The Mesozoic magmatic sources and tectonic setting of the Zijinshan mineral field, South China: Constraints from geochronology and geochemistry of igneous rocks in the Southeastern Ore Segment

Abstract

The Zijinshan mineral field is a large porphyry-epithermal CuAuMoAg ore system located in the southwestern Fujian Province, China. The Southeast (CuMo) Ore Segment (SOE) is a newly discovered section located in the southeastern portion of the Zijinshan mineral field. Zircon UPb and molybdenite ReOs ages, as well as the geochemical compositions of intrusive and volcanic rocks and zircon Hf isotope data are used to constrain the timing, petrogenesis, tectonic setting and the relationship between magmatism and metallogenesis in the SOE and Zijinshan mineral field.Major and trace element geochemistry indicates that igneous rocks from SOE are mainly shoshonitic granitoids that are LILE enriched (Rb, Ba, Th, U) with HFSE depletion (Nb, Ta, Zr, Hf and Y), indicating a subduction related setting. New UPb zircon analyses yield weighted mean ages of ca. 160Ma for the Caixi pluton, ca. 110Ma for the Sifang granodiorite, ca. 107–105Ma for the Luoboling granodiorite porphyry, ca. 105Ma for the quartz diorite porphyry, ca. 108–105Ma for the dacite porphyry, ca. 98Ma for the rhyodacite, and 97Ma for the rhyolite in the SOE. These dates, integrated with previous geochronological data, indicate that there were two main magmatic events in this area: (1) a Middle to Late Jurassic event (165–150Ma), and (2) a Cretaceous event (110–95Ma) which can be further divided into two groups, i.e., the late Early Cretaceous event (110–105Ma) and the early Late Cretaceous event (100–95Ma). The late Early Cretaceous intrusions contain higher εHf (t) values (0.1 to −3.6), with TDM2 varying from 1.4 to 1.0Ga, which may come from mixing between crustal and subduction-related mantle derived melts in contrast to the Middle to late Jurassic intrusions that may come from the partial melting of the Cathaysia Block basement. The early Late Cretaceous intrusions display a broader range of εHf (t) values (−0.9 to −9.4), with TDM2 varying from 1.8 to 1.2Ga, suggesting a similar source to the late Early Cretaceous rocks but with more crustal material.The molybdenite ReOs ages for mineralization range from 110Ma to 105Ma, consistent with ages of the late Early Cretaceous rocks. Zircon trace elements analyses imply that the early Late Cretaceous rocks (Ce4+/Ce3+=113–1800) are characterized by higher oxygen fugacity with higher mineralization potential than the late Early Cretaceous (Ce4+/Ce3+=9–545) and the Middle to Late Jurassic rocks. Variability in ages, geochemistry and oxygen fugacity indicate that the Middle to Late Jurassic magmatic event represents pre-mineralization intrusions whereas the late Early Cretaceous is syn-mineralization and the early Late Cretaceous intrusions post-mineralization. We propose that both the Middle to Late Jurassic and the Cretaceous magmatic events may relate to the subduction of the Pacific slab, and the changing direction of the subducted slab may result in the different sources for the two magmatic events.