Mineralization of the Luanling gold deposit in the southern margin of the North China Craton: Insights from mineralogy and mineral chemistry of sulfides, tellurides and oxides

The Luanling gold deposit, a typical Te‐Au deposit in the Xiong'er terrane, the southern margin of the North China Craton, contains two types of ores, namely altered rock‐type and quartz vein‐type ores. Gold is hosted by pyrite and As‐bearing pyrite in the altered rock‐type ores, whereas tellurides are the main gold carriers in the quartz vein‐type ores. In order to investigate their origins, we calculated the Gibbs free energies of formation and reaction of related sulfides, tellurides and oxides from different ore types. The phase diagrams of these minerals were constructed at 300 °C, a temperature at which gold was mainly precipitated in the Luanling deposit. According to phase relations among sulfides, logfS2(g) is constrained between −10.4 and −6.5. In addition, we propose that [Au(HS)2]− was the dominant carrier of gold in ore‐forming fluid, and its decomposition formed pyrite and arsenian pyrite, in which gold mainly exists as an invisible phase. In the quartz vein‐type ores, logfTe2(g) and logfO2(g) are constrained to range from −13.68 to −7.9 and −36.8 to −31.1, respectively, based on the phase relations between tellurides, sulfides and oxides, and gold was mainly transported as [Au(HTe)2]− and its break down formed gold‐bearing, including sylvanite, petzite and Au‐Ag‐tellurides. Sylvanite and petzite, the two stable Au‐Ag tellurides, were probably precipitated primarily from hotter ore‐forming fluids. According to phase relations among tellurides, the two phases were decomposed into native gold, hessite and more stable Au‐Ag‐tellurides with the changes of physicochemical conditions in the later stages. In addition, we proposed that the ore‐forming fluids relevant to the altered rock‐ and the quartz vein‐type ores were from the same source and continuously evolved.