Cu-Mo deposits are commonly associated with talc-alkaline porphyries and derived from subduction-modified lithosphere in magmatic arcs. Here we report the Wangjiazhuang Cu-Mo deposit, which is peculiarly associated with an alkaline quartz monzonite that originated from a metasomatized asthenospheric mantle within an intracontinental setting in the eastern North China Craton (NCC). The deposit was formed at ca. 128.3 +/- 0.7 Ma (2 sigma), basically coincident with the emplacement of the host quartz monzonite (128.8 +/- 1.0 Ma, 2 sigma). Pegmatitic vein-type and stockwork-/disseminated-type mineralizations were identified in the deposit, of which the former is shallowly situated and characterized by coarse and well-crystallized Cu- and Mo-bearing sulfides + quartz + biotite + K-feldspar assemblages, whereas the latter is deeply situated and characterized by stockworks or Cu- and Mo-bearing sulfides disseminated in the altered quartz monzonite. Separation between Cu-rich and Morich ore bodies at shallow depth was also observed. The Cu-Mo mineralization is typically associated with potassic-silicic alteration. High-temperature fluid inclusions with halite/sylvite daughter minerals (having homogenization temperatures of 287-466 degrees C and salinities of 33.8-55.3 wt% NaCl equivalent) suggest a magmatic origin of the ore-forming fluids. Differentiation of the fluids into vapor-rich and liquid-rich phases occurred during the magmatic-hydrothermal processes, as indicated by the coexistence of vapor-rich, liquid-rich and daughter mineral-rich fluid inclusions with similar homogenization temperatures. LA-ICP-MS analysis of the fluid inclusions shows that Cu is highly partitioned into the vapor-rich fluid inclusions, whereas Mo is mainly concentrated in the daughter mineral-rich fluid inclusions. This implies that phase differentiation of the oreforming fluids was likely responsible for the separation of Cu-rich and Mo-rich ore bodies. The presence of hematite in the fluid inclusions also suggests high oxidation state of the ore-forming fluids. Based on the variations of delta S-34 in sulfides and Al contents in quartz, continuous reduction of the oxidized ore-forming fluids, coupled with pH change of the fluids during water-rock interaction, led to the deposition of the ore-forming metals.

The host quartz monzonite shows adakitic geochemical signatures. Fractional crystallization of low-Mg amphiboles contributed to the adakitic signatures. According to the mixing modeling using Sr-Nd isotopes, the quartz monzonite was mainly derived from asthenospheric mantle-derived magma mixed by similar to 15-20% lower crust components at the base of the lower crust. Zircon saturation thermometry and Ti-in-quartz thermobarometer suggest that the rock was emplaced at the temperature of similar to 746 degrees C and the depth of similar to 4 km. High water (H2O >= 4 wt%) and metal (Cu is up to 840 ppm and Mo is up to 11.9 ppm) contents as well as high oxidation state (f(o2) > fayalite-magnetite-quartz oxygen buffer) are featured in the quartz monzonite, indicating that the asthenospheric mantle source had been hydrated and oxidized by the water released from the stagnant Paleo-Pacific slab in the transition zone (410-610 kin). Such modification by the deep stagnant oceanic slab beneath the eastern NCC thus not only triggered the remarkable thinning of the Archean lithospheric keel (loss of > 120 km), but also promoted the asthenospheric mantle to become the favorable magma and metal source for the Cu-Mo deposits in the study region.

 

 

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