Petrogenetic setting of andsites in Rongna ore block, Tiegelong Cu (Au-Ag) deposit, Duolong ore concentration area, Tibet: Evidence from zircon U-Pb LA-ICP-MS dating and petrogeochemistry of andsites

Abstract: In this paper, the authors studied the andsites in Rongna ore block, Tiegelong Cu (Au-Ag) deposit, Duolong ore concentration area, Tibet, and used the zircon U-Pb data and petrogeochemistry of andsites to identify the petrogenetic age and tectonic setting of the andsites. The weighted average age of zircon U-Pb is 110.1±0.7 Ma (MSWD=2.3) , which represents the petrogenesis age. An analysis of the whole rock sample shows that SiO2 values are 59.5%-61.48%, Al2O3 values are 16.14%-17.11% and Na2O+K2O values are 6.02%-6.63%; Mg# is of intermediate value. LILE such as Ba, Rb, Th, K are rich, HFSE such as Ta, Nb, P, Ti are relatively poor and characterized by Rb positive anomaly and obvious Zr, Hf negative anomalies. Petrological and geochemical studies show that the andesites are high potassium calc-alkaline rocks exhibiting the characteristics of Andean-type active continental margin volcanic rocks. They resulted probably from partial melting of subducted oceanic slab as well as miscibility with crustal material during the upward migration of the magma in the case that subducted oceanic slab didn’t interact with the mantle. They were the magma products of the northward subduction of Bangong Co-Nujiang Ocean and Lhasa and the collision collage process of Qiangtang terrane. In combination with other dating data and drilling core catalog data, the authors hold that the formation of andesites was later than the rock-forming and ore-forming stage of the Tiegelongnan high sulfur epithermal Cu (Au-Ag) deposits, which seems to have been the important condition for the preservation of the ore deposit.