Geochemical factors revealing the differences between the Xitian and Dengfuxian composite plutons, middle Qin-Hang Belt: Implications to the W-Sn mineralization

摘要

The Xitian and Dengfuxian ore fields, located in the middle Qin-Hang Belt, are known for hosting large-scale WSn polymetallic deposits. The Xitian ore field hosts a series of small to large scale SnW deposits, including skarn-, quartz vein- and structurally altered rock-type, whereas the Dengfuxian ore field mainly contains a medium-scale quartz vein-type W deposit. These granitic rocks are composed of ore-barren Triassic and ore-related Jurassic granites. In this study, LA-ICP-MS zircon UPb dating of three representative samples from Dengfuxian Triassic granites (DTG), Xitian Jurassic granites (XJG) and Dengfuxian Jurassic granites (DJG) yields weighted mean 206 Pb/U-238 ages of 231.2 +/- 2.5 Ma, 152.5 +/- 1.2 Ma and 155.9 +/- 1.6 Ma, respectively. Chemical compositions of the Xitian Triassic granites (XTG) and DTG indicate that they belong to relatively lower fractionated granites, whereas the XJG and DJG are highly fractionated granites. The XTG and DTG have similar LuHf isotopic compositions, which have peak eHf(t) values of ca. -5.9 and T-DMC ages of ca. 1.57 Ga, and peak epsilon(Hf)(t) values of ca. -5.7 and T-DMC ages of ca. 1.57 Ga, respectively. However, the XJG have higher peak epsilon(Hf)(t) values of ca. -5.4 and younger T-DMC ages of ca. 1.52 Ga than those of the DJG with peak epsilon(Hf)(t) values of ca. -7.6 and T-DMC ages of ca. 1.70 Ga, respectively. Furthermore, the XTG have epsilon(Nd)(t) values of -10.5 to -9.40 and Nd model ages (T-DM2) of 1.86 to 1.76 Ga, which are similar to those of the DTG with epsilon(Nd)(t) values of -11.7 to -9.67 and Nd model ages (T-DM2) of 1.95 to 1.79 Ga (Wu et al., 2016, Cai et al., 2015). The XJG have epsilon(Nd)(t) values of -11.0 to -9.1 and younger T-DM2 ages of 1.691.84 Ga, which are fairly distinct to the reported Nd isotopes of the DJG (Cai, 2013). It is proposed that the XTG and DTG were originated from the similar magma chamber and formed from partial melting of the Proterozoic basement rocks of the South China Block (SCB) at low temperatures (ca. 690 degrees C) and oxygen fugacity (log(fO(2)) values of ca. -17), involved with a certain amount of mantle derived magma; Magmas of the XJG and DJG might be mainly originated from the partial melting of Proterozoic basement rocks of the SCB. In contrast, the formation of XJG and DJG are involved with a considerable and little mantle-derived magma, respectively. In addition, the magma temperatures and log(fO(2)) values of XJG are of ca. 856 degrees C and ca. -15, respectively, which are higher than those of the DJG (ca. 716 degrees C and ca. -17). That the Xitian and Dengfuxian Triassic granites did not induce WSn mineralization is likely attributed to their low fractionated signatures. The key factors leading to the SnW mineralization in the Xitian ore field and W mineralization in the Dengfuxian ore field might be magma source and temperature, since additional Sn could be provided by the mantle and/or released by melting of those Sn-bearing minerals at high temperatures (over than 780 degrees C).