Dawsonite deposition and metal transport during natural carbon capture and storage in a geothermal system

Dawsonite is an Al-Na carbonate that is documented in several geological environments. This mineral gained global attention due to its potential role in Carbon Capture and Storage (CCS) via long-term mineral trapping, but its genesis in nature is incompletely constrained. In this work, we present a case of dawsonite mineralization that provides constraints on its genesis. The study area is the Mt. Amiata geothermal system (Italy), which hosts intense CO2 venting and an historical district of Hg-Sb mineral deposits. Here dawsonite is long known to form within the cap rocks of the geothermal system. We combine remote sensing data, field mapping, microthermometry and Raman spectrometry of fluid inclusions, mass transfer calculations, elemental imaging by Laser Ablation-Inductively Coupled Plasma-Time of Flight Mass Spectrometry, C-O isotopic compositions, and geochemical modeling to provide constraints into the process of dawsonite deposition. We show that at Mt. Amiata dawsonite deposition is extensive, fracture hosted, and is probably a product of chemical vapor deposition, i.e., mineral deposition from heterogeneous reactions involving a vapor phase. Dawsonite deposited from an effervescing H2O-NaCl-CO2 fluid at c. 100 °C, PCO2≥2 MPa, and aNa+/aH+> 106 together with a group of transition metals (Ni, Co, Fe, Cr, Cu, Pb, Zn) and geothermal pathfinder elements (Hg, Sb, Ga, Tl, Ag), which occur as oxides or as trace elements in the host rock minerals. Dawsonite bearing joints and veins are interpreted as escape structures of fracture-controlled, metal-bearing, low-T hydrothermal fluids in an area of intense CO2 venting of this geothermal system.