Geodynamic Setting And Metallogenic Mechanism Of Porphyry Molybdenum Deposits In Eastern Tianshan,NW China

The Eastern Tianshan belt,which is the southern part of the Central Asian Orogenic Belt between the Siberian Craton and Tarim Craton,has a great ore-forming potential and prospecting potential for porphyry Mo mineralization.The Donggebi and Baishan giant deposits located in the central and eastearn part of the Kanggur-Huangshan ductile shear belt,respectively,are the most significant molybdenum prospecting achievements.Based on the field geological surveys,laboratory microscope observation,as well as previous data,we have conducted the detailed studies on the Donggebi and Baishan Mo deposits,and discussed the associated magmatism,geodynamic setting and their genetic mechanism.Compared with research on the Tuwu-Yandong porphyry Cu deposit,we have further discussed the genetic relationship between the magmatic rocks and the porphyry Cu or Mo mineralization.This study is very important to provide guidelines for regional porphyry Mo?Cu?exploration in Eastern Tianshan.The following achievements have been withdrawn in this study.1.The intermediate-felsic intrusions in the Eastern Tianshan belt can be divided into four stages: Silurian-Devonian?440³70 Ma?,Carboniferous?350³00 Ma?,Permian?299²52 Ma?and Triassic?246²27 Ma?.Molybdenite Re-Os and secondary ion mass spectrometry?SIMS?zircon U-Pb dating constrain that the Mo mineralization and associated magmatism in the Donggebi and Baishan areas occurred at ca.236²27 Ma,which were formed in an intracontinental setting in Eastern Tianshan.2.The Donggebi granites are characterized by high SiO₂ and K2 O,low MgO concentrations,and high differentiation index,belonging to high-Kcalc-alkaline to shoshonitic granites.They show strong negative Eu anomalies,enrichment in large ion lithophile elements,and depletion in high field strength elements,similar to high fracioned I-type granites.The Sr-Nd-Hf-O isotopic data show that they have relatively high ⁸⁷Sr/⁸⁶Sr ratio and ?_Nd?t?value,positive ?_Hf?t?value,young crustal model age?TDM C?,and high zircon ?¹⁸O value.These features indicate that the Donggebi granites were derived from the parial melting of the lower crustal materials that involved some mantle components.3.The Baishan granites show features of high SiO₂ and K2 O,low MgO and CaO contents,and high differentiation index,belonging to calc-alkaline to high-K calc-alkaline granites.They also show pronounced enrichment in LREE,moderately negative Eu anomalies,high Sr/Y ratio and low Y concentrations,geochemical similar to adakites.The Sr-Nd-Hf-O isotopic data display that they have relatively consistent ⁸⁷Sr/⁸⁶Sr ratio,positive ?_Nd?t?and ?_Hf?t?values,young TDM C age,and relatively low zircon ?¹⁸O value.These features indicate that the Baishan granites were probably derived from the lower crust,with some input of mantle-derived materials.4.The ore bodies in the Mo deposits mainly occur in the Lower Carboniferous Gandun Formation,controlled by the buried intrusions and fracture system.The Mo mineralization is closely related to the silicification,potassic alteration and sericitization assemblages.The ore-forming fluids of the Donggebi deposit are characterized by high temperature,moderate salinity and CO₂-bearing,belonging to a H₂O-NaCl±CO₂ system,whereas the ore-forming fluids of the Baishan deposit are characterized by high temperature,moderate salinity and CH₄-bearing,belonging to a H₂O-NaCl±CH₄ system.Hydrogen and oxygen isotopic compositions of quartz indicate that the ore-forming fluids were from the magmatic and meteoric water,and the inflow of abundant meteoric water into the hydrothermal system was critical for the formation of ore-bearing quartz veins.Carbon and oxygen,sulfur and lead isotopes suggest that the ore-forming materials came predominantly from a deep-seated magma source from the lower continental crust that mixed some upper crustal materials.5.Compared with research on the Tuwu-Yandong porphyry Cu deposit,the porphyry Cu mineralization was mainly hosted in the intrusive rocks,whereas the porphyry Mo mineralization mainly occurred in the outer contact zones of the intrusions.The whole-rock major and trace elements of the magmatic rocks indicate that the ore-forming magma affinity and geodynamic setting are different.The magma source characteristics and magma evolution probably are the important factors to control the Cu or Mo mineralization occurred in different spatial locations.