Magmatism And Mineralization Of Mazhuangshan-Nanjinshan Gold Ore Belt In Eastern Tianshan Area

The Eastern Tianshan orogenic belt,located in the southern margin of the Central Asian orogenic belt,lies between the Tarim plate and the Siberian plate.It has undergone multi-stage magmatism and mineralization during the Paleozoic and is an important polymetallic metallogenic belt in China.For a long time,it has been a hot topic for scholars.However,the tectonic setting and metallogenic process of the Late Carboniferous remain controversial.In this study,The Mazhuangshan-Nanjinshan gold ore belt in the eastern Tianshan metallogenic belt was selected as study object.We analyze the age,petrogenesis and tectonic background of magmatic rocks and constrain the genesis and metallogenic process by petrology,geochemistry,fluid inclusion and stable isotope methods.Finally,we preliminarily summarize the metallogenic regularity,change and conservation of these deposits,which can provide a reference for theoretical research and practical exploration of this type of deposit in the region.The magmatic rocks exposed in Mazhuangshan-Nanjinshan gold deposit belt are mainly volcanic-subvolcanic rocks and intrusive rocks.Volcanic-subvolcanic rocks mainly include tuff,tuffaceous breccia and quartz porphyry,and intrusive rocks mainly consist of granodiorite and diorite.The quartz porphyry yielded a U-Pb concordant age of 324.2±0.8Ma–315.4±0.6Ma Ma,the granodiorite yielded a U-Pb concordant age of 320.2±0.9Ma and the diorite yielded a U-Pb concordant age of 309.4±2.7 Ma.The magmatic rocks have medium?REE contents and medium REE fractionation,with apparently enrichment of light rare earth and slightly negative Eu anomaly.They are also riched in Rb,K,Th,U,Pb elements and depleted in Sr,Ba,P,Nb and Ti,which is similar with the characteristic of I type granites.Sr-Nd-Hf isotopic compositions indicate that magmatic rocks are characterized by high(⁸⁷Sr/⁸⁶Sr)_i values?0.69970–0.71447?,low?_Nd?t?values?-6.7–1.89?,with two stage model ages(T_DM2)ranging from 0.9 to 1.2Ga.The?_Hf?t?values of zircons are-3.6–6.9 and the corresponding T_DM2 model ages are 1.51–1.28Ga.These features illustrate that the magmas are derived from the partial melting of the ancient crustal materials that involved some mantle component.Based on the geological,geochemical and chronological characteristics,it is considered that the ore belt is in the continental margin arc and island arc environment formed by subduction during 324–312Ma.The typical deposits in the study area belong to epithermal deposits.The Mazhuangshan and Xiuwenghala gold deposits are mainly hosted in quartz porphyry,whilst the Nanjinshan gold deposit is mainly hosted in tuff breccia and tuff of Baishan Formation.Ore bodies are mainly vein-like and lenticular.Metallic minerals are mainly pyrite,chalcopyrite,arsenopyrite,galena and sphalerite.Hydrothermal alteration types at Nanjinshan include silicic,sericite,chlorite,adularia,pyrophyllite and carbonate.Hydrothermal alteration styles at Xiuwenghala consist of silicic,sericite and chlorite.The Mazhuangshan gold deposit mainly include silicic,sericite,chlorite,kaolinite and pyrophyllite.The ore-forming fluids of the deposits belong to medium-low temperature and medium-low salinity fluids,which are consistent with the fluid characteristics of epithermal deposits.Ore-forming materials of Mazhuangshan and Xiuwengha gold deposits are mainly derived from subvolcanic rocks?quartz porphyry?,while the Nanjinshan gold deposit are mainly from volcanic clastic rocks?tuff and tuffaceous breccia?of Baishan Formation.Therefore,it is proposed that Xiuweng Haha and Nanjinshan gold deposits are low-sulfur epithermal deposits and Mazhuangshan gold deposits are high-sulfur epithermal deposits.Auriferous pyrite samples from Mazhuangshan deposit yielded Re-Os isochron age of 312.2±3.2Ma.In conclusion,the Mazhuangshan can be classified as a high-sulfidation epithermal deposit,whilst Xiuwenghala gold deposit is best calssified as a low-sulfidation epithermal deposit.They are formed by the subduction of the Paleo-Tianshan Ocean during Late Carboniferous.The Nanjinshan gold deposit is a low-sulfidationr epithermal deposit formed in the post-collision extension environment in the Middle Triassic.In addition,from west to east,the distribution characteristics of ore belts show that the type of ore deposits changes from high sulfidation to low sulfidation and the denudation depth weakens.