Tourmaline geochemistry insights into Au–Sb separation and precipitation at the Ludousou deposit, West Qinling, China

The Au–Sb assemblage is a common characteristic in low-temperature mineralisation, yet scientific investigations into this association remain limited. The West Qinling Orogen in central China hosts more than 50 Au deposits, with the Ludousou Au deposit in its northwestern part being closely related to reduced intrusive rocks characterised by a low oxidation state. Previous studies have largely focused on the geochronology and evolution of magmatic-hydrothermal fluids involved in the formation of the deposit. However, the relationship between Sb and Au has not been thoroughly examined. The Ludousou Au deposit is notable for its abundant hydrothermal tourmaline, a mineral with stable geochemical properties that make it a valuable tool for tracing the evolution of ore-forming magmatic-hydrothermal fluids and associated changes in physicochemical conditions. This study presents detailed in-situ analyses of major and trace elements in tourmaline to better understand the evolution of hydrothermal fluids and the separation of Au closely associated with pyrite and Sb that occurs as stibnite during regional Au mineralisation in the orogen. Tourmaline geochemical data reveal that Al substitution in tourmaline is governed by the mechanisms of Fe 3+ Al −1 and [AlO][(Fe, Mg)(OH)] −1 . Meanwhile, V contents in tourmaline decrease from 978 ppm in the core to 96 ppm at the rim, and Cr contents drop from 2,177 ppm to 64 ppm, indicating a decline in the oxygen fugacity of the ore-forming fluids. Additionally, the concurrent decrease in Na and Ca contents in tourmaline, along with an increase in X-site vacancies, indicates increasing fluid salinity. These data also indicate that the mineralising fluids evolved from an initial magmatic-hydrothermal stage characterised by moderate temperatures, high salinity, and low oxygen fugacity, to a later stage marked by lower temperatures, reduced salinity, and higher oxygen fugacity. Geochemical analyses of tourmaline associated with pyrite and stibnite indicate that the segregation and precipitation of Au and Sb were driven by changes in physicochemical conditions caused by the influx of meteoric water. Antimony (Sb) shows a stronger temperature-dependent solubility than Au, resulting in its mineralisation occurring at later stages and under lower temperature conditions. This pattern is also observed in nearby giant Au deposits such as Zaozigou, Yangshan, and Zhaishang, contributing to a broader understanding of Au mineralisation in the region. Furthermore, similar Au–Sb relationships have been documented in various types of Au–Sb deposits worldwide, highlighting key processes involved in the differentiation and mineralisation of Au and Sb.