Petrogenesis and Cu metallogenic potential of the Zhawala pluton in the Western Sichun, China

The evaluation of the Cu-metallogenic potential of plutons is a very important work in the study of porphyry Cu deposits. In this paper, the geochronology and petrogeochemistry and mineral geochemistry of the Zhawala pluton in the Western Sichuan have been carried out in order to reveal the petrogenesis and Cu-metallogenic potential of this pluton, and to provide an example of how to effectively carry out the porphyry copper mtallogenic potential assessment. The Zhawala pluton is located in the Zhongzha micro massif in the Yidun terrane. Its zircon U–Pb dating results show that it was formed at (226.9±1.1) Ma. Combined with the regional tectonic evolution history, it can be concluded that this pluton was formed in the collision or post-collision stage between the Zhongza micro massif and the Changdu-Simao micro massif. The pluton was product of magma derived from the partial melting of the middle and lower crust induced by the upwelling of mantle-derived basic magma. The Zhawala pluton belongs to the I-type granite based on its whole-rock geochemical characteristics. Compared to typical adakites, the Zhawala pluton has relatively low Sr/Y, (La/Yb)N and δEu values. Based on the calculated magmatic oxygen fugacity using zircon composition (△FQM=0.1) and a set of geochemical criteria including δEu, δCe values and S contents of apatites and Ce/Nd and Ce4+/Ce3+values of zircons, the parental magma of the Zhawala pluton was more reductive than those of the typical Cu-mineralized plutons. The calculated magmatic water contents (w(H2Omelt)>4.5%) and emplacement depths (7-8 km) based on compositions of amphiboles, and the relationship between XCl/XOH and XF/XCl values of apatites of the Zhawala pluton indicate that the large scale hydrothermal fluid exsolution could have occurred extensively in the evolution process of the magma. In addition, the calculated Cl contents of the magma based on Cl contents of apatites are more than 2000×10-6, indicating that the evolved magma system still had high Cl contents during or after the hydrothermal fluid exsolution. It can thus be concluded that the parental magma of the Zhawala pluton should be a Cl-rich system. In general, our study has shown that the parental magma of the Zhawala pluton was characterized with enriched water and Cl and had experienced significant hydrothermal fluid exsolution in the evolution process of magma. Based on these findings, it is inferred that the magma of the Zhawala pluton could have a certain potential to form the porphyry Cu deposit, but was unlikely to form a large or super-large porphyry Cu deposi due to the relatively low oxygen fugacity of the magma. Finally, based on the previous results and the Sr–Nd isotopic characteristics of the whole rock, we speculated that the relatively shallow depth of the magma source which was mainly derived from the ancient lower crust with lack of added components of mantle or juvenile crust could be the reason resulting in the limited magmatic fertility of the Zhawala pluton.