Systems and methods for sequestering carbon, evolving hydrogen gas, producing iron oxide as magnetite, and producing magnesium carbonate as magnesite through sequential carbonation and serpentinization/hydration reactions involving processed olivine- and/or pyroxene-rich ores, as typically found in mafic and ultramafic igneous rock. Precious or scarce metals, such nickel, cobalt, chromium, rare earth elements, and others, may be concentrated in the remaining ore to facilitate their recovery from any gangue material.