Characterising the hyperspectral SWIR features of tourmaline from two western Tasmanian granites

Cassady L. Harraden1, Wei Hong2,3, Cari Deyell-Wurst1

1Corescan Pty Ltd, Vancouver, BC, Canada, V6E 2E9 2Centre for Ore deposit and Earth Sciences (CODES), University of Tasmania, Hobart, TAS, Australia, 7001 3Department of Earth Sciences, School of Physical Sciences, The University of Adelaide, Adelaide, SA, Australia, 5005

Western Tasmania is one of the most important resource suppliers for Sn and W production in Australia and is host to numerous world-class high-grade deposits. The Heemskirk Granite, a highly productive batholith in this terrane, is associated with numerous skarns and greisens, which together have yielded a total of more than 104,000 tons of Sn. The nearby Pieman Heads Granite is of similar age (365-360 Ma; Hong et al., 2017) and has similar geochemical and mineralogical characteristics, but has not been associated with any known mineralisation. Both plutons contain abundant tourmaline in a variety of forms including magmatic hydrothermal veins, cavities, orbicules. This study compares the geochemical and spectral features of the tourmaline in both plutons to identify potential indicators of fertile magmatic systems.

Tourmaline samples were previously analysed by Hong et al. (2017) using Electron Microprobe (EMPA) and Laser-Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) to quantify the major and minor element compositions. Results showed that tourmalines are dominated by schorl varieties with a narrow range of chemical compositions (enriched in Fe and Al, moderately enriched in Mg, and poor in Ti, Li and Zn). Some systematic increases in Fe, Al and Li, and decreases in Mg and Ti are recognised from early- to late-stage tourmaline occurrences.

These analysed samples were also scanned using the Corescan Hyperspectral Core Imaging (HCI-3) system to obtain high resolution hyperspectral imaging data between 450nm and 2500nm (representing the visible to near-infrared (VNIR) and short-wave infrared (SWIR) regions). Characteristic absorption features in the SWIR are attributed to BOH, AlOH, and MgOH stretching vibrations: ~2200nm (AlOH), ~2250nm (BOH), ~2320nm (MgOH), and ~2360nm (BOH) (Hunt et al., 1973; Clarke et al., 1990; Bierwirth, 2004). Results show very little variation in individual wavelength values across samples for all four major SWIR absorptions. However, using combination of spectral features and specialised imaging capabilities, differences in tourmaline spectra between the two plutons are apparent. Tourmaline from the Pieman Heads Granite has relatively high ~2375nm wavelength positions while the ~2200nm and ~2250nm wavelengths are shifted to lower wavelengths. The Heemskirk Granite tourmalines display mid-range wavelength positions for all three absorption features. Additionally, variations in specific reflectance ratios in the SWIR region highlight zoning patterns that could reflect small-scale Fe variations.

Distinguishing between the barren Pieman Heads and fertile Heemskirk Granites has implications for Sn exploration activities in Western Tasmania. The major-element compositions of the tourmalines in these two granites are very similar but discrete variations in key SWIR absorption feature relationships are apparent using high resolution hyperspectral imaging. This demonstrates the potential to apply hyperspectral imaging techniques to differentiating compositionally similar intrusive bodies with different mineral potential for mapping and exploration activities in the region and beyond. 


Biography

Cassady’ current work with Corescan is focused on developing new geometallurgical and geotechnical applications using data collected from hyperspectral imaging technology. This was also the focus of her PhD research. Before completing her PhD she worked as an exploration geologist in Alaska, Colorado, Nevada, Utah, and Arizona.

About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

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