The role of LWIR hyperspectral data in the characterisation of high-sulphidation epithermal systems and the implications to mineral exploration

• False-colour composites delineate the quartz-alunite-pyrophyllite lithocap. • Alunite is distinguished from pyrophyllite using non-overlapping emissivity minima. • Sericitic halo is defined by quartz-white mica with minor albite, chlorite, kaolinite. • Laboratory data supports kaolin-group differentiation and fluid chemistry inference. • LWIR-based mineralogy enables fluid chemistry evolution tracing across the system. In high-sulphidation epithermal systems, fluid chemistry can be inferred by mineral zonation from residual quartz (advanced argillic alteration) to higher pH sericitic assemblages outwards. Visible and near infrared and shortwave infrared (VNIR-SWIR) airborne hyperspectral surveys are commonly used to map these spatial patterns, but two gaps remain: quartz is not directly detected, and key minerals have overlapping features in the shortwave-infrared (e.g., alunite vs pyrophyllite; white mica vs the kaolin group). Longwave infrared (LWIR) data offers non-overlapping emissivity minima for these phases that might address both issues. We test whether LWIR hyperspectral data can (i) map the silica core together with quartz-alunite-pyrophyllite in the lithocap, (ii) distinguish alunite from pyrophyllite to assess magmatic sulphur input, and (iii) identify kaolin-group occurrences at or beyond white-mica zones as indicators of late, low-temperature neutralisation. Using LWIR airborne and laboratory hyperspectral imagery, we test this approach at Alunite Hill, the lithocap environment of the Yerington mining district, Nevada (USA). We use false-colour composites of targeted relative band-depth ratios with validation from laboratory LWIR hyperspectral data acquired on ground samples. The false-colour composites enable us to directly map the silica core and the quartz-alunite-pyrophyllite domain, defining the advanced argillic alteration zone. Due to non-overlapping emissivity minima in the LWIR wavelength range, the composites also enable us to separate alunite from pyrophyllite occurrences. Surrounding the advanced argillic core, we identify a sericitic envelope dominated by quartz-white mica with minor albite and kaolin-group minerals. While kaolin sub-types like kaolinite and dickite are not fully resolved in the airborne data, our hypothesis is confirmed by laboratory spectra indicating proximal dickite (within the advanced argillic alteration zone) and more distal kaolinite (within the sericitic alteration zone), consistent with their relative stability under different pH and temperature conditions. We demonstrate, for the first time, that mineral assemblages derived from LWIR-only hyperspectral data can assist in better understanding of fluid chemistry evolution and thus supporting early-stage mineral exploration surveys in high-sulphidation epithermal systems.