On the origin of oldest Iron-Oxide-Copper-Gold (IOCG) deposits at the transition from Archean drip to plate tectonics

Abstract

Geological evidence supports a significant change in Earth’s behaviour in the mid- to late Archaean, between 3.2 and 2.5 Ga, reflecting stabilization of the lithosphere and replacement of vertical tectonics by linear imbricated belts. At the heart of this change, the oldest (c. 2.75 Ga) Iron-Oxide-Copper-Gold deposits (IOCG) were formed in the Carajás Mineral Province (CMP) of the Amazon craton. U-Pb ages, Lu-Hf isotopes and trace element composition of detrital zircons from modern drainages record the crystallization ages of the exposed rocks of the CMP. Combined with the geochemistry of Archean granitoids in the CMP, we recognize four different age and compositional groups: 3.01-2.92 Ga TTG, 2.87-2.83 Ga transitional TTG + sanukitoid + K-rich granitoids, 2.78-2.72 Ga A-type crustal granites accompanying IOCGs, and 2.59-2.53 Ga alkaline high-K intrusions accompanied by renewed IOCG mineralization. The first two groups have a dominantly juvenile isotopic signature whereas the last two have evolved Hf-isotope signatures, accompanied by increase in K2O/Na2O, reflecting addition of old crustal components in the melting sources over time. The older juvenile granitoids are associated with dome-and-keel structures typical of granite-greenstone terranes, whereas the younger granitoids were emplaced along a linear shear belt associated with new mafic-ultramafic intrusions and remelting of older TTG. Based on the tectono-magmatic evolution, we argue that metasomatism and fertilization of the underlying lithospheric mantle by incompatible elements, necessary for the development of IOCG deposits, were related to vertical drip-tectonics during development of the TTG proto-continent. This proto-continent made the lithosphere rigid enough to allow linear translithospheric deformation to localize at c. 2.85 Ga, allowing decompression melting of the metasomatized lithospheric mantle in a restricted extensional setting to form abundant mafic and A-type granitoids at c. 2.75 Ga, and the first IOCG deposits on Earth.