Anatomy of a world-class epizonal orogenic-gold system: A holistic thermochronological analysis of the Xincheng gold deposit, Jiaodong Peninsula, eastern China

Abstract

Xincheng is a world-class orogenic-gold deposit hosted by the Early Cretaceous Guojialing granitoid in the Jiaodong Peninsula, eastern China. A zircon U–Pb age of 126±1.4Ma, together with previous data, constrain the emplacement of the Guojialing intrusion to 132–123Ma. The granitoid underwent subsolidus ductile deformation at >500°C following its intrusion. The small difference in age between the youngest zircon U–Pb age of unaltered granitoid (~123Ma) and the ca. 120Ma 40Ar/39Ar ages of sericite, associated with breccias and gold mineralization within it indicate initial rapid cooling from magmatic temperatures to those prevalent during brittle deformation and associated gold mineralization at ~220–300°C. Evidence of a direct association between granitic magmatism and gold mineralization, such as at least localized near-magmatic depositional temperatures and metal zoning evident in undoubted intrusion-related gold deposits, is absent. The 40Ar/39Ar age of ~120Ma coincides with the mineralization age of many other orogenic-gold deposits along the Jiaojia Fault. Sixteen zircon fission-track (ZFT) ages across the ore and alteration zones range from 112.9±3.4 to 99.1±2.7Ma. The long period of cooling to the ~100Ma ZFT closure temperatures recorded here suggests that ambient temperatures for hydrothermal alteration systems lasted to ~100Ma, possibly because of their focus at Xincheng within the young Guojialing granitoid as it cooled more slowly below approximately 300°C to 220°C. However, the restricted number of auriferous ore stages, combined with the presence of cross-cutting gold-free quartz-carbonate veins, indicate that gold itself was only deposited over a restricted time interval at ~120Ma, consistent with studies of orogenic gold deposits elsewhere. This highlights the complex interplay between magmatism, deformation and the longevity of hydrothermal systems that cause genetic controversies. Based on apatite fission-track (AFT) ages, the Xincheng gold deposit was then uplifted and exhumed to near the surface of the crust at 15Ma, probably due to movement on the crustal-scale Tan-Lu Fault. Recognition of such exhumation histories along gold belts has conceptual exploration significance in terms of the probability of discovery of additional exposed or sub-surface gold ore bodies as discovery is as much a function of preservation as formation of the deposits.