Formation of Clays and Chlorites in the Upper Icelandic Crust
Hjalti FRANZSON and Einar GUNNLAUGSSON
[Iceland GeoSurvey ISOR, Iceland]
Zeolites, clays and chlorites form the basis of the alteration zonation in the upper part of the Icelandic geothermal environment. Detailed studies using XRD, binocular and petrographical microscope data from Nesjavellir high-temperature field in SW-Iceland show that the dominant clay formation occurs as smectite within the smectite-zeolite zone both as direct precipitation as well as the alteration of volcanic glass. Near the base of that zone a distinct coarse-grained clay crystallizes overlying the fine-grained clay lining in the voids. This clay crystallization occurs at same depth as zeolites become unstable and appear to grow from underneath the gradually dissolving zeolites. The final alteration of the zeolites is represented by their replacement mainly into quartz and less wairakite. This transformation, observed at the upper boundary of several high-temperature systems, occurs at a location where a pH change takes place from a slightly alkaline (low-temperature) to a slightly acidic (high-temperature) environment. The progressive hydrothermal alteration is dominantly caused by the suppression of the volcanic strata into the normal geothermal gradient but can also be affected by the propagation of the high-temperature system into the former. This causes the gradual transformation/recrystallization of the already formed smectite clays into mixed layer clays and later into chlorites. Clay analysis show that smectites and mixed layer clays persist to much higher temperatures. Detailed studies combining geological structures and permeability variation within the geothermal reservoir show that their persistence to higher temperature is closely linked to low permeability, while their transition is fast where permeability is high.
|        Topic: Geology||Paper Number: 12007|