Zeolite
facies mineralisation in the Hvalfjördur area, Iceland
Research
Collaboratore:
Dr. Rune Selbekk, Natural Museum Oslo, Norway
The
Hvalfjördur area, 30 km north of Iceland´s capital
Reykjavik, belongs to the sequence of late Tertiary to early Quaternary
flood basalts with minor interlayer of hyaloclastites and rhyolites.
Rutten 1958). The basalts are affected by a low temperature zeolite
facies metamorphism, caused by the burial of the lava succession and
higher heat flow influenced by the Laxárvogur and the
Hvalfjördur located central volcano (Weisenberger &
Selbekk 2009). Low-grade zeolite facies metamorphism of basaltic lavas
in the Hvalfjördur field area results in two distinct mineral
paragenesis that can be correlated to events in the burial and
hydrothermal history of the lava pile. Stage 1a marks near surface
alteration in which celadonite and silica were precipitated along
primary pores. During burial, hydrolysis of olivine and glass led to
the formation of mixed layers chlorite/smectite clays. The chlorite
content of stage 1b precipitation increases with increasing burial
depth, respectively temperature. Stage 2 occurred after burial and is
marked by the zeolite mineralisation, caused by higher heat flow, from
the Laxárvogur central volcano. Altogether eleven different
zeolites were found in the Hvalfjördur area: analcime,
chabazite, epistilbite, heulandite, laumontite, levyne, mesolite,
stilbite, stellerite, thomsonite and yugawaralite Selbekk &
Weisenberger 2005, Weisenberger & Selbekk 2009). Based on the
work done by Walker (1960), zeolites were grouped into zeolite zone. In
total three separate depth and temperature-controlled
“zeolite zones” are described in the
Hvalfjördur area: the upper chabazite/thomsonite zone, the
middle mesolite zone and the lowest laumontite zone. The mineralisation
temperature for zeolites increases from the upper chabazite/thomsonite
zone to the lower laumontite zone. Empirical correlation between the
depth distribution of zeolite zone and the temperatures of formation of
zeolites in the geothermal system, a geothermal gradient of
133°C/km can be estimated, usual for central volcanoes. This
indicate the occurrence of a Laxárvogur central volcano,
which can be supported by geochemistry of volcanic rocks and
tectonic features in the Hvalfjördur area.
References:
Rutten M.G. (1958) Geological reconnaissance of the
Esja-Hvalfell-Armannsfell area, southwestern Iceland. Verhandelingen
van het Koninklijk Nederlands Geologisch-Mijnbouwkundig Genootschap,
Geologische Seris XVII, 219-298
Walker G.P.L. (1960) Zeolite zones and dike distribution in relation to
the structure of the basalts of eastern Iceland. Journal of Geology 68,
515-528
Selbekk R.S. & Weisenberger T. (2005) Stellerite from
the Hvalfjördur area, Iceland. Jökull 55, 49-52
Weisenberger T. & Selbekk R.S. (2009) Multi-stage
zeolite facies mineralization in the Hvalfjördur area,
Iceland. International Journal of Earth Sciences, 98, 985-999 (pdf) |
Fig. 1: Geology of Iceland
Fig. 2: Simplified
geological map of
the
Hvalfjördur area
Fig. 3: Zeolite stability
Fig. 4: Spatial and temporal development of
pore-filling mineral
assemblages in the Hvalfjördur area. The vertical axis depicts
depth below land surface at the time of each event depicted in the
figure. Time elapsed after eruption increases to the right. No scales
are implied on the axis |
Iceland
spar from Helgustadir: geology, mineralogy and the
influence to science development
Research
Collaboratores:
Dipl. Geol. Simon Spürgin, Hauri, Mineralstoffwerk,
Bötzingen, Germany,
Dr. Runes S. Selbekk, Natural Museum Oslo, Norway
The
rock formations exposed in Iceland consists mostly of basalt lava
sequences erupted from fissures or within central-volcanoes.
Low-temperatures alteration minerals like zeolites and calcite are
abundant in the tertiary basalts, especially in zones of alteration
associated with central volcanoes. A site at Helgustadir
(65°0´, 14°01´),
Reydarfjördur supplied
large quantities of transparent cleavage rhombs of calcite (commonly
called Iceland spar). These crystals played a significant role in the
early development of several fields in the physical sciences.
Reference:
Weisenberger T., Spürgin
S. & Selbekk R.S. (2008) Die Fundstelle Helgustadir (Island):
Geologie, Mineralogie und die bedeutende Geschichte des
Isländischen Doppelspats für die Wissenschaft.
Aufschluss 1, 53-63 |
Fig. 1: Helgustadir road sign
|
Fig. 2: Map of low temeprature alteration
in East Iceland
|
Fig. 3: Iceland spar
|
Fluid control on low-temperature mineral formation in volcanic rocks of
Kahrizak, Iran
Research
Collaboratores:
Kousehlar M., Tutti F.,
& Mirnejad H., University of Teheren, Iran
The Kahrizak volcanic field, south Tehran, in Iran, is composed
dominantly of basalt and basaltic andesite that have experienced
variable degrees of alteration due to the low-grade metamorphism (Stage
I) and hydrothermal activity (Stage II). Stage I alteration, which
occurred in response to the burial of volcanic rocks and their
interaction with heated groundwater, is characterized by the formation
of low-temperature zeolite facies minerals in vesicles consisting
mainly of fine-grained mafic phyllosilicate (smectite,
chlorite/smectite mixed layer) and zeolites (thomsonite, chabazite,
gonnardite, natrolite, analcime, heulandite, and mordenite). Stage II
mineralization occurred due to the activity of hydrothermal fluids that
formed large crystals of heulandite, stilbite, mesolite/scolecite,
natrolite, and analcime along with quartz and calcite in cavities and
fractures. The elements necessary for the formation of these alteration
minerals in Kahrizak were derived from the hydrolysis of olivine and
volcanic glass as well as the alteration of plagioclase. Various
mineral assemblages formed during stages I and II reflect changes in
temperature, pressure, and fluid composition. The change from mafic
phyllosilicates to zeolites species is caused by the decrease of Mg and
Fe fluid activities. Zeolite assemblages of stage I, known to be formed
at lower temperatures, show the general sequential order from older to
younger: chabazite, thomsonite, gonnardite, and natrolite. This
sequence is consistent with a hypothetical fluid evolution path with
increasing Na+ relative to Ca2+ activity. The change to stage II, which
consists of zeolites species that formed at higher temperatures, can be
attributed to a temperature increase and fluid influx caused by
hydrothermal activity related to a later magmatic event in the region.
Kousehlar M., Weisenberger T.B., Tutti F.,
& Mirnejad H. (2012)
Fluid control on low-temperature mineral formation in volcanic rocks of
Kahrizak, Iran. Geofluids 12 (4), 295-311. doi:
10.1111/gfl.12001 (pdf)
|