Title: Hydrothermal alteration in the ASK-57 and ASK-86 exploratory wells at Hoffell/Miðfell within the Tertiary Geitafell volcano

Abstract

The Hoffell/Miðfell area is part of a hydrothermal system located on the south flank of the
Tertiary Geitafell volcano. Several wells have been drilled in the area and various studies
have been carried out, showing interaction between hot intrusions and meteoric water that
led to the development of prograde and retrograde alteration. However, there is a lack of
detailed mineral chemistry data from the area, as from most Icelandic geothermal systems.
The ASK-57 and ASK-86 core sections show volcanic sequences affected by progressive
high-temperature hydrothermal metamorphism that was followed by low-temperature
zeolitization after a drop in regional temperatures. These events can be identified from the
hydrothermal mineral assemblages formed and precipitated in vesicles, veins and the matrix
of the rocks analyzed. Stage I is characterized by the crystallization of fine-grained chlorite
interbedded with fine-grained epidote in veins, small vesicles and filling the bottom parts of
geopetal structures. Stage II occurred after a progressive temperature increase that formed
chlorite + epidote + wairakite + calcite + quartz + albite ± K-feldspar ± actinolite ±
clinopyroxene ± magnetite ± ilmenite ± titanite. Stage III occurred after temperature drop
and is marked by zeolite mineralization that includes laumontite, yugawaralite, stellerite,
stilbite, and chabazite. Altogether 15 different mineral phases were identified, and there are
at least three separate temperature-controlled “alteration zones” in the Hoffell/Miðfell core
sections