Rock alteration mapping in and around fossil shallow intrusions at Mt. Ruapehu New Zealand with laboratory and aerial hyperspectral imaging

Abstract

Hydrothermal alteration of minerals occurs when water, heated and ionized by shallow magma bodies or volcanic activity, circulates in the surrounding rocks. Different types of hydrothermal alteration and surface weathering can either weaken volcanic materials, potentially resulting in localised rock fall or large-scale flank collapse, or locally strengthen volcanic rocks, potentially resulting in blocked degassing pathways that can drive explosive events. Therefore, defining the types and spatial extent of rock alteration processes are critical for a thorough understanding of volcanic activity and associated hazards.

Hyperspectral data (how different substances absorb and reflect different wavelengths of light) can be used to identify specific minerals or mineral associations. However, it is unknown how accurate hyperspectral mapping can be for identifying alteration mineral compositions or quantifying mineral alteration. This study compared microscopic observation of minerals with with hyperspectral remote sensing data at laboratory and aerial scales to characterise the abundance of surface hydrothermal rock alteration in and around a shallow fossil intrusion on Pinnacle Ridge, Mt. Ruapehu, New Zealand.

Our analysis shows a moderate correlation exists between microscope-observed and laboratory-based hyperspectral analytical methods. Meanwhile, aerial hyperspectral data failed to provide any clear correlations to field-mapped alteration, and we interpret this as being due to ‘oversampling’ of surface alteration, spectral mixing, and sensor limitations (e.g., bandwidth, signal-to-noise ratio). However, a hyperspectral image-derived alteration map can loosely be translated to a geotechnical map where porosity and permeability play a major role in localizing hydrothermal fluid flow and the formation of alteration mineral associations.