The behaviour of metals in deep fluids of NE Iceland

Abstract

In this contribution, we present some of the first data on the elemental signature of deep crustal fluids in a basalt-hosted, low-chloride magmatic-hydrothermal system. Down-hole fluid samples (850–1600 m) from wells in the Theistareykir and Krafla geothermal fields in the Northern Volcanic Zone of Iceland were combined with well-head samples of condensed vapor, cuttings of altered rock, and fresh basalt (being some of the first concentration data for volatile and semi-volatile elements (Sb, Tl, Bi, Cd and As) for this area of Iceland). Results show that the deep fluids are relatively enriched in base metals and (semi)-volatile metals (in particular Te, Hg, Re and Tl) compared to local basalt. We interpret this enrichment in volatile metals to reflect a significant element input from magma degassing. Boiling of this deep fluid results in a well-head fluid composition that is significantly depleted in most elements. This well-head fluid has a distinct elemental signature, including a depletion in Sb that is mirrored in the altered rocks, and a depletion in the base metals that shows their selective sequestration in scale minerals, likely sulphides. As expected, the element content and patterns in surface fluids can thus not be interpreted to directly reflect that of the deep reservoir fluid. The behaviour of elements in Theistareykir and Krafla fluids is consistent, and largely agrees with similar data obtained for the Reykjanes geothermal system in SW Iceland. We therefore posit that our results are representative for this geological setting and indicate a significant magmatic degassing cation input to deep fluids, variably modified by water–rock interaction.