• Neil Pendock

Can thermal map sulfides?

Politics is not the only field of human endeavour plagued by fake news. I've lost count of the number of times I've been told that thermal imaging of sulfides is not possible. A frequently quoted justification is "Thermal Infrared Sensing for Exploration and Mining – An Update on Relevant Systems for Remote Acquisition to Drill Core Scanning" by Richard Bedell et. al. presented at Exploration 2017 in Toronto, which includes the table I've lifted to illustrate this article.

In fact, somewhat embarrassingly, I may have been an author on the paper that gave birth to the table as the quoted 2015 reference is to Harris, P., P.Linton, N. Pendock, K.C. Nicholson, and W. Howe-Watson,, 2015, Hyperspectral Drill Core Imaging: An Emerging Technology for the Digital Era: Keynote lecture FEM.

In fact the body of published work shows quite the opposite, that it is very possible to distinguish sulfides as a group.

I suppose one could still argue the efficacy of distinguishing different types of sulfides (pyrite from chalcopyrite, for example). But a simple look at the longwave infrared [LWIR] spectra for these minerals shows them to be quite different - so logically, there is no reason why they wouldn't be distinguishable. Expecially if you can do spectral unmixing.

Even if they're not, for exploration work, finding sulfides of any kind is usually significant. If you knew an anomaly was pyrite, chalcopyrite, or pentlandite, you'd check it out, whatever it was. 

The proof of concept for thermal imaging of sulfides actually happened in the early 2000s. E.g. the following, from Rivard et al. (2001) published in GEOPHYSICS, VOL. 66, NO. 6 (NOVEMBER-DECEMBER 2001); P. 1691–1698. Ben Rivard is also an author on the Bedell paper with the infamous table.

"The relationship demonstrates the capability to estimate total sulfide concentration using TIR reflectance data."

Ben found that thermal could discriminate sulfides, as distinct from clays, other silicates, etc. But he didn't yet have the resolution to differentiate types of individual sulfides.

Bolin & Moon (2003) found similar results for sulfides at Stillwater, reported in GEOPHYSICS, VOL. 68, NO. 5 (SEPTEMBER-OCTOBER 2003); P. 1561–1568. At the time they found that their work was detecting at least some of the sulfide - although not all:

"The band shape characteristics for all four of these spectra are similar to the expected spectrum for sulfide minerals showing stronger absorption in the UV wavelengths and higher reflectance towards the IR wavelengths."

These results were from spectrometers rather than satellite data. But the physics is the same. Here's a more-recent similar conclusion for satellite imagery, from Lorenz et al (2019) published in Sensors 2019, 19, 2787; doi:10.3390/s19122787 

"For example, the FX data allowed to discriminate the quartz, feldspar, and sulfide classes that actually have no distinct spectral features in the covered VNIR and SWIR range."

Of course the most convincing proof has been the field findings of many explorationists in different terrains. Detecting the anomaly, then going out and walking on sulfides is the most definitive proof for any Doubting Thomas. Of this there are many examples, some of which I have posted as case studies on LinkedIn.

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