Junming Yao

Published Sn isotope data along with 150 new analyses of cassiterite and four granite analyses constrain two major tin isotope fractionation steps associated with (1) separation of tin from the magma/orthomagmatic transitional environment and (2) hydrothermal activity. A distinct Sn isotope difference across deposit type, geological host rocks, and time of ore deposit formation demonstrates that the difference in the mean δ¹²⁴Sn value represents the operation of a unified process. The lower Sn isotope values present in both residual igneous rocks and pegmatite suggest that heavier Sn isotopes were extracted from the system during orthomagmatic fluid separation, likely by F ligands with Sn. Rayleigh distillation models this first F ligand-induced fractionation. The subsequent development of the hydrothermal system is characterized by heavier Sn isotope composition proximal to the intrusion, which persists in spite of Sn isotope fractionating towards isotopically lighter Sn during hydrothermal evolution.

The Shibaogou pluton, located in the Luanchuan orefield of western Henan Province in China, is a typical porphyritic granite within the Yanshanian “Dabie-type” Mo metallogenic system. It is mainly composed of porphyritic monzogranite and porphyritic syenogranite. Zircon U-Pb dating results indicate emplacement ages of 150.1 ± 1.3 Ma and 151.0 ± 1.1 Ma for the monzogranite and 148.1 ± 1.0 Ma and 148.5 ± 1.3 Ma for the syenogranite. The pluton is characterized by geochemical features of high silicon, metaluminous, and high-K calc-alkaline compositions, enriched in Rb, U, Th, and Pb, and exhibits high Sr/Y (18.53–58.82), high (La/Yb)_N (9.01–35.51), and weak Eu anomalies. These features indicate a source region from a thickened lower crust with garnet and rutile as residual phases at depths of approximately 40–60 km. Sr-Nd-Pb isotopic analyses suggest that the magmatic source is mainly derived from the Taihua and Xiong’er Groups of the Huaxiong Block, mixed with juvenile crustal rocks from the Kuanping and Erlangping Groups of the North Qinling Accretion Belt. Combined with geological and isotopic characteristics, it is concluded that the Shibaogou pluton formed during the compression–extension transition period associated with the collision between the Yangtze Block and the North China Craton, reflecting the complex partial melting processes in the thickened lower crust. The present study reveals that the magmatic–hydrothermal activity at Shibaogou lasted approximately 5 Ma, showing multi-phase characteristics, further demonstrating the close relationship between the pluton and the Mo-W mineralization.

The Dashui porphyry granitoids in the western Qinling Orogen are spatially associated with the Dashui gold deposit, but their ages and tectonic setting have not been well defined yet in previous studies. This paper reports the results and understandings obtained from our original study in geochronology and isotope and element geochemistry. SHRIMP zircon U–Pb dating yielded ages of 213.7 ± 2.7 Ma for granitic stock and 215.1 ± 2.5 and 210.2 ± 1.6 Ma for granitic dikes. The granitoids are characterized by high LREE but low HREE and HFSE. They have (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.706 to 0.709 and εNd(t) values of −5.3 to −7.6. The majority of zircons in the Dashui granitoids have εHf(t) values ranging from −6.2 to 6.7 and T_DM2(Hf) ages from 743 to 1464 Ma. The isotope signatures, together with La/Y and Mg^# values, show that the magmas are mainly sourced from partial melting of a subducted slab, and possibly assimilated immature sediments during upward migration. Thus, the Dashui granitoids most likely formed in a volcanic arc where supra‐subduction zone melts assimilated lower crustal material no later than 215 Ma. Copyright © 2014 John Wiley & Sons, Ltd.