| Title | On the occurence of iron-magnesium carbonates in hydrothermal systems |
|---|---|
| Authors | Giordano Montegrossi |
| Year | 2026 |
| Conference | World Geothermal Congress |
| Keywords | hydrothermal alteration, carbonates, magnesite, siderite |
| Abstract | The deep reservoir of Mount Amiata Geothermal system (Italy) is made of schists with a content of Ca ranging 7-1.5%, Mg 0.8-3%, Fe 7-9%, and a CO₂ content in the geothermal fluid of about 2% w/w. Over time, an hydrothermal alteration consisting of a mineral assemblage of Quartz, Muscovite, Clinochlore, Illite, albite, paragonite, carbonates (dolomite, ankerite, ferroan magnesite and magnesian siderite), chlorite (Mg, Fe) and pyrite has been developed. Hydrothermal and metamorphic chlorites are replaced by magnesian siderite and other carbonates after the CO₂ – driven alteration. The development of carbonate spotting (magnesian siderite) and carbonate flooding (dolomite–ankerite and ferroan magnesite–magnesian siderite) is reflected geochemically by increases in carbonation indices such as [CO₂ /(Ca + Mg + Fe)]. The primary (Mg,Fe)CO₃ solids undergo a dissolution−recrystallization process driven by two forces: the preferential partition of Fe(II) toward the (Mg,Fe)CO₃ solid solution and the oxidation of aqueous Fe(II) species to form Fe(III)-bearing solids. Under limited oxygen supply, the oxidation of Fe(II) is a slow process, particularly in the presence of chlorine and inorganic carbon. In absence of oxidation, the initial solid solution should become increasingly Fe-rich as the supersaturation decreases and the system approaches equilibrium. Here, the secondary (Mg,Fe)CO₃ solids become the wrong way round, Mg-rich. The gradual oxidation of the aqueous Fe(II) species eventually leads to precipitation of magnetite, which is also metastable because the aqueous solution is maximally supersaturated with respect to hematite. |