关键词: 锆石U-Pb年龄, 全岩化学成分, Sr-Nd-Pb同位素, 闪长斑岩, 安徽沙溪矿区

Abstract: The Shaxi Cu (Au) deposit is the largest porphyry copper deposit with proved resource reserve in the Middle-Lower Yangtze River Valley metallogenic belt till present. This paper discusses the genesis of the diorite porphyry by integrating the analysis of chronology, whole-rock chemical composition and isotopic geochemistry. The LA-ICP-MS zircon U-Pb dating results reveal that the diorite porphyry was the product of early cretaceous magmatic activities, with a weighted mean 206Pb/238Pb age of 128.3±1.5 Ma. The diorite porphyry is rich in alkali (ALK=6.12~7.53 wt%), K and magnesium (Mg#=38.99~51.53), and present metaluminous characteristic (A/CNK=0.90~0.99), which can be grouped into calc-alkalic-high-K calc-alkalic rock series. Light rare earth elements (LREEs) are enriched, heavy rare earth elements (HREEs) are depleted (LaN/YbN=12.63~17.63), and almost no Eu anormaly (δEu=0.84~1.14) is shown. In addition, an enrichment of large ion lithophile elements (LILEs) (e.g., Ba, Sr) as well as Pb, and a depletion of high field strength elements (HFSEs) (e.g., Nb, Ta, Ti) are observed. The diorite porphyry have (87Sr/86Sr)t values of 0.7051~0.7056, and εNd(t) values range of -6.09~-3.42; (206Pb/204Pb)t values are 17.51~18.16, (207Pb/204Pb)t values are 15.49~15.63, and (208Pb/204Pb)t values are 37.57~38.45, which indicate the rock-forming materials were derived from melted crust with involvement of mantle materials. Around 130 Ma, the stretching of the translational Tanlu fault caused large-scale magmatism. The magma from the deep intruded along the Tanlu fault belt and experienced contamination of considerable amount of crustal materials, finally the magma emplaced at relative shallow depth and resulted in the formation of the Shaxi diorite porphyry and quartz diorite.

Key words: Zircon U-Pb dating, whole-rock chemical composition, Sr-Nd-Pb isotopes, the diorite porphyry, the Shaxi porphyry Cu (Au) ore-field, Anhui Province