| dc.description.abstract | The migration of hydrothermal CO2-bearing fluids from sedimentary crustal units into tectonically emplaced ultramafic mantle rocks has for a long time been an important topic in geology, due to its association with the formation of economically significant ore deposits. Furthermore, it represents a natural analogue for CO2 sequestration, increasingly recognised for its significant storage capacity (> 30 wt. % CO2) and its potential role in mitigating anthropogenic greenhouse gas emissions. Therefore, it is necessary to further investigate natural examples of CO2 consuming fluid-rock reactions to better understand reaction mechanisms, extent and favourable conditions. To this end, this thesis will investigate samples from the recently discovered sulphide-bearing listvenite-altered peridotite at Gråberget, Røros, east-central Norway. At Gråberget, listvenite is present along the contact with the surrounding metasedimentary Røros schist. Samples for this study were collected along a 180 m long transect beginning in partially serpentinized peridotite that transitions into serpentinite, ophicarbonate (serpentine + Mg-carbonate), a thin zone of soapstone (talc + Mg-carbonate), and finally a several hundred-meter wide listvenite zone (quartz + Mg-carbonate). The spatial relationship of the alteration zones is consistent with the theory of metasomatic zoning, indicating a gradual decrease in fluid CO2 concentration from the assumed point of infiltration at the contact with the local schist towards the central part of the ultramafic body. Fluid inclusion data indicate the presence of low-salinity reactive fluids (< 5 wt. % NaCl eq.) and a minimum formation temperature of 265-295 oC. Given the PT-conditions of 280 oC and 3.2 kbar, the total fluid volume required for the formation of listvenite is estimated between 2.6 and 8.8 million cubic meters. The carbonate shows distinct reaction microtextures in the different zones. In the ophicarbonate, carbonate is close to the magnesite endmember composition and chemically homogeneous, whereas it exhibits a complex chemical zonation with variable proportions of Mg and Fe-rich endmembers in the soapstone and listvenite. The formation of Fe-rich carbonate is coupled to the breakdown of magnetite during the carbonation and proceeds as reductive dissolution in the presence of dissolved sulphur, consistent with the presence of abundant Fe- and Ni-sulphide minerals in the listvenite. These observations suggest that coupled sulphidation-carbonation reactions may be favourable to reach the maximum carbonation potential in listvenite. | |
