Yufei Wang, PhD

Y. Wang^1,2, Z. Wang², D. Gregory¹, S. Zou³, D. Xu³

¹Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada

²Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring, Ministry of Education, School of Geosciences and  Info–Physics, Central South University, Changsha, Hunan, China

³State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang, Jiangxi, China

Hydrothermal Co–Au deposits represent an uncommon metal assemblage, and the paragenetic relationships and mechanisms responsible for their co–formation remain poorly understood. The Tuolugou Co–Au deposit in the East Kunlun Orogenic Belt is a representative stratiform system overprinted by multistage tectonic–hydrothermal events, providing an ideal case to unravel Co–Au mineralization processes. To constrain fluid evolution and the physicochemical controls on sequential Co–Au precipitation, we integrated microtextural observations with EPMA and LA–ICP–MS analyses and characterized fluid evolution using Raman spectroscopy and fluid-inclusion microthermometry. Thermodynamic modeling was further applied to reconstruct variations in critical physicochemical parameters.

The early Caledonian exhalative–syngenetic stage is dominated by H₂O-rich fluid inclusions in quartz (187–232 °C; 8.7–14 wt.% NaCl_eqv.), consistent with SEDEX-type mineralization and forming Co–Au–rich Py I (up to 4.42 wt.% Co; 284 ppm Au). Indosinian overprinting comprises five sub-stages (Da–De). Da–Db are marked by Co-bearing pyrite and siegenite under plastic deformation. The Dc stage represents the primary Co mineralization, characterized by intense Co precipitation as Co-rich pyrite (up to 5.36 wt.% Co) and cobaltite under plastic–brittle transitional conditions, with negligible Au (Au/Ag ≈ 963). Dc fluids contain both H2O-rich and CO₂-rich inclusions (268–330 °C; 5.4–9.9 wt.% NaCl_eqv.). In contrast, the Dd stage marks the primary Au mineralization, featuring native gold (Au/Ag ≈ 859), negligible Co, and the presence of CH₄-bearing inclusions, with coexistence of H₂O-rich and CO₂-rich fluids (226–305 °C; 2.8–9.2 wt.% NaCl_eqv.). The progressive decrease in Al content of quartz, coexistence of H₂O-rich and CO₂-rich fluids, and elevated Mg–Fe–Ca in quartz and chlorite compositions indicate fluid boiling and enhanced fluid–rock interaction during Dc–Dd, leading to increased pH. Replacement textures in sulfides and reactions with carbonaceous host rocks during Dd further suggest a significant reduction in fluid fO₂. Thermodynamic modeling confirms that increasing pH reduces Co solubility and promotes Co precipitation, whereas large-scale Au deposition requires substantial lowering of fO₂ at moderate hydrothermal temperatures.

These results demonstrate that the sequential Co and Au precipitation in the Tuolugou deposit was controlled by the combined effects of fluid boiling (pH increase) and redox buffering by carbonaceous rocks (fO₂ decrease), providing new insights into the metallogenic processes of global Co–Au systems. These findings emphasize that the integration of Co-rich stratiform precursors, carbonaceous host rocks, and indicators of fluid boiling and redox buffering constitute an effective exploration model for Co–Au deposits. Such criteria may be directly applied to targeting concealed orogenic Co–Au systems in Paleo–Tethyan and comparable metallogenic belts.