Hydrothermal alteration batch experiments were conducted on olivine bearing mine tailings in order to investigate two potential valorization methods: the ex-situ CO2 sequestration and hydrogen production. The originality of this work lies in the simultaneous investigation of these two processes. We reacted powdered mine tailings with CO2-saturated water at three different sets of P/T conditions, 473 K/15 MPa, 523 K/30 MPa and 573 K/30 MPa. After 25 days of reaction, CO2 was sequestered in the form of Fe-bearing magnesite, (Mg,Fe)CO3 in all the experiments. Maximum carbonation yield was achieved at 523 K and 30 MPa, which was 53.8 wt.% of product, equivalent to the trapping of 320.5 g of CO2 per kg of mine tailings. Hydrogen gas was produced via the oxidation of Fe2+ in olivine. The highest quantity of hydrogen (H2) was produced at 573 K/30 MPa which was 0.57 g of H2 per kg of mine tailings. It suggests that the temperatures between 523 K and 540 K at pCO2 = 30 MPa are favorable for simultaneous ex-situ CO2 mineral sequestration and hydrogen production from New Caledonian mine tailings. The combined method of ex-situ CO2 storage and hydrogen production proposed by this study offsets 90% of New Caledonia's annual CO2 emissions while compensating ∼10% of New Caledonia's annual energy demand. More globally, it has implications for cost effective disposal of industrial CO2 emissions and production of hydrogen gas (clean energy) at a large scale; those two processes could be combined using the residual heat provided by a third one such as the high temperature smelting of ore.