuch weathering can be expected to Reverse Sombrero responses. Figure 1 illustrates howreduce the density of the mineralisation carbonate (or silica) flooding associateddramatically and preferentially within Standard practice in assessing the resultswith sulphide mineralisation can renderthe weathered zone. Figure 2 illustrates of a geophysical survey is to gradethat part of the target horizon muchthe resultant gravity expression, a the anomalies by size and intensity toless electrically conductive with ancombination of a broad positive response define exploration priorities. But is thisattendant diminution or even absencefrom the deep unweathered sulphides best practice? With the sophisticatedof an EM response. Yes, the biggest EMand a sharper negative response from geophysical survey techniques andanomaly may be expected to indicatethe preferentially weathered zone. The processing capabilities now available tothe best concentration of conductivemineralisation gravity response now us, I think we can do much better. Andsulphides within the host horizon.comprise a negative anomaly centred the key to this is an understanding ofHowever, a better understanding ofwithin a broader subdued positive the geology of the mineralisation and itsthe geology may lead you to seekanomalythe Reverse Sombrero environment, and how this impacts theexactly the opposite situation, where ain cross-section. An assessment of geophysical results. Here are a couplediminution or even absence of responsegravity survey results based on the ofexamples. in an otherwise anomalous formationalintuitive expectation of only positive response may be a target, so much thegravity responses over denser sulphide Geophysical exploration for strata-boundmore so if there are significant structuralmineralisation may well ignore mineralisation typically focusses onelements near-by. An assessment ofvalidtargets. favourable geological formations. Whereairborne EM survey results based only these are pyritic and carbonaceous,on the intuitive expectation of strongestSo, in assessing mineral exploration airborne electromagnetics providesEM response over concentrations ofgeophysical survey results, it would be an excellent mapping tool to assesssulphide mineralisation may well ignoreprudent to consider the geology and their structural disposition and relativevalidtargets. keep an open mind.conductivity. A localised increase in sulphide mineralisation associated withBut things can go even further. target mineralisation might reasonablyGeophysical anomalies can sometimes be be expected to result in increased EMthe reverse of those expected. Consider anomalismBigger is Better.However,the use of gravity in the exploration for the mineralising process itself maysulphide mineralisation. Massive sulphide Figure 1. Schematic long section of stratiform mineralisation and EM response. Figure 2: Cross section of modelled gravity response of weathered sulphides.34 PREVIEWJUNE 2024'