Characterizing the rock alteration associated with vein-hosted Cu sulphide mineralization using hyperspectral reflectance spectroscopy; A case study from the Allihies region, SW Ireland.

The Allihies region on the Beara Peninsula, SW Ireland possesses a mining history for vein-hosted Cu sulphide mineralisation. Structural and chronological control of the deposit has been studied extensively (Fletcher, 1969; Lang et al., 2020; Reilly, 1986; Sheridan, 1964). However, the spatial distribution of fluid alteration in the host rock and associated mineralogy remain unstudied. Several alteration minerals linked with the sulphide mineralisation have been recorded, such as chlorite, muscovite, siderite, calcite, dolomite, kaolinite, montmorillonite, and goethite (Fletcher, 1969).

Reflectance spectroscopy can be used for identifying alteration minerals. Hunt (1977) showed, due to the different electron and molecular structure of the compounds, most minerals absorb unique amounts of energy upon the incident of electromagnetic radiation, thus the reflected energy show characteristics absorption features in the spectra. Certain mineral groups exhibit unique features in the visible-near (400 – 900 nm) and short-wave infrared (900 – 2500 nm) wavelength ranges (Clark et al., 1990; Hunt, 1977). High-spectral resolution (hyperspectral) imaging (HSI) techniques provide a large amount of spectral information where each pixel contains hundreds of narrow, contiguous wavelength bands (Goetz et al., 1985; Lodhi et al., 2019). This gives the ability to identify wavelength positions of mineral absorptions and their subtle deviations that reveal the compositional variations.

Consequently, HSI can be used for analysing the host-rock alterations around the Mountain Mine, Allihies, which will reveal the spatial patterns. The target sulphide mineralisation/lodes are oriented in E-W and N-S (Reilly, 1986), and systematic sampling from the mineralized vein across the alteration zone  will help determine if the fluid alteration has a recognisable detectable spectral signature. Mineral groups such as chlorites, carbonates, and clays (Clark et al., 1990) possibly be differentiated of the existing propylitic and sericitic alteration phases (Fletcher, 1969) as  one moves away from the veins into the country rock.

The current study will use laboratory HS data from a rock scanner for initial analysis, followed by a HS drone survey for extending the spatial scale. Principal Component Analysis will be used for extracting the relevant spectral information (Burger & Gowen, 2011). Subsequently, Minimum wavelength mapper can be incorporated for further analysis of dominating mineral occurrences (Hecker et al., 2019), by studying unique absorption features and their feature depths, for mapping variations across the samples. Specifically, the wavelength range of 2100 - 2400 nm contains the diagnostic absorption features for phyllosilicates and carbonates that highlight the different alteration stages the region has undergone.

The research model has the potential to be further developed for identifying regions with similar spectral responses with mineral exploration potential.