Metallogenic age and genesis of the Zhaishang gold deposit in Minxian, Gansu Province

Zhaishang gold deposit is located in the concentrated area of large-scale carlin-like gold deposits in West Qinling Mountains, and it is the second super-large gold deposit discovered in the past 20 years. Rb, Sr, Re, Os isotopes of the pyrite, the gold-bearing mineral of Zhaishang gold deposit, in six ore samples were tested in this paper to determine their metallogenic ages and ore-forming materials. The measured Rb-Sr and Re-Os isochron ages of the pyrite are 273 +/- 10Ma and 273 Ma +/- 22Ma, which are older than the ages of two metallogenic stages of the Zhaishang deposit obtained by the predecessors, i. e. , a earlier metallogenic stage ( the first stage of 273 Ma) might be exist in this area. This new chronological data may broaden the time axis for searching gold deposits in this area since no same metallogenic ages have ever been reported in the explored gold deposits in West Qinling. Furthermore, analyses results of Sr, Os, S isotopes, REE and trace elements compositions indicate that the ore-forming material of the Zhaishang gold deposit is the mixture of deep source and crust source materials. Tectonically, the Wujiashan Rise which is located in the Minli fore-arc basin experienced a strong geothermal uplift and extension in the Late Hercynian Period, which resulted in the hydrothermal fluids rising along the fault channels to the folded belts of the basin with a high Au background, and the activated Au enriched in the formation and unloaded in suitable structural spaces to form the first stage Zhaishang gold deposit. The later two stages of W and Au mineralization superimposed on the early gold deposit. Compared with regional geological evolutionary events, the age of gold mineralization in the first stage of Zhaishang gold deposit is consistent with the time of the closure of the A'nyemailen ocean basin in the West Qinling, the subduction collision of Gonghe aulacogen, and the intrusion of Jiangligou and Zhongchuan Himalayan-type plutons, which obeys the rule "crustal thickening is conducive to the formation of gold deposits and Himalayan granites are closely related to gold deposits". It is of indicative significance for searching contemporary gold deposits in this area.