Using synthetic mixtures of rocks and lichens for detecting the spectral properties of gossans in the Cape Smith Belt, northern Quebec, Canada

Kurzfassung

Ore deposits are sometimes covered by oxidized surfaces that form at or near surface where ferrous iron and sulphide minerals are unstable in the presence of weathering agents. Subsequently these minerals are converted to oxyhydroxide, oxide and sulphate minerals to form gossans, where specific iron oxide assemblages can be associated with ore bodies and potentially correlated with a definite host rock composition. Owing to the links between gossans and ore deposits, much attention has been paid to mapping gossans by remote sensing means. In some studies the spectral shape of gossans in the visible-near infrared wavelength region has been exploited for determining their mineralogical composition. However, little attention has been paid to the possible effects of spectral mixing of oxidized surfaces with lichens or host rock substrates. In this study we have investigated the effects of lichens and host rocks on the spectral properties of gossans by means of near-infrared and short-wave infrared hyperspectral data acquired from samples and from airborne data for the Ni-Cu bearing Cape Smith Belt in northern Canada. Fifty-eight rock samples, twenty-three of which are gossans, were collected from the study area across sedimentary, mafic and ultramafic rocks. Spectral measurements were acquired from these samples by means of the ASD FieldSpec 3 spectrometer and Sisurock imaging system that acquires sample imagery at high spatial resolution (0.8 mm in the visible-near infrared region). These measurements were used to build a series of synthetic spectral mixtures comparing gossans to lichens and rocks that do not have an oxidized surface. The analysis was constrained by means of X-ray diffraction, which provided information on the mineralogy of the samples. The synthetic mixtures were used to investigate the effects of lichens and rock substrates on spectra of gossans. Furthermore, the laboratory spectrometry of the gossan samples was compared to 10.7 km2 of hyperspectral airborne imagery acquired over the study area where the objective was to study how lichens affect the spectral properties of gossans at a large scale. Results show that different lichen species have an impact on the spectral shape of gossans in the near infrared wavelength region, consistent with previous studies. Moreover, some gossans are optically thin, meaning that the thickness of the oxidized surface does not completely prevent the transmission of light onto the host rock mineralogy. Implications include: i) the host rock mineralogy can be spectrally mixed with the gossan mineralogy and ii) the host rock mineralogy can be detected by remote sensing means. The latter finding is important from a mineral exploration point of view, especially in areas where there is a known association between specific rock types and ore deposits. However, our findings also suggest that the spectral shape of the iron oxide minerals can only be relied upon as a mineral identification method in the absence of lichens and when the gossans are optically thick.