Petrogenesis and metal fertility of Yinzu pluton in the Daye district, eastern China: Insights from whole-rock and mineral (zircon, apatite and amphibolite) geochemistry

Abstract

The Daye district in eastern China is featured by numerous granitoid intrusions of Early Cretaceous ages that intrude Paleozoic to Early Triassic marine carbonate and clastic rocks. Several dozen skarn Cu-Fe-Au deposits occur along the intrusive contact zones of the Echeng, Tieshan, Jinshandian, Lingxiang, Tongshankou and Tonglushan intrusions. However, no economic skarn mineralization has been recognized in other intrusions and whether or not these intrusions are metal fertile remain unresolved. In this paper, we use whole-rock and mineral (zircon, apatite and amphibole) geochemistry to provide significant new insights into the petrogenesis and metal fertility of the Yinzu granitoid pluton.The Yinzu pluton mainly consists of quartz diorite and diorite phases that have zircon U-Pb ages of 148.4 ± 1 Ma and 146.6 ± 1.6 Ma, respectively. Whole-rock Sr-Nd isotopic compositions ((87Sr/86Sr)i = 0.70635 to 0.70741; εNd(t) = −8.74 to −3.73) and zircon hafnium isotopes (εHf(t) = −9.1 to −4.8) overlap with values of the Cu-fertile Tongshankou and Tonglushan granitoids, implying a common enriched lithospheric mantle source. The whole-rock geochemical data suggest that both the diorite and quartz diorite have experienced variable degrees of fractional crystallization of olivine, amphibole, titanite and magnetite. The Ce4+/Ce3+ ratios (16–445) and oxygen fugacity (logfO2) calculated using trace element concentrations of zircon (ΔFMQ = +0.2 to +2.4) suggest that both intrusive phases crystalized from oxidized melts similar to Cu-fertile magmas in the world, with the quartz diorite being well comparable to the Tongshankou and Tonglushan granitoids. Based on available melt-apatite partitioning models, the estimated S and Cl contents for the magmas parental to the Yinzu pluton are akin to those of the Cu-fertile granitoids in the Daye district. Rocks of the Yinzu pluton display high apatite Sr/Y (0.38–3.01) and Eu/Eu* (0.35–0.71) ratios as well as high zircon Eu/Eu* (0.51–0.86) and 10000*(Eu/Eu*)/Y (2–27) ratios, indicating significant fractionation of amphibole with limited plagioclase fractionation and high magmatic water contents (>4 wt% H2O). This view is consistent with the calculated melt H2O content of 4.3–6.1 wt% based on the amphibole compositions.Collectively, results presented here indicate that magmas generating the Yinzu pluton are hydrous, oxidized and S-rich, characters similar to the Cu-fertile granitoids in Daye and elsewhere. Thus, the Yinzu pluton is considered to be Cu-(Au) fertile. This consideration is partially supported by occurrences of several vein-type Cu prospects within and disseminated Au ores in Silurian clastic sedimentary rocks immediately to the southeast of this pluton. Based on the regional metallogeny and results of this study, we suggest that the Yinzu pluton is a promising target of future Cu-(Au) exploration.