Spectralucation July: Beyond Porphyry-Epithermal

Spectralucation July Beyond Porphyry Epithermal
A common comment that we receive is that ‘spectral is great for porphyry and epithermal, but it cannot help with other deposits.’
At LKI Consulting, we wholeheartedly disagree with this statement so for our last Spectralucation July post we would like to debunk some myths and explore the range of deposit types that are seen in full living color using hyperspectral technologies.

General Limitations & Some Myth Busting
We won’t be coy – like every technique there are limitations to hyperspectral technology. In order to determine if your minerals of interest can be identified consistently, it is important to understand what part of the electromagnetic (EM) range you are working in (UV, VNIR, SWIR, MIR, LWIR…) and what the technology’s spectral and spatial resolution are. Your provider should be able to help guide you with this and like in geochemistry, you will be rewarded in the long run by doing an orientation survey. On to the myth busting
Working in the VNIR-SWIR, anhydrous quartz and unaltered feldspar are the most lamented “missing” minerals. However, these minerals can be mapped as a single group as they share a consistent, featureless negative slope between  ~850nm and ~2400nm. It is also important to note that feldspars in hydrothermal systems are typically altered… despite the fact that you may see a perfect euhedral crystal it may no longer be feldspar per se, but altered to clay. Having a record of clay speciation and content for downstream activities may be essential.
Sulfide mapping is not a strength of hyperspectral analysis because these minerals lack diagnostic absorption features (apart from sphalerite and molybdenite) and the spectra are noisy. That said, we have seen some spectroscopists do great work mapping non-disseminated sulfides using hyperspectral core imaging (HCI).
Another mineral that comes quickly to mind is magnetite – true, where there is mineral mixing, the magnetite absorption feature can be masked, but it does have a broad absorption feature with a minima at ~1050nm due to Fe2+ (attributed to crystal field absorptions).
Carbonates can 100% be mapped and speciated (using 2340nm feature) in the SWIR.  Fe-carbonates have additional features at ~1050nm and 1450nm, attributed to Fe2+absorptions.

More than just porphyry and epithermal
Returning to the title of this post, a common cop-out for those geoscientists not inclined to use new technology because it’s time consuming and difficult to start new protocols (yes we went there!) is to layer reason upon reason of why “it will not work.” We are here to tell you that the old adage that ‘hyperspectral only works in porphyry and epithermal’ is a lie.
A really cool reference for this is to head to the Minnesota Department of Natural Resources’ Corescan project. Available are summaries, interpretations, raw data (.csv), and a link to view the drillhole RGB, HCI and geotech data on Coreshed.  As you will see while browsing the site, the MDNR scanned greenstone gold, iron, vanadium, manganese, and SEDEX deposits.
In addition to these five deposit types, we have also seen the successful application of hyperspectral to VMS, IRGS, skarn (40+ minerals – incredible!), Ni-laterite, IOCG, layered ultramafic, diamond, uranium, sediment-hosted Cu, a large variety of Fe deposits, emeralds…
Now just because hyperspectral has been used on a deposit similar to your own, doesn’t necessarily mean it is fit-for-purpose for your project. Start by asking yourself, “what is my question?” If you’re question is along the lines of needing to map talc distribution then you’re in the right market, whereas if you’re question is along lines of quantifying sulfides then you need to pivot towards another more appropriate technology.
The joy of utilizing hyperspectral is that now you have all this rich mineralogical information… think of it as the same as when your eye looks at a piece of core and interprets the history of the rock, except now you’re not misidentifying minerals (yup, we said it) before you interpret the history of the rock. Remember back to our second Spectralucation post when we talked about speciation? Because now, in addition to identifying the distribution of a mineral, you can also map its sub-species, for example, alunite and whether it is natroalunite or alunite.
So… are you ready to reinterpret your paragenesis yet?