| Summary: | This research aims to identify the sources, mineral speciation and natural budget of C and S in the Nesjavellir geothermal field as a base for further research into the sequestration potential of CO2 and H2S by reinjection into the hydrothermal system. Chemical analysis performed on drill cuttings from the Nesjavellir geothermal field were combined with geochemical modelling efforts to identify the natural processes governing the precipitation of secondary C and S species and calculate the natural C and S budget of the system. Secondary carbonate and sulfide minerals precipitate from the Nesjavellir hydrothermal system upon adiabatic depressurization boiling and/or fluid-rock interaction. The models show that boiling in combination with minor fluid-rock interaction, i.e., basalt dissolution, will cause supersaturation of carbonates and sulfides in the boiled fluid resulting in extensive precipitation of these minerals along the boiling depth zone. Outside the boiling zone low reaction temperature and the presence of volcanic gas enhance secondary calcite and pyrite stability and precipitation upon fluid-rock interactions. However, a delicate balance between C and S sequestration and leaching was discovered. At high reaction progress and reaction temperatures ≥200°C pyrite and calcite are unstable and mineral dissolution becomes inevitable. This puts an important time constraint on C and S sequestration efforts using natural occurring mineral processes in the hydrothermal system. It was calculated that the Nesjavellir geothermal field naturally buffers 0.35 kt/y of CO2 and 1.7 kt/y of S which amounts to 0.7 and 7.8% of the total annual fluxes of C and S, respectively. Markmið rannsóknarinnar er að skilgreina uppruna, kristalla myndun og náttúrulegan forða C og S í jarðhitakerfinu á Nesjavöllum til að leggja grunn að frekari rannsóknum á bindingu á CO2 og H2S með niðurdælingu í jarðhitakerfið. Notast var við efnagreiningar á borkjörnum og jarðefnafræðilegar hermanir til að skilgreina náttúruleg ferli sem stjórna ...
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