Preliminary description of rocks and alteration in IDDP-2 drill core samples recovered from the Reykjanes geothermal system, Iceland

The Iceland Deep Drilling Project (IDDP) well IDDP-2 was drilled to 4,659 m in the seawaterrecharged and basalt-hosted Reykjanes geothermal system in Iceland. Spot drill cores were recovered between drilling depths of 3,648.00 m and 4,657.58 m. Temperature and pressure conditions at the base of IDDP-2 were over 426°C and 340 bar immediately following drilling, exceeding the critical point of seawater (406°C and 298 bar). The IDDP-2 cores are the first samples ever recovered from the supercritical roots of an active basalt-hosted hydrothermal system. We provide some preliminary hand sample descriptions, supplemented where possible by thin section petrography and mineral composition analyses for the IDDP-2 drill cores. The cores recovered between 3,648 m and the bottom of the hole at 4,659 m are from a sheeted dike complex and are generally pervasively altered. Despite the extensive alteration, veining is relatively minor and open space veins are very rare. Veins tend to be discontinuous and anastomosing and lack sharp wall rock contacts. They are interpreted as hydrothermal replacement veins formed in the transition zone between brittle and ductile deformation. Important initial findings include the transition from epidote-actinolite alteration to hornblende hornfels alteration at approximately 3,650 m, and the development of hydrothermal biotite in rocks below ∼4,250 m. Felsic (plagiogranite) segregation veins are not common on the Reykjanes peninsula west of the Hengill volcanic system, but are present in minor amounts in most of the dikes cored below ∼4,300 m. Detailed petrographic and geochemical analysis of the samples is on-going. We have also sampled what appears to be hypersaline supercritical/magmatic brine trapped in pore spaces of porous felsite veins and adjacent wall rock, which manifests as a yellow potassium-iron chloride salt that precipitates on the cut edge of the core as pore fluid evaporates. Some of the core at these depths was stained by hematite that formed on the outer core surface by oxidation of ferrous iron in the formation fluid reacting at elevated temperature with oxygenated surface water used as drilling fluid. Further evidence for supercritical brine is apparent in complex fluid inclusions within quartz that contain multiple solid phases. The drill core samples are of immense scientific value for studying chemical conditions in the supercritical roots of high-enthalpy geothermal resources and submarine hydrothermal systems, with implications for improved understanding of ore-forming processes.