Magnetite formation in the serpentinization of ultramafic rocks from Bangong-Nujiang suture zone, Tibetan Plateau

Serpentinized ultramafic rocks are widely distributed in the Bangong-Nujiang suture zone (BNSZ), and show close relationship with the regional mineralization. Here we investigate the serpentinized ultramafic rocks from Angwu in the central segment of the BNSZ, where the primary minerals include olivine, clinopyroxene, chromite, lizardite, magnetite, and chlorite. The alteration process of the ultramafic rocks, including serpentinization, can be divided into three stages: (I) formation of relatively Fe-rich serpentine (Mg#, 0.82), with no precipitation of magnetite; (II) formation of relatively Mg-rich serpentine (Mg#, 0.93), with precipitation of magnetite in serpentine veins; and (III) alteration of serpentine into chlorite. We computed the reactions associated with alteration process for the above stages. In addition, thermodynamic modeling in multicomponent mineral equilibrium phase diagrams indicate that during the serpentinization process, the desilication of pyroxene in caused the enhancement of 5102 activity and restricted the transformation from olivine to magnetite. The reaction temperature for the serpentinization of these rocks was relatively high, and within this temperature interval (100-300 degrees C), lizardite was stable, and precipitation of magnetite was not favored.