b'Minerals geophysics Minerals geophysicsare other factors to considerparticularlyThis conventional wisdom can be the environment of the mineralisation. misleading on two counts:The increased sensitivity of modernWith respect to gravity, while all common EM techniques does offer possibilitieszinc ore minerals are anomalously dense, for indirect detection of disseminatedthe environments in which they occur, mineralisation where, for instance,particularly the carbonate sediment environmental resistivities have beenenvironments associated with some lowered by mineralising processes.styles of zinc mineralisation, can generate Furthermore, where deep weatheringdensity variations which may mask (typical of much of Australia) is present,gravity anomalies directly attributable to preferential weathering of disseminatedzinc orebodies. For example, recognition sulphides, and the development of anof relatively subtle orebody-related Terry Harveyassociated supergene enrichment zone,gravity anomalies within major gravity Associate Editor formay provide targets for direct detectiongradients associated with the margins of Mineralsgeophysicswith EM. The Ernest Henry IOCG deposit,carbonate sequences can be problematic.terry.v.harvey@glencore.com.au first identified from a ground EM anomalyGeophysical exploration in the Northern derived in part from native copper withinFlinders Ranges in South Australia for a supergene zone associated with themassive hydrothermal willemite (zinc Conventional wisdom in mineralpalaeo-weathering profile beneath youngersilicate) mineralisation, for which gravity exploration transported cover, is an excellent example. really is the go to method, illustrates IP-Resistivity should not be used forthe need for lateral thinking (Groves and When I see the term conventionalmassive sulphide mineralisation. Carman, 2003). Here the Beltana orebody wisdom or the like used in the contextis associated with a discrete gravity high of mineral exploration geophysics, I canThe thinking here in part harks back toas would be expected, but the nearby become a little uneasy. Yes, there areearly concerns about the diminutionReliance orebody is associated with a petrophysical properties you can relyof IP effects with decreasing resistivity.discrete gravity low! The confounding effect upon: magnetite is magnetic, galena isRemember the Metal Factor (defined as IP/ is the karsting of the carbonates hosting dense, graphite is electrically conductive,resistivity multiplied by a constant) whichthe mineralisation, or more specifically the pyrite is IP-anomalous, pitchblende iswas designed to compensate for thisnature of the karst fillmineralised material radioactive, etc. But there is more toproblem? By following the conventionalat Beltana, sand at Reliance.an orebodys geophysical responsewisdom, access to the capability that than a simple sum of the petrophysicalIP offers to potentially discriminateWith respect to electrical methods, while properties of the constituent minerals. between mineralised and un-mineralisedit is true that the zinc sulphide sphalerite conductors is denied to you. is not a metallic conductor, ore-grade In the same vein (sorry about that),sphalerite rarely occur in isolation, and type examples of orebody geophysicalIn practice, massive sulphide mineralisationthe typically associated sulphides such signatures are really just that - examples forstill contains boundaries between regimesgalena, pyrite, marcasite, chalcopyrite, etc. individual orebodies. While they unarguablyof metallic and ionic electrical conduction.are most definitely metallic conductors. provide valuable information, a slavish, inWhile the associated IP effects may be lessElectrical and EM geophysical techniques particular quantitative, adherence to suchanomalous than might be expected fromtherefore have the potential for detection type examples in geophysical explorationthe sulphide content, some IP anomalismof zinc mineralisation via the directly and interpretation can lead to missedcan be expected. In addition, the presenceassociated suite of conductive, IP-opportunities and unwanted outcomes. of a strongly conductive zone in theanomalous sulphide minerals.accompanying resistivity results should alert Both of these concerns are a function ofyou to the presence of possible massiveIn summary, I most certainly am not the inherently wide variations in bothsulphide targets. The down side is thatsuggesting that the conventional wisdom mineralisation styles and the geologicalIP-resistivity does not provide the degreeis wrong, just that it should guide rather environments in which they occur. Letsof source body geometry delineation thatthan constrain our thinking, and that the illustrate this theme with a few examples. is typically derivable from the results of abigger picture should be considered when detailed electromagnetic survey. selecting geophysical techniques for an Electromagnetics (EM) cannot be used forexploration programme.disseminated sulphide mineralisation. One flow-on from this thinking is that, At first glance this conventional wisdomunder appropriate circumstances, there may be merit in considering IP-resistivity asReferenceseems a given; disseminated sulphidesa possible follow-up technique for airborne are not interconnected electrically andEM surveys, rather than ground EM asGroves, I. and Carman, C., 2003. therefore cannot support the flow of theconventional wisdom would have you do. Geophysics as an exploration tool for induced electric currents necessary for thewillemite - a case study of the Beltana-EM method to work. And, by following thisGravity is the method for massive zincReliance-Aroona zinc deposits, conventional wisdom, the use of airbornemineralisation; zinc ore minerals arentNorthern Flinders Ranges, South EM, for instance, for rapid and systematicelectrically conductive therefore electricalAustralia, ASEG Extended Abstracts, investigation of your area of interestmethods, including EM and IP-resistivity,2003:3, 223-232, DOI: 10.1071/appears to be off the table. However, therearent going to work. ASEGSpec12_137 PREVIEW AUGUST 2021'