Petrogenesis of Ore-Bearing and Ore-Barren Intermediate-Acid Intrusive Rocks from Jilongshan Au-Cu Skarn Deposit , the Middle-Lower Yangtze River Metallogenic Belt, Eastern China and their Geological Implications

Jilongshan Au-Cu skarn deposit in Edong-Jiurui ore district , Middle-Lower Yangtze River Metallogenic Belt(MLYRB) , eastern China ,contains 44 t gold and 32 Mt of copper ores. The mineralization is dominated by massive skarn ores, most of which occurs along the contact zone between the lower Triassic dolomitic limestones and Jilongshan granodioritic intrusion. However, Baiguoshu pluton, no more than 1 km western, has been not found any mineralized occurrence with the same strata. The ore-bearing and ore-barren intrusive rocks are granodiorite porphyries, could not be identified by petrographic characters. Besides, Zircon U-Pb dating results demonstrate that Jilongshan and Baiguoshu intrusion emplaced at 140 ± 1Ma and 141 ± 1 Ma respectively, coeval with the Early Cretaceous magmatism in Edong-Jiurui area. Elements geochemistry present that they are both characterized by high Al2O3, rich Na2O (Na2O/K2O>1.0), enrichment of LILE (Rb, Ba, K, Sr) and depletion of HFSE (Nb, Ta), and weak negative Eu anomalies, which suggest they may be originated from partial melting of enriched mantle and associated with crust-mantle interaction ,evidenced by the Sr-Nd-Hf isotopic composition as well. Although the two are partly geochemically similar with each other, they have some obvious differences. The former have higher K2O and Y, Yb ,lower MgO, and Cr, Ni contents, and more obvious differentiation degree between light and heavy REEs with (La/Yb)N=10.55-15.95 than the latter with (La/Yb)N=8.67-10.47. It is indicated that the magmas of the Jilongshan intrusive rocks were probably derived from deeper source than that of the Baiguoshu, also supported by mineralogical data of biotite. In addition, Jilongshan intrusive rocks have a relatively higher initial Nd (ɛNd (t) = -8.2 - -9.4) and Sr ((87Sr/86Sr)i=0.70822-0.70897) isotopic composition than Baiguoshu (-9.2 - -9.7 and 0.70855-0.70881), as same as Lu-Hf isotopic composition. Therefore, combined with previous studies, we suggest that the Jilongshan granodiorite porphyry originated as partial melts of an enriched mantle source experienced more sufficient interaction with lower crust materials during magma ascent compared with Baiguoshu granodiorite porphyry, which may lead to Cu-Au enriched magmas, contributing to subsequent mineralization.