Constraints on metallogenic age from cryptoexplosive breccia in Naruo Cu(Au) deposit, Xizang

This paper is the result of mineral exploration engineering.

Objective The Naruo deposit is one of four super large deposits in Duolong ore district. It is composed of porphyry and breccia type orebodies. Previous researches have focused on porphyry orebody, while the research on cryptoexplosive breccia ore bodies is relatively weak. The temporal relationship between cryptic explosion processes and mineralization remains inadequately studied. The cryptoexplosive breccia is generally produced in a cylindrical shape, with the breccia composed of feldspar quartz sandstone and granodiorite porphyry, and the cement mainly composed of rock powder.

Methods This study is to explore the age relationship between the formation and mineralization of cryptoexplosive breccia through chronological evidence.

Results Zircon mineralogy shows that the zircon edge of the granite diorite porphyry (NR1) is characterized by dissolution, weak oscillation zone, and incomplete recrystallization. Zircons in cement (NR1) is characterized by fine crystalline form, clear oscillatory zone. The trace element characteristics of these two types of zircons show that the source of two stages of magma is continental crust. The rare earth distribution curves are relatively steep, with enrichment of heavy rare earth elements, loss of light rare earth elements, significant positive Ce and negative Eu anomalies. LA−ICP−MS zircon U−Pb dating results show that the zircon ²⁰⁶Pb/²³⁸U age of granite diorite porphyry is between 117.3 Ma and 125.4 Ma, with weighted average age of (120.8±1.4) Ma, indicating the diagenetic age of 120.8 Ma. The zircon ²⁰⁶Pb / ²³⁸U age of cement is between 113.5 Ma and 119.9 Ma, with weighted average age of (116.4±1.2) Ma, indicating the cryptoexplosive age of 116.4 Ma.

Conclusions The results in this study show two stages of diagenesis in Naruo deposit, ~120 Ma and ~117 Ma, respectively. Mineralization occurred between 116 Ma and 117 Ma, rather than ~120 Ma. The chronological data shows that the mineralization and cryptoexplosion are controlled by contemporaneous magmatism, with cryptoexplosion occurring first and mineralization occurring later, and there are differences in the time and space of magma evolution.