Re-Os Isotopic Dating on Molybdenite Separates from the Xiaodonggou Porphyry Mo Deposit,Hexigten Qi,Inner Mongolia

Abstract

Located at the easternmost part of the Early Paleozoic tectono-magmatic belt of the Ondur Sum-Ongniud Qi along the northern edge of the Northern China craton, the newly discovered Xiaodonggou deposit is a medium-scale porphyry molybdenum deposit occurring in the central-eastern Inner Mongolian Autonomous Region. During the Mesozoic Yanshanian orogeny, intensive tectonic and igneous activities resulted in the large-scale granitoid magmatism in the Xiaodonggou and its neighboring area. The Xiaodonggou granitoid porphyry stock was emplaced in the volcano-sedimentary sequence of the Upper Permian Randifang Group, controlled by NS-trending fault zone. It has an outcrop area of 0.22 km2,and consists of mainly porphyritic biotite granite and fine-grained biotite granite that have no difference in mineral assemblage. Rock-forming minerals are mainly quartz, microcline, microperthite and plagioclase associated with minor biotite and sericite. Accessory minerals are of magnetite, apatite, zircon, sphene, and allanite. The porphyry stock is petrochemically characterized by an average SiO2 of 75.12% and K2O +Na2O of 8.75% with K2O/Na2O ratio being 1.28, belonging to silicon-high, potassium-rich rock of calc-alkaline series. Molybdenum mineralization occurs entirely within the Xiaodonggou porphyritic granitoid stock as veins, veinlets and disseminated blocks. The molybdenum ore consists of pyrite, chalcopyrite, molybdenite, sphalerite, pyrrhotite, scheelite, wolframite, K-feldspar, quartz, sericite, chlorite and epidote. Re-Os isotopic age dating for six molybdenite separates from the No.Ⅰ orebody give an isochron age of 135.5±1.5Ma, and model age ranging from 135.7 to 138.8Ma with an average value of 137.2Ma. As the Re-Os isochron age is in agreement with field geological evidences, and the molybdenite has co-existing relations with chalcopyrite and pyrite, it has been suggested that the Re-Os isochron age represents the ore-forming time of the porphyry Mo mineralization occurring in the Xiaodonggou deposit and its neighboring area. Combined with field geological observations and petrological evidences, it has been suggested that the ore-forming materials may be derived from a mixed source of depleted mantle- and crustal-derived magma or fluids. The Re-Os isochron age of the molybdenite separates at Xiaodonggou indicates that Early Cretaceous intra-plate granitoid magmatism and relevant Mo mineralization in the southernmost part of Da Hinggan Mountain was active after the Permian collision between the Siberian plate and Northern China craton. Therefore, the Xiaodonggou district and its neighboring area have a great potential for Mesozoic Yanshanian granitoid porphyry molybdenum deposits.