Geology, C-H-O-S-Pb isotope systematics and geochronology of the Yindongpo gold deposit, Tongbai Mountains, central China: Implication for ore genesis

The Yindongpo gold deposit is located in the Weishancheng Au-Ag-dominated polymetallic ore belt in Tongbai Mountains, central China. The ore bodies are stratabound within carbonaceous quartz-sericite schists of the Neoproterozoic Waitoushan Group. The ore-forming process can be divided into three stages, represented by early barren quartz veins, middle polymetallic sulfide veinlets and late quartz-carbonate stockworks, with most ore minerals, such as pyrite, galena, native gold and electrum being formed in the middle stage. The average δ18Owater values changed from 9.7‰ in the early stage, through 4.9‰ in the middle stage, to - 5.9‰ in the late stage, with the δD values ranging between - 65‰ and - 84‰. The δ13CCO 2 values of ore fluids are between - 3.7‰ and + 6.7‰, with an average of 1.1‰. The H-O-C isotope systematics indicate that the ore fluids forming the Yindongpo gold deposit were probably initially sourced from a process of metamorphic devolatilization, and with time gradually mixed with meteoric water. The δ34S values range from - 0.3‰ to + 5.2‰, with peaks ranging from + 1‰ to + 4‰. Fourteen sulfide samples yield 206Pb/204Pb values of 16.990-17.216, 207Pb/204Pb of 15.419-15.612 and208Pb/204Pb of 38.251-38.861. Both S and Pb isotope ratios are similar to those of the main lithologies of the Waitoushan Group, but differ from other lithologic units and granitic batholiths in the Tongbai area, which suggest that the ore metals and fluids originated from the Waitoushan Group. The available K-Ar and 40Ar/39Ar ages indicate that the ore-forming process mainly took place in the period of 176-140 Ma, during the transition from collisional compression to extension and after the closure of the oceanic seaway in the Qinling Orogen. The Yindongpo gold deposit is interpreted as a stratabound orogenic-style gold system formed during the transition phase from collisional compression to extension. The ore metals in the Waitoushan Group were extracted, transported and then accumulated in the carbonaceous sericite schist layer. The carbonaceous sericite schist layer, especially at the junction of collapsed anticline axis and fault structures, became the most favorable locus for the ore bodies.