Minerals Geophysics

Minerals geophysics

Minerals geophysics

resistive IP-anomalism, and the intense Pyrolusite: pyrolusite (manganese pyritisation in the phyllic alteration zone dioxide) has long been the bane of peripheral to porphyry copper deposits geochemists’ lives because of its can be the lead-in to the somewhat ability to scavenge other base metals. lower magnitude IP-anomalous copper But pyrolusite, surprisingly (to me, at mineralisation with which it is associated. least), can be electrically conductive and strongly IP-anomalous, so much so The inability to discriminate between that IP-resistivity is now an accepted the IP responses of different sulphides geophysical technique in manganese (and of graphite) was the original driving exploration. Apparently manganese force for investigations into spectral IP. dioxide is (nearly) a semiconductor. In the time domain this led to the Cole– Cole mathematical model, and in the I originally subscribed to the mantra ‘only complex resistivity domain to the Zonge metallic sulphides (except sphalerite) Terry Harvey classification system. That the measurable and graphite were really IP anomalous’ Associate Editor for Minerals geophysics differences in spectral IP appeared to and I hadn’t even considered pyrolusite. [email protected] relate more to mineral grain size and To those who proposed magnetite as distribution styles rather than mineral IP-anomalous, I had examples where species was disappointing. The Holy disseminated magnetite had failed to Grail of sulphide species identification respond to IP-resistivity. My mantra using IP remained unrealised, but out changed with the publication of Banes and mantras, of these investigations came a much geophysical data from the iron oxide learning never stops better understanding of the IP effect and copper gold mineralisation (IOCG) with improvements in IP instrumentation. its associated massive concentrations Those of you who have read some of of iron oxides – particularly haematite my previous pieces may have deduced Graphite: graphite is strongly conductive – in the Stuart Shelf deposits such as that I’m middle-of-the-road when it and strongly IP anomalous. Graphite, Olympic Dam, Prominent Hill, etc. comes to choosing between alternatives. particularly in faults and shears, is Here was a new (at least to me) class of Uncharitably this may be called fence- frequently mentioned as an unwanted IP anomalous minerals – metallic iron sitting, but I prefer to see it as taking the anomaly source in Canadian electrical oxides. These were not necessarily as considered approach – cherry-picking the geophysics case histories. Of course, if IP-anomalous as metallic sulphides and better aspects that different options might you’re looking for graphite itself with graphite, but capable of generating intense have to offer. There’s something good to electrical geophysics, its conductive low resistivity IP-anomalous responses if be had out of most things. And so it is IP-anomalism provides the ideal target. there was enough material present. with those sometime banes of galvanic Otherwise graphite’s response can be electrical geophysics – pyrite, graphite an unwanted distraction, unless you are So my new mantra is ‘metallic sulphides and pyrolusite. using it to trace structures or map out a (except sphalerite), graphite, pyrolusite prospective formation. and iron oxides (if there are enough of Pyrite: pyrite may not be the most them), can all generate significant IP electrically conductive mineral, nor have Put the two (graphite, or at least higher anomalies’. the strongest induced polarisation (IP) grade carbonaceous material, and pyrite) response, but it is so darn common. together, as in many carbonaceous rock But what of the other metallic oxides? How many promising low-resistivity IP- types, and you have a nightmare situation What about pitchblende (uraninite), for anomalous base metal targets have been for electrical geophysics. A decent base example? Is that IP-anomalous? Could drilled only to find the source material is metal sulphide deposit should generate we use IP-resistivity to search directly predominantly pyrite? Pyrite is no longer a detectable electrical geophysics for buried uranium deposits? What an economic source of iron or sulphur, response, but it won’t be recognisable about cassiterite (in the rutile group so it’s not the target you seek. However, in this environment of intense electrical with pyrolusite), chromite (in the spinel pyrite detection can have indirect anomalism. Perversely, if silica and/ group with magnetite), and ilmenite (in applications in mineral exploration. The or carbonate flooding accompany the the haematite group)? Is anyone using silica-pyrite that accompanies some styles mineralisation, a locally resistive, less IP-resistivity to search for these minerals? of gold mineralisation may provide a IP-anomalous zone might be the target – I don’t know, but I’d be interested to find realistic electrical geophysics target – good luck with that! out. Learning never stops.

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