Genesis of Cretaceous igneous rocks and its related large scale porphyry Cu-Au mineralization in Chating, the Middle-Lower Yangtze River Metallogenic Belt: The geochemical constrains

Abstract

The newly discovered large Chating porphyry Cu–Au deposit is associated with early Cretaceous intrusive rocks in the Middle-Lower Yangtze River Belt (MLYRB), eastern China. Two types of intrusive rocks in the Chating area have been identified, i.e., the quartz-diorite porphyry, related to the Cu–Au mineralization, and the diorite porphyry that is ore-barren. Zircon LA–ICP–MS U–Pb dating yields 145–136 Ma for the Chating intrusions. They both belong to High-K calc-alkaline series, with enrichments of large-ion lithophile elements (LILEs) and light rare earth elements (LREEs), depletions of high field-strength elements (HFSEs) and heavy rare earth elements (HREEs), and no or slightly negative Eu anomalies. The quartz-diorite porphyry shows high Sr contents and Sr/Y ratios; low MgO, Cr and Ni contents; medium (La/Yb)N and Mg# (34.6–54.6) and low K2O/Na2O (<1) values; decoupling of Sr/Y and (La/Yb)N; and low Th/U (3.49–6.08) values, thus exhibiting characteristics of slab-derived adakitic rocks. In addition, it has slightly enriched Sr–Nd–Hf isotopes ((87Sr/86Sr)i = 0.70655–0.70717, εNd(t) = −6.9 to −8.8, and εHf(t) = −7.6 to −10.6) and high radiogenic Pb isotopes ((206Pb/204Pb)i = 18.22–18.70, (207Pb/204Pb)i = 15.63–15.66 and (208Pb/204Pb)i = 38.49–38.97), reflecting the involvement of subducted sediments in the magma source. Therefore, the fertile quartz-diorite porphyry was most likely generated by the partial melting of the slab-derived, subducted sediments and a partial degree of metasomatized mantle-derived magma with crustal assimilation. As to the barren diorite porphyry, it has similar Sr–Nd–Pb isotope and trace element features as the fertile adakitic samples. However, it shows relatively lower SiO2 contents; high contents of MgO (3.89–4.06 wt%), Cr and Ni (avg. 47.6 ppm and avg. 15.01 ppm); high Mg# (51.2–69.8); and more enriched Hf isotope characteristics (−8.5 to −12.9), which imply that this barren diorite porphyry could be generated from the partial melting of enriched metasomatised lithospheric mantle assimilated with old continental crust materials. The δ34S values of the pyrite and chalcopyrite of the ores in Chating indicate a magmatic-derived source, resembling other porphyry-skarn Cu–Au deposits in the MLYRB. The fertile Qtz-diorite porphyry adakitic rocks are characterized by higher oxygen fugacities than the barren diorite porphyry, which are closely related with the partial melting of the subducted oceanic slab in an oxidizing environment, thus making them favorable for Cu–Au mineralization.