Hydrothermal Alteration on Composite Volcanoes -Mineralogy,
Hyperspectral Imaging and Aeromagnetic Study of Mt Ruapehu, New Zealand
Abstract
Prolonged volcanic activity can induce surface weathering and
hydrothermal alteration that is a primary control on edifice
instability, posing a complex hazard with its challenges to accurately
forecast and mitigate. This study uses a frequently active composite
volcano, Mt Ruapehu, New Zealand, to develop a conceptual model of
surface weathering and hydrothermal alteration applicable to long-lived
composite volcanoes. The rock samples were classified as non-altered,
supergene argillic alteration, intermediate argillic alteration, and
advanced argillic alteration. The first two classes have a paragenesis
that is consistent with surficial infiltration and circulation of the
low-temperature (40 degree C) neutral to mildly acidic fluids, inducing
chemical weathering and formation of weathering rims on rock surfaces.
The intermediate and advanced argillic alterations are formed from
hotter (100 degree C) hydrothermal fluids with lower pH, interacting
with the andesitic to dacitic host rocks. The distribution of weathering
and hydrothermal alteration has been mapped with airborne hyperspectral
imaging through image classification, while aeromagnetic data inversion
was used to map alteration to several hundred meters depth. The joint
use of hyperspectral imaging complements the geophysical methods since
it can numerically identify hydrothermal alteration style. This study
established a conceptual model of hydrothermal alteration history of Mt
Ruapehu, exemplifying a long-lived and nested active and ancient
hydrothermal system. This study highlights the need to combine
mineralogical information, geophysical techniques and remote sensing to
distinguish between current and ancient hydrothermal and supergene
alteration systems, to indicate the most likely areas of future debris
avalanche initiation.