Chronology and geochemical composition of cassiterite and zircon from the Maodeng Sn-Cu deposit, Northeastern China: Implications for magmatic-hydrothermal evolution and ore-forming process

Abstract

Primary tin deposits usually contain little or no copper due to the distinct geochemistry of Sn and Cu. However, examples of copper-rich tin deposits in many tin provinces around the world are also well known. The genesis of copper-rich tin deposits remains controversial. The Maodeng Sn-Cu polymetallic, a typical Cu-rich Sn deposit in the southern Great Xing’an Range (SGXR) Northeast China, offers an excellent opportunity to reveal the genesis of coupled copper-tin deposits. The ore mineralization is associated with the granite porphyry, complement phase of the Alubaogeshan complex, which emplaced into the volcanic rocks of the Lower Permian Dashizhai Formation. Herein, we report new zircon and cassiterite U-Pb ages and their trace elements compositions, with the aim of constraining the metallogenic chronology framework, and clarifying the indicative effects of the ore-forming fluids on mineralization in different ore-forming stages, and thus establishing the genetic model for the Sn-Cu deposit. LA-ICP-MS U-Pb dating of zircon from the granite porphyry yields a weighted mean U-Pb age of 134.6 ± 0.4 Ma, which consistent to the zircon U-Pb age (ca. 138 Ma) of porphyry monzogranite (main phase of the Alubaogeshan complex) and cassiterite U-Pb ages (137–140 Ma), suggesting an Early Cretaceous Sn-Cu mineralization under the Paleo-Pacific plate slab roll-back setting. Granite porphyry displays more evolved characterisrics, with higher SiO2 contents (71.5 ∼ 77.4 wt%), Rb/Sr ratios (3.39 ∼ 10.33) and lower Nb/Ta ratios (10.86 ∼ 13.06) compared to those of porphyry monzogranite (SiO2 contents of 70.4 ∼ 72.1 wt%; Rb/Sr and Nb/Ta ratios of 1.03 ∼ 1.72 and 13.34 ∼ 15.83, respectively). This is further supported by the trace elements contents of zircons from granite porphyry and porphyry monzogranite, because the former has a stronger Eu anomaly, higher Hf concentrations, lower Zr/Hf and Th/U ratios than the latter. Our new data, integrated with previously published geochemical data, suggest that the granites associated with Cu-rich Sn deposits are characterized by higher oxygen fugacity, lower differential degree and aluminum saturation index (ASI) than those of Sn-W deposits. This could also use to address the Sn-Cu deposits usually have relatively small Sn mineralization potential. The trace elements of the cassiterites are characterized by high Fe (up to 3358 ppm), Ti (up to 1894 ppm) and abnormal high In (∼2500 ppm) concentrations, but low Nb, Ta contents. From early to late stage, the W and U contents and Nb/Ta and Zr/Hf ratios are increased, reveal that the ore-forming process experienced the cooling, increasing volatile contents and fluid-rock reaction. Finally, a new metallogenic model was established, which highlight oxidized Cu-rich fluids exsolved from the upwelling mantle magma would add to the reduced, Sn-rich magma chambers. This contribution indicate that, the Cu and Sn metals come from the mantle magma and crustal granitic magma, respectively, and that Cu-rich Sn deposits are more likely a product of spatial coupling.