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This review focuses on the solubility, origin, abundance, and degassing of carbon dioxide (CO₂) in magma–hydrothermal systems, with applications for those workers interested in intrusion-related deposits of gold and other metals. The solubility of CO₂ increases with pressure and magma alkalinity. Its solubility is low relative to that of H₂O, so that fluids exsolved deep in the crust tend to have high CO₂/H₂O compared with fluids evolved closer to the surface. Similarly, CO₂/H₂O will typically decrease during progressive decompression- or crystallization-induced degassing. The temperature dependence of solubility is a function of the speciation of CO₂, which dissolves in molecular form in rhyolites (retrograde temperature solubility), but exists as dissolved carbonate groups in basalts (prograde). Magnesite and dolomite are stable under a relatively wide range of mantle conditions, but melt just above the …