Borehole geology and hydrothermal alteration of well KJ-28, Krafla high-temperature area, NE-Iceland

Abstract

The report describes the study of drill cuttings of a 1003 m deep drill hole located in the Krafla high-temperature area. The strata penetrated by the drillhole consists of fine- to medium-grained basalts (olivine tholeiites & tholeiites) and altered glassy basalts and basaltic tuffs and breccias which are referred to as hyaloclastites. Intrusions appear at 525, 635, 700, 725, 750 and 780 m depth. There are more than 15 aquifers. Both high-temperature (> 200°C) and low-temperature (40-200°C) hydrothermal minerals are present in the well. According to the distribution of deposition minerals, four alteration zones have been identified. They are a smectite-zeolite zone (< 200°C), down to 180 m depth, a mixed-layer clay zone (200-230°C) down to the depth of 525 m, a chlorite zone (230-250°C) down to 600 m depth and a chlorite-epidote zone (250-280°C) which is continuous down to the depth of 808 m. No cuttings were collected below 808 m to the bottom of the well because of total circulation loss (>40 l/s). The increase in temperature is indicated for example by the transformation of low-grade clays to relatively coarse-grained clays. With increasing depth the smectite becomes interlayered with chlorite and high-temperature minerals, such as wairakite. With increasing depth and temperature, epidote, albite and sphene appear. Calcite, pyrite and quartz are distributed in all the alteration zones. Zeolites are most common above 250 m depth. Comparison of well KJ-28 with other drillholes in the area shows that it is located within a major upflow zone.