Thermochronologic constrains on the processes of formation and exhumation of the Xinli orogenic gold deposit, Jiaodong Peninsula, eastern China

Abstract

The Xinli orogenic gold deposit, with gold resources of 40t, located in the northwestern part of the giant Jiaodong gold province, eastern China, is controlled by the Sanshandao Fault and is one of a few deposits hosted by the Early Cretaceous ~129–128Ma Guojialing granitoid. Soon after intrusion, the granitoid underwent ductile shearing at >400–500°C marked by recrystallized quartz ribbons and bending of plagioclase lamellae. With rapid cooling from zircon crystallization temperature of ~750–800°C at ~129–128Ma (U–Pb) to closure temperature of 300±50°C for biotite (40Ar/39Ar method) at 124±1Ma, the ductile deformation lasted <4 million years. Mineralization was associated with subsequent brittle reactivation of normal movement on the fault, indicated by a SE plunging, downdip lineation on the fault. A hydrothermal sericite 40Ar/39Ar age of ~121Ma, inferred to date the alteration associated with gold mineralization, has been previously obtained for the adjoining Cangshang gold deposit also controlled by the Sanshandao Fault. At the Xinli deposit, the 121.5±1.3Ma and 120.5±1.2Ma 40Ar/39Ar plateau ages of weakly-fractured igneous K-feldspar record closely the time of normal faulting and cooling within the range of mineralization temperatures (350–250°C).Two zircon fission-track (ZFT) ages of 91±4Ma and 90±3Ma constrain the time of cooling through ~240±50°C. Unimodal distribution of apatite fission-track (AFT) lengths with a slightly negative skew and mean fission-track lengths of 12.3±0.2μm indicate relative slow continuous cooling through 125–60°C at 60±6Ma. The slight acceleration of cooling around ~90Ma constrained by the 40Ar/39Ar, ZFT and AFT data, and thermal modelling may have resulted from the late normal reactivation of the Sanshandao Fault. In summary, extension and normal faulting not only created the channelways for the mineralizing fluids, but also gave rise to the subdued topography and post-mineralization exhumation that preserved the deposit.