Geology, geochronology, and stable isotopes of the Triassic Tianjingshan orogenic gold deposit, China: Implications for ore genesis of the Qinzhou Bay–Hangzhou Bay metallogenic belt

Abstract

The Qinzhou Bay–Hangzhou Bay metallogenic belt (QHMB), South China, contains numerous variable deposits, including Au, Cu, Pb, Zn, Ag, W, Sn, U-polymetallic ore deposits. The Tianjingshan gold deposit is located in the northern segment of the QHMB and is a typical large-scale lode gold deposit. Auriferous quartz veins occur along the contact zones between the Neoproterozoic greenschist-facies metamorphic rocks and gneissic granite and are structurally controlled by brittle faults. Mineralization-related hydrothermal alteration mainly includes silicification, pyritization, sericitization, and chloritization. Four stages of alteration and mineralization are recognized: quartz (I), quartz–pyrite (II), quartz–polymetallic sulfide (III), and quartz–carbonate–fluorite (IV) stages. Gold mineralization occurs mainly in stages II and III, as visible gold.Sericite that formed in association with mineralization stages II and III has well-defined step heating plateau 40Ar/39Ar ages of 243.4 ± 2.3 and 239.5 ± 2.3 Ma, respectively, suggesting that the gold mineralization took place during the Middle Triassic, consistent with the timing of the regional Indosinian intra-continental orogeny. In contrast, the Hanjia gneissic granite and Mi’ao monzogranite in the Tianjingshan deposit have zircon U–Pb ages of 789 ± 7 and 147 ± 2 Ma, respectively, which indicates that both intrusions were not involved directly in the gold mineralization. δ18O and δD values from fluid inclusions of auriferous quartz range from −1.1 to +2.6‰ and from −76.4 to −62.8‰, respectively, suggesting that the mineralizing fluid was likely of metamorphic origin. Gold-bearing sulfide minerals have δ34S values ranging from +0.6 to +9.8‰ with a mean of +7.1‰, which are similar to δ34S values of Neoproterozoic greenschist-facies metamorphic rocks and of typical orogenic gold deposits; therefore, we suggest that sulfur is derived mainly from regional metamorphic country rocks. The above mentioned characteristics, together with geological features, suggest that Tianjingshan is a typical orogenic Au deposit that formed during the Indosinian orogeny. Combined with knowledge of other similar deposits, the age and characteristics of the Tianjingshan gold deposit imply that Middle Triassic orogenic gold deposits in the QHMB could be common and of high economic potential.