Borehole geology and hydrothermal alteration in well B-9, Namafjall geothermal field, NE-Iceland

Abstract

A study was made of the drill cuttings from well B-9, a 1312 m deep drillhole in the Bjarnarflag sector of the Namafjall high-temperature area, NE Iceland. The strata which the drillhole dissects consists of fine- to medium-grained basalt below 422 m depth, the top being covered by a succession of fresh-looking basaltic breccia, glassy basalt and tuff, these being referred to as hyaloclastite. Basalt intrusions occur below 500 m. The hydrothermal mineralogy predominantly reflects a high-temperature environment (>200 C) although an overlap with an intermediate temperature environment (150-200 C) is present in the upper 700 m of the drillhole. The first hydrothermal event, which involved high-temperature conditions is evidenced by change from lowgrade clays into more coarsely crystalline clays (smectites) with depth and increasing temperature, wairakite, chlorite, prehnite, epidote, albite, actinolite and possibly garnet. The second hydrothermal event so far restricted to the top 700 m is evidenced by the zeolites mordenite and laumontite. An overlap of laumontite (temperature range 100-200 C) with wairakite and prehnite (>200 C), epidote (250-280 C), occurs at about 434-460 m depth. Both the petrographic observations and XRD analyses over the same depth range confirmed these findings.