Multi-stage zeolite facies mineralization in the Hvalfjördur area, Iceland

Abstract

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 intercalations of hyaloclastites and rhyolites. The basalts are affected by progressive low-temperature metamorphism, caused by the burial of the lava succession and higher heat flow from nearby central volcanoes. Low-grade zeolite facies metamorphism of basaltic lavas in the Hvalfjördur area results in two distinct mineral parageneses that can be correlated to events in the burial and hydrothermal history of the lava pile. Stage Ia represents syn-eruptive near-surface alteration in which celadonite and silica were precipitated along primary pores. During regional burial metamorphism (stage Ib), hydrolysis of olivine and glass led to the formation of mixed-layer chlorite/smectite clays. The chlorite content of stage Ib phyllosilicate vesicle rims increases with increasing burial depth and temperature. Stage II occurred after the burial and is marked by zeolite mineralization caused by higher heat flow from the Laxárvogur and Hvalfjördur central volcanoes. Altogether 11 different zeolites were found in the Hvalfjördur area: analcime, chabazite, epistilbite, heulandite, laumontite, levyne, mesolite, stilbite, stellerite, thomsonite and yugawaralite. In total, three separate depth and temperature-controlled ''zeolite zones'' occur in the Hvalfjördur area.

Differential alteration of basaltic lava flows and hyaloclastites in Icelandic hydrothermal systems

“…The massive observation of zeolites in Tertiary areas is in contradiction with the mineralogy observed in high temperature areas. This had been explained by a progressive low-grade metamorphism due to the burial of lava succession and higher heat flow caused by the influence of central volcano (Neuhoff et al 1999;Weisenberger and Selbekk 2009). In those areas, altered hyaloclastites exhibit a wide range of permeability, from 1°10 −17 and 1.5°10 −11 m 2 , positively correlated to the porosity (Frolova et al 2005).…”

“…When compared at a similar temperature, hyaloclastites are usually completely altered, whereas intrusive and compact basalts appear rather fresh (Kristmannsdottir 1979;Lonker et al 1993;Gudmundsson and Arnorsson 2002;Gudmundsson and Arnorsson 2005;Marks et al 2010). For instance, in Hvalfjördur area, secondary minerals mainly occur in the top and bottom parts of basaltic flow units (Weisenberger and Selbekk 2009). (2) Low-temperature geothermal areas, in the older Quaternary and Tertiary rock formations, in which rocks may have experienced previous low-grade metamorphic events.…”

“…The subsurface stratigraphy of Iceland mainly consists in a succession of hyaloclastite formations and lava flows (Weisenberger and Selbekk 2009;Banik et al 2014) sometimes intersected by basalt intrusions (Lonker et al 1993;Mortensen et al 2014). The thickness of individual layers is variable and irregular (Watton et al 2013), varying between a few meters and a few hundred meters (Alfredsson et al 2013).…”

“…Arnorsson (1995) estimated with the available permeability data that the age of geothermal waters emerging in wells was no more than a few tens of years, and a few hundreds of years in areas having the lowest permeability. Weisenberger and Selbekk (2009) Fluid analyses from boreholes indicate the presence of gases (e.g., Stefansson and Arnorsson 2002;Kaasalainen and Stefansson 2011). The source of gas is not very well established but is commonly associated with young magma intrusions (Arnorsson 1995;Gudmundsson and Arnorsson 2002).…”

“…These earlier events could have been caused not only by magma intrusions but also by hydrostatic pressure increase due to the progressive burial of lava succession or the ice load during Pleistocene. The occurrence of wollastonite in Krafla (Gudmundsson and Arnorsson 2005), garnet in Krafla and Namafjall (Gudmundsson and Arnorsson 2005), or some zeolites in Hvalfjördur (Weisenberger and Selbekk 2009) are assumed to be the result of contact metamorphism rather than of purely hydrothermal alteration.…”