Caledonian tungsten deposits in Dayaoshan area of South China

Caledonian magmatic rocks in South China were believed to be barren, but since the 1980 s, the fact that some deposits have been found broke this view. Especially in recent years, many tungsten polymetallic deposits related to Caledonian magmatic rocks have been discovered in Dayaoshan area, which further indicates that Caledonian magmatic rocks can also be mineralized. In the Dayaoshan area, nine Caledonian tungsten deposits (occurrence) have been discovered, including two large deposits, Shedong and Yupo. The WO₃ resource has been reported to be close to 200 000 tons. The types of deposits include skarn-, quartz vein- and porphyry-types. The metallogenic time concentrated between 432 and 466 Ma. The metallogenic elements is W, with Mo, Cu, Bi, Ag, Pb, Zn, and Au in selective deposits. The main ore mineral is scheelite. In addition to being closely related to the space-time of magmatic rocks of pyrrhotite, sericite, epidote and diopside, porphyry-type and quartz vein-type deposits are also associated with a set of steep small faults and joints in the EW and NW directions. The skarn deposit is closely related to the limestone interlayer in the Cambrian and has elemental zones. The geochemistry of the deposit indicates that the homogenization temperature in the main metallogenic stage concentrated between 225~ 370℃, and the salinity ranged from 5% to 25% w（NaCl_eq）. H-O-S isotope tracing shows that the ore-forming fluids mainly come from magmatic rocks. The variation of temperature, salinity, pH, and fluid mixing are the factors intriguing the ore precipitation. The granodiorites related to mineralization belong to the I-type granite with low level of differentiation and reduction. These rocks are outcropped as stocks and apophysis, and often distributed in groups and belts. The concentrated rocks generated between 432~480 Ma. The granodiorites lack pegmatite, contain hornblende without aluminum-rich or volatile minerals found. Geochemical discrimination indicates that these magmatic rocks can be classified to the calcium alkaline to high potassium calcium alkaline series, weak over-aluminum granite with low differentiation index, low Li, B, La/Yb and Rb/Sr, high CaO/(Na₂O+K₂O), Mg/(Mg+Fe) and high consolidation index, high mafic mineral content, and weak Ba, Sr, P, and Ti losses, and the higher Zr/Hf ratio from both whole rock and zircon. The rare earth element distribution curve is obviously right-sloping with weak negative Eu anomaly. All these indicate that the rock mass related to tungsten mineralization is low-differentiated rock mass, which is obviously different from the traditionally considered tungsten-bearing rock mass as high-differentiated granite. Elevated zircon O isotope, weak negative ε_Hf(t) value, and T_DM2 age of 1.5~1.6 Ga indicate that granodiorite originated from the partial melting of Mesoproterozoic crustal material and mixed with a certain amount of mantle material. The extensive Caledonian magmatism and mineralization in the Dayaoshan area could have been related to the collision caused by the final closure of the "South China Rift". The intensity, scope and mineralization of the Caledonian magmatic activity in this area are comparable to those of the Yanshanian period, which has great potential for prospecting. The fact that the recently discovered tungsten deposits forms an ore concentration area in the Dayaoshan area updated the previous understanding that the granite of the Caledonian in South China is generally barren, and is important for further understanding the Caledonian tectono-magmatic evolution and mineralization in South China.