¹George L. Carson,¹Lindsay J. McHenry,²Brian M. Hynek,¹Barry I. Cameron, ¹Chase T. Glenister
American Mineralogist 108, 409-429 Link to Article [http://www.minsocam.org/msa/ammin/toc/2023/Abstracts/AM108P0409.pdf]
¹Wisconsin-Milwaukee, 3209 N. Maryland Avenue, Milwaukee, Wisconsin 53211, U.S.A.
²Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, 1234 Innovation Drive, Boulder, Colorado 80303, U.S.A
Copyright: The Mineralogical Society of America

Iceland’s Námafjall geothermal area exhibits a range of alteration environments. Geochemical and
mineralogical analyses of fumaroles and hot springs interacting with Holocene basaltic lavas at Hverir,
and with Pleistocene hyaloclastites atop nearby Námaskar∂, reveal different patterns of alteration
depending on the water/rock ratio, degree of oxidation, and substrate composition and age. The focus
of this study is a transect of a Hverir fumarole that has formed a bullseye pattern of alteration of a
Holocene basaltic lava flow. Surface samples and samples collected from shallow pits were analyzed
by X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM) to
constrain changes in mineral assemblage and major elemental composition with both distance and
depth. Elemental sulfur is concentrated near the vent, with leached deposits with amorphous silica
and anatase nearby and kaolinite, hematite, and jarosite/alunite-group sulfate minerals farther out,
with smectites and less altered material at the margins, though smaller-scale mineralogical diversity
complicates this pattern.
Silica phases include amorphous silica (most samples), cristobalite (some samples in the leached
part of the apron), and quartz (minor constituent of a few samples). The silica was concentrated through
residual enrichment caused by leaching and is accompanied by a significant enrichment in TiO2 (in
anatase). The presence of abundant cristobalite in a surface fumarole-altered Holocene basaltic lava
flow most likely reflects cristobalite formed during the devitrification of volcanic glass or precipitation
from fumarolic vapors, rather than high-temperature processes. Minor, localized quartz likely reflects
diagenetic maturation of earlier-formed amorphous silica, under surface hydrothermal conditions.
Natroalunite, natrojarosite, and jarosite are all present and even exhibit compositional zonation within
individual crystals, showing that under surface hydrothermal conditions, these minerals can form a
significant solid solution.
The high iron content of the substrate basalt and the prevalence of Fe-sulfates and Fe-oxide spherules
among the alteration products makes this geothermal area an especially useful analog for potential
martian hydrothermal environments. The residual enrichment of silica in the leached deposits of the
Hverir fumarole apron could serve as an acid-sulfate leaching model in which amorphous silica forms
without appreciable sulfur-bearing phases in many samples, a possible analog for silica-rich soils in
the Columbia Hills on Mars. The coexistence of hematite spherules and jarosite-group minerals serves
as an intriguing analog for a volcanic/hydrothermal model for hematite and jarosite occurrences at
Meridiani Planum.