GRSG Conference 2022: Orbit to Outcrop

Title: Hyperspectral imagery application in hydrothermal muscovite-pyrophyllite intergrowth identification over the volcanic-hosted lithocap part of the Yerington district

Author: Bruno Portela


Hydrothermal alteration minerals associated with the formation of hydrothermal mineral deposits are active in the visible and infrared range. Spectral absorption features can be analysed to identify the mineralogy associated with different alteration events. Some hydrothermal events are responsible for mineral commodities, while other events create hydrothermal alteration unrelated to the introduction of base and precious metals.

Therefore, this research investigates spectrally active alteration minerals which occur together but are potentially associated with several different hydrothermal alteration events, generating mineralisation or not. High spatial resolution airborne and laboratory-based hyperspectral images are combined to detect and visualise muscovite-pyrophyllite intergrowth in the shortwave infrared (SWIR) imaging spectroscopy survey over the Buckskin Range, the volcanic-hosted lithocap part of the Yerington porphyry district, Nevada (USA).

Spectral wavelength maps in different SWIR ranges are used to map the hydrothermal alteration mineralogy at the laboratory (26 μm) and airborne (1 m) scales. The airborne spectral data define outward zoning from alunite ± pyrophyllite to muscovite characterized by variable wavelength positions of its Al-OH absorption feature. The 1650–1850 nm wavelength range is used to differentiate zones of pyrophyllite predominance over alunite within the inner domain. The laboratory data improve the characterisation of the hydrothermal alteration mineralogy, which includes alunite, pyrophyllite, muscovite, dickite, chlorite, topaz, and zunyite.

The textural relationship of muscovite replacing pyrophyllite is addressed by developing a novel spectral index, the pyrophyllite-muscovite index (PMI). The characterisation of the intergrowths of pyrophyllite and muscovite at the laboratory scale is based on two aspects: (1) the definition of pervasive versus veinlet-controlled textures and (2) a subtle shift detection in the wavelength position of the Al-OH absorption feature of muscovite from 2189 to 2195 nm. The combination of the spatial patterns with the textural relationship of the pyrophyllite-muscovite association allows the identification of areas that contain the muscovite replacement of pyrophyllite.

The recognition of a late muscovite replacement of pyrophyllite suggests that advanced argillic alteration reflecting intense acid leaching is followed by late near-neutral pH magmatic-hydrothermal fluids, adding K+ and potentially other alkali elements and metals in the epithermal environment. As a result of this study, we document the hydrothermal muscovite-pyrophyllite intergrowth relationships in the study area, thus contributing to an improved understanding of the lithocap epithermal system and a better assessment of its exploration potential for Au, Ag, and Cu mineralisation