Hyperspectral characterisation of alteration in a covered terrain in the Southern Gawler Ranges, South Australia

摘要

Locating ore deposits buried under cover is crucial to keep up with the demand for metals, as many surficial deposits have already been discovered and developed (Schodde, 2014). A method that has shown promise in locating buried mineral deposits is remote sensing (eg Huntington, 1996; Pour and Hashim, 2012). Hyperspectral remote sensing can provide the necessary spatial and spectral information to locate alteration minerals that can direct explorers towards mineral deposits. Throughout Australia, satellite and airborne hyperspectral imagery has been successfully used to comprehend the regolith overlying basement mineralisation (eg Lau, 2004; Laukamp et al, 2011; Laukamp, Salama and Gonzalez-Alvarez, 2016). This work aimed to spectrally characterise alteration in a covered terrain in the southern Gawler Ranges in the central Gawler Craton, South Australia (Figure 1). This study area has been selected as it contains the Paris Silver deposit and several silver and porphyry copper targets across a tenement owned by Investigator Resources. Mineralogy for spectral detection was chosen based on prior geological knowledge of the advanced argillic alteration style; this included pyrophyllite, alunite, kaolinite and dickite (Anderson, 2014). Reference mineral spectra originating from the United States Geological Survey spectral library and diagnostic features identified by AusSpec International Ltd (2008) were selected for each mineral for analysis. Spectral Feature Fitting within ENVI~R software was successfully applied to the hyperspectral image to map the specific alteration mineralogy. Surface validation of the study area included soil sampling and spectral analysis using an Analytical Spectral Devices FieldSpec Pro 3~R Spectrometer. Alunite and kaolinite are the most widespread in the cover sequence, illustrating the pervasive nature of the advanced argillic alteration (Figure 2). Conversely, clinochlore and pyrophyllite occur in discrete zones of outcrop and are known to be vectors to mineralisation near porphyry copper deposits. Some concentrated zones of alteration coincide with specific outcrop. Notably,- the greatest likelihood of pyrophyllite corresponds with an outcrop in the centre of the study area known to be Katunga Dolomite, the host rock for the nearby Paris Silver deposit (Figure 2d). Results from field validation improve interpretation of the soils and mineralogy in the landscape using field spectra rather than pure reference spectra. This work has shown that it is possible to characterise alteration using a targeted mineral mapping method in a covered and vegetated terrain. An approach to de-risking exploration for the mining industry.