Zircon U-Pb dating and geochemistry of the volcanic rocks in the Dishuishan gold deposit in Gansu Province and its tectonic setting

Located in the west part of North Qilian orogenic zone, the Dishuishan gold deposit is one of the altered cataclastic rock type deposits hosted in volcanic rocks. LA-ICP-MS zircon U-Pb chronology indicates that the andesitic crystal tuff was emplaced at (479.7±6.9) Ma, the granodiorite was emplaced at (426.9±6.7)Ma, and the gold bearing beresite was emplaced at (482.7±9.5)Ma, which represent the ages of ore-bearing volcanic rocks, whereas metallogenic age should be later than (426.9±6.7)Ma. The geochemical data suggest that the trace elements of volcanic rocks are rich in K, Rb, Th, and relatively depleted in Nb, Ta, P, Ti. The volcanic rocks and related mineral ores have similar chondrite-normalized REE patterns, exhibiting right-dip curve, the LREE are relatively enriched and Eu negative anomaly is weak. Combined with the trace elements and the REE characteristics, the authors infer that the volcanic rocks were formed in an island-arc setting, related to subduction of Qilian oceanic crust. The volcanic magma contaminated by the upper crust probably came from the partial melting of the mantle wedge, which had undergone metasomatism induced by the subduction fluid. The chemical composition of the granodiorite reveals SiO₂ values ranging from 67.48% to 72.68%, K₂O+Na₂O values ranging from 7.15% to 7.44%, Rittmann index ranging from 1.78 to 2.07, and aluminous index A/CNK values ranging from 0.95% to 1.19%; the trace elements show enrichment of K, Rb, Th, U and relative depletion of Nb, P Ti. The LREE are relatively enriched and Eu negative anomaly is medium. The granodiorite belongs to Na calc-alkaline series weakly peraluminous I-type granitoid, and was formed in a volcanic island-arc setting of arc-continent collision and indicated crust and mantle source. According to regional geological data, the metallogenic tectonic background of the Dishuishan gold deposit was developed from the subduction of the North Qilian ocean basin beneath Alxa land block in Early Ordovician, which formed island-arc volcanic rock, and acted as main source bed→arc-continent collision in Middle Silurian period, which formed syncollisional granodiorite after Late Silurian period, and Qilian orogeny and intraplate extension in the post orogenic stage led to the enrichment of gold.