Structure and Origin of an Active Hydrothermal System in NW Svalbard: A Combined Magnetotelluric and Gas‐Water Isotope Study

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Abstract
The Woodfjorden area in northwestern Svalbard hosts the world's northernmost onshore thermal springs, extinct Quaternary volcanoes (the Bockfjorden Volcanic Complex), late Miocene lava flows (the Seidfjellet Formation) that overlie a Devonian sedimentary basin, and a long‐lived regional fault zone. To contribute to a better understanding of subsurface fluid flow in this volcanic region, we conducted a 3D magnetotelluric (MT) survey at 12 sites, combined with water and gas sampling at the thermal spring clusters (Trollkjeldene and Jotunkjeldene). While the MT survey has a limited aperture due to the remoteness of the region and complex logistics, the identified structural features put several constraints on the evolution of this poorly studied hydrothermal system. The electrically conductive zones (1–10 m), under Trollkjeldene and the Halvdanpiggen volcanic center, are interpreted as signatures of deep hydrothermal fluids or mineralized zones; the resistive anomalies (1–10 km) at 1–4 km depth under the Sverrefjllet volcano indicate buried magmatic or basement rocks. Stable isotope data indicate both meteoric and crustal components for thermal waters. Carbon and noble gas isotope ratios support a mixed gas source involving crustal, atmospheric, and mantle components. Helium isotope ratios suggest a mantle‐derived fraction of 6%–8%. These findings suggest extinct magmatic activity and ongoing hydrothermal circulation, likely prolonged due to fluid compartmentalization at depth by low‐permeability magmatic or metamorphic rocks.

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