Ore genesis of the Nanban gold deposit, West Qinling Orogen: Insights from monazite geochronology and geochemistry of sulfides

Abstract

The Nanban quartz vein-hosted gold deposit is located in the West Qinling Orogenic Belt, and the NS-trending faults control gold mineralization. This study integrates deposit geology, hydrothermal monazite U-Pb dating, trace element composition of sulfides, and sulfur isotope characteristics of the Nanban gold deposit, in comparison with the nearby Ludousou reduced intrusion-related gold deposit, to establish the genetic model of the deposit. Hydrothermal monazite U-Pb dating indicates that gold mineralization occurred at 229.8 ± 2.5 Ma. The study has identified two generations of pyrite (Py1 and Py2) and two generations of arsenopyrite (Apy1 and Apy2). The early Py1 and Apy1 samples exhibit compositional homogeneity, with minimal fractures and mineral inclusions. In contrast, Py2 and Apy2 exhibit a greater abundance of mineral inclusions and are extensively fractured, with quartz, galena, and sphalerite filling the fractures. Geochemical analyses show that Py1 and Apy1 have higher concentrations of Co and Ni, while Py2 and Apy2 are enriched in Au, As, Cu, Ag, and Pb. The Ag/Co ratio of Py1 is less than 0.1, while the Ag/Co ratio of Py2 is generally greater than 0.1. All these characteristics indicate that the Py1 and Apy1 might have formed under mild boiling conditions, and Py2 and Apy2 may have occurred under highly variable physicochemical conditions, such as violent fluid boiling. Fluid boiling likely destabilized gold-sulfur complexes, leading to gold precipitation. The sulfur isotope data of the pyrite range from +2.17 ‰ to +6.54 ‰, indicating a deviation from the values observed in nearby magmatic-hydrothermal deposits but demonstrating a similarity to those of authigenic pyrite within the underlying Cambrian black shale. The ore-forming fluids are inferred to originate from metamorphic devolatilization of underlying strata during the Late Triassic syn-collisional event. Differences in mineralization age, ore-forming material sources, and geochemical characteristics between the Nanban and nearby Ludousou gold deposits further support a non-magmatic-hydrothermal origin. The results support the classification of the Nanban gold as orogenic gold deposit. This study further demonstrates that vein-type gold deposits hosted in granitoids do not necessarily have a genetic relationship with the granitoids. Through the integration of geochronological research with sulfur isotope analysis, a more precise determination can be achieved.