Molybdenum isotope tracing petrogenesis of adakitic rocks and associated ore-forming process

Abstract

Molybdenum (Mo) isotopes can be fractionated during redox-related hydrothermal and Earth’s surface processes. Distinct Mo isotope features of different reservoirs, thus, make Mo isotopes as a potential tracer for the recycling of crustal materials, particularly subduction sediments. Here, we report the Mo isotope compositions of 61 early Cretaceous high-K calc-alkaline rocks (defined as adakitic rocks) with distinct source affinities from central-eastern China, including low-Mg adakitic rocks from the Dabie orogen, ore-barren high-Mg adakitic rocks from the South Tan-Lu Fault and ore-bearing high-Mg adakitic rocks from the Lower Yangtze River Belt. Low-Mg adakitic rocks have the low δ98MoNIST3134 of −0.48‰ to −0.03‰, and both ore-barren and ore-bearing high-Mg adakitic rocks display heavier isotope compositions and larger variations: for example, −0.38‰ to 0.41‰ and −0.58‰ to 1.39‰, respectively. Magma evolutions (e.g., hornblende crystallizations) appear to dominate the Mo isotope variation in low-Mg adakitic rocks but do not account for those in high-Mg adakitic rocks. Some ore-bearing high-Mg adakitic rocks characterized by high S contents and hydrothermal alteration minerals, display positive correlations between S and Mo, Cu contents, δ98Mo values, reflecting the effects of isotopically heavy hydrothermal fluid.Having eliminated the effects of magmatic evolution and hydrothermal alteration, both ore-barren and ore-bearing high-Mg adakitic rocks display multiple-end-member mixing trends according to the correlations of Ce/Mo, Th/Yb, and Ba/Th ratios to δ98Mo. First, a common end-member with extremely low Ce/Mo, low Th/Yb, high Ba/Th and mantle-like δ98Mo values was identified, representing an enriched mantle origin metasomatized by fluids from subduction oxidized sediments. The other end-member of ore-barren high-Mg adakitic rocks displays high Ce/Mo, Ba/Th ratios and low δ98Mo values similar to low-Mg adakitic rocks, thus, it represents the lower continental crust derived melt component. By contrast, the other end-member of ore-bearing high-Mg adakitic rocks displays extremely high δ98Mo (>1.5‰), implying that initial adakitic melts involve reduced sediments in addition to the oceanic mafic crust. Notably, the common enriched mantle end-member for both ore-barren and ore-bearing adakites is characterized by high Mo and Cu concentrations, implying that the incorporation of oxidized sediments into the mantle likely plays a key role in the initial enrichment of the chalcophile metals in the mantle. This work demonstrates that Mo isotopes can be useful for understanding adakitic source affinities and associated Cu-Au-Mo ore-forming processes.