Zircon U-Pb Age, Geochemical Characteristics and Geological Significance of Tonalite in Saibagou Gold Deposit, the Northern Margin of Qaidam

This paper carried out the LA-ICP-MS zircon U-Pb age testing and analyzed the geochemical charateristics of Saibagou tonalite.The results show that the formation age of tonalite is(470.4±5.2)Ma,belonging to the Middle Ordovician. The SiO₂ content of tonalite is between 62.46% and 64.19%,with an average of 63.20%; Al₂O₃ is between 14.39% and 15.01%,with an average of 14.71%; the total alkali content (K₂O+Na₂O) is 2.32% to 3.55%,with an average of 3.08%,rich in Na and poor in K; the aluminum saturation index A/CNK is between 1.09 and 1.26,being peraluminous granite of low potassium calcium-alkaline. In the original mantle standardized trace element cobweb diagram, Ba and Th are enriched,and Nb,P and Ti are strongly depleted.The total rare earth element (ΣREE) of tonalite is low and the degree of light and heavy rare earth fractionation is also low; Eu is no abnormal,the rare earth element distribution pattern is slightly "right-inclined" in the standardized spheroidal meteorite. Considering the regional tectonic evolution and the formation age of tonalite,it is believed that Saibagou tonalite is the I-type granite formed in the continental marginal arc tectonic environment of the subduction zone. Based on the chronological study of Saibagou gold deposit,it is concluded that there is no genetic relationship between the mineralization and the magmatism of tonalite in the middle Ordovician.