In situ apatite U-Pb dating for the ophiolite-hosted Nianzha orogenic gold deposit, Southern Tibet

Mineralization age dating is essential for understanding orogenic-type gold metallogeny, but suitable minerals for dating are not always available. Apatite can incorporate considerable amount of U and Th, making it a potential U-Pb geochronometer to study hydrothermal alteration and mineralization processes. The newlydiscovered Nianzha is a large orogenic Au deposit (~25 t @ 3.08 g/t Au) and located in the contact fault zone between ultramafic rocks and diorite in the Renbu tectonic me ' lange of southern Tibet. Two different types of apatite were identified in the Nianzha gold deposit: type I magmatic apatite hosted in diorite and syenite, and intergrown with other magmatic minerals; type II hydrothermal apatite hosted in mineralized diorite. These apatite grains are coarse euhedral granular and closely associated with auriferous sulfides (e.g., pyrite, chalcopyrite, galena). Type I apatite is F- and SO3-rich, whereas type II apatite is distinct from type I apatite by its significantly higher Cl, Mn, rare earth elements (REE), U, Th, and As contents, indicative of a hydrothermal origin. Thus, formation age of type II apatite reflects the timing of gold mineralization. Two type I magmatic apatite samples yielded similar discordia U-Pb ages of 80.35 +/- 1.56 Ma (MSWD = 1.3; n = 86) and 79.53 +/- 1.27 Ma (MSWD = 0.91; n = 72), respectively, whilst type II hydrothermal apatite yielded a discordia age of 44.60 +/- 1.45 Ma (MSWD = 1.2; n = 64). The gold mineralization age is consistent with that of nearby orogenic gold deposits (e.g., Mayum, Bangbu, Zhemulang, and Juqu) in the region. Therefore, we suggest that hydrothermal apatite U-Pb dating can effectively constrain the timing of orogenic Au mineralization events.