Characterising mineral chemistry variation as a proxy for fluid composition using EMIT spaceborne hyperspectral data

Abstract

Epithermal and porphyry-Cu systems exhibit well-defined hydrothermal alteration zonation, reflecting the mineral systems’ fluid chemistry and thermal evolution. High-sulphidation deposits are characterised by advanced argillic alteration (alunite ± pyrophyllite) in the lithocap environment, which can be overprinted by sericitic alteration (white micas) due to fluid evolution. Mapping these alteration patterns is critical for mineral exploration, as they potentially indicate mineralising events. While airborne hyperspectral imaging has been widely used to remotely map alteration patterns, its high cost and limited coverage restrict large-scale applications. Recently launched publicly available spaceborne hyperspectral sensors such as PRISMA, EnMAP, and EMIT offer systematic, cost-effective data acquisition but at reduced resolution. PRISMA and EnMAP have proven effective for hydrothermal alteration mapping, but EMIT remains largely unexplored.

We assess EMIT’s ability to map hydrothermal alteration mineralogy and characterise mineral chemistry variations for fluid composition analysis in the lithocap of the Yerington mining district, Nevada. Using wavelength and decision-tree classification maps, we identify a typical high-sulphidation zonation, where advanced argillic alteration is enveloped by sericitic alteration. Additionally, we map an outward shift from Al-rich to Mg-Fe-rich white micas, serving as a pH proxy. For the first time from space, we differentiate alunite- from pyrophyllite-rich zones, previously thought to require high-resolution airborne data. This distinction is crucial, as alunite-rich zones indicate high-temperature magmatic sulphur input, forming acidic environments, while pyrophyllite-rich zones reflect evolving fluid conditions. Our findings confirm EMIT’s ability to characterise mineral chemistry variation as a proxy for fluid composition despite its coarser resolution when compared to PRISMA and EnMAP. This capability reinforces the role of spaceborne hyperspectral data for mineral exploration in high-sulphidation epithermal-Au and porphyry-Cu systems.