Episodic Au-Mo mineralization events in the Xiaoqinling district, southern margin of the North China Craton

Abstract

The Xiaoqinling district, located in the southern margin of the North China Craton, is one of China’s most significant gold districts (resources of > 630 t Au). A complex series of overprinting Mesozoic tectonic and related hydrothermal events associated with the metallogenic evolution in this region has led to controversial interpretations and models. Here we present new geological, mineralogical, ore texture, geochronological, and in-situ S isotope data from the Dahu Au-Mo, Qinnan Au-Mo, and Tonggou Au deposits from this region which indicate that the Mo and Au were introduced in two discrete mineralization events. The Mo-bearing mineral assemblage mainly comprises quartz, anhydrite, pyrite, molybdenite, and rutile that yielded an age of 216.1 ± 9.7 Ma at Dahu. In-situ LA-ICP-MS δ34S values for the molybdenite and related pyrite cluster tightly at -10 to -9‰. On the other hand, the Au-bearing mineral assemblage mainly comprises quartz, pyrite, galena, sphalerite, chalcopyrite, native gold, and Te (±Bi) minerals. Hydrothermal rutile and monazite associated with auriferous pyrite yielded ages of 124.7 ± 3.9 Ma (Dahu), 121.5 ± 1.3 Ma (Qinnan), and 122.1 ± 2.0 Ma (Tonggou). The in-situ δ34S values of gold-related sulfides fall within a range of -4 to + 3 ‰. Although Mo and Au ores are both present in the same ore veins, their mineral assemblage, mineralization age, fluid properties and sources are different. Textural relationship shows that the earlier Mo-bearing quartz-anhydrite veins were overprinted by later auriferous fluids. Several lines of evidence support a multiphase mineralization process of Mo and Au, which are genetically linked to the post-collisional extension of the Qinling orogen and the Paleo-Pacific post-subduction tectonics, respectively. The episodic hydrothermal mineralization in Xiaoqinling represents a protracted and complex lode system.