Epidote–clinozoisite as a hyperspectral tool in exploration for Archean gold

Abstract

Hyperspectral analysis at seven gold deposits within the eastern Yilgarn Craton of Western Australia has revealed significant occurrences of previously unrecognised clinozoisite, and spatial relationships between the distribution of clinozoisite, epidote and gold deposits. Here we report the development of an index to allow the systematic spectral mapping of the epidote–clinozoisite solid solution. The combination of the wavelength position and depth of the 1550 nm absorption was used to characterise the solid- solution series spectrally. The spectral responses from CSIRO HyChips™, fitted with an Analytical Spectral Devices (ASD) FieldSpec-3 spectrometer, and a SisuCHEMA™ spectral-imaging camera were calibrated against electron microbe analyses of epidote–clinozoisite. The spectral-imaging camera helped resolve correlations for samples with complex paragenetic histories. Textural studies found genetic links between epidote and Mg-chlorite, and between clinozoisite and Fe-chlorite, with each mineral combination part of separate, diagnostic hydrothermal assemblages. Spectra from epidote–clinozoisite-dominated veins showed that shifts in the 2250 nm absorption correlate with epidote–clinozoisite composition and not with chlorite composition, and that coexisting amphibole phases have a closer compositional tie than chlorite in the given samples. The genetic affiliation, yet compositional discordance, between coexisting epidote–clinozoisite and chlorite suggests that the compositional spectral index associated with each are wholly independent, but in combination are diagnostic for the mapping of separate hydrothermal assemblages. Of the newly defined compositional relationships, vein-hosted clinozoisite was found to be a proxy for pre-existing structurally-controlled hydrothermal tschermakite. A comparison of spectral and stable isotopic characteristics from diamond drill hole CD5026, St Ives mining camp, shows correlations between the epidote–clinozoisite spectral index and δ13C of carbonate and δ34S of sulfide. Such correlations imply a redox control on the distribution of clinozoisite and epidote, and mean that the spectral logging of epidote–clinozoisite transitions can serve as a proxy for mapping paleoredox gradients.