Geological Characteristics And Genesis Of Hongyuntan Iron Deposits In The Estern Tianshan, Xinjiang

Iron deposit hosted in volcanic rock becomes an important hotspot of academic research in China recently. Xinjiang Aqishan-Yamansu iron ore belt, located in Jueluotage Block of East Tianshan orogenic belt, is between Junggar plate and Tarim plate, where has developed numerous large-medium-small sized iron deposits. Overall study of the iron deposits, hosted in the volcanic rocks is stalled, the genesis, the ore-forming processes, the regional mineralization law and mechanism of the deposits are not clear. Therefore, this paper selects Hongyuntan iron deposit, located at the western part of this iron belt, to carry out deeply field geological investigation, according to Electron probe, Lithogeochemistry, zircon U-Pb dating, Hf isotope, Fluid inclusion temperature measurement and Stable isotope, this paper analyse geological characteristics, tectonic background of metallization, ore-forming material source and metallogenic epoch etc. Thus, we achieve the thorough understanding of the genesis of Hongyuntan iron deposit:Hongyuntan iron deposit host in Lower Carboniferous Yamansu group volcanic rocks, ore body is laminar, like-layered, lentoid, not located in intrusive rocks contact zone; massive, ore types are mainly disseminated, banded ore, brecciaous and veined ore is partial; Mineralization evolution experienced three stages: Diopside-Garnet skarn stage, Magnetite-Actinolite-Amphibole-Epidote-Chlorite degenerate alteration stage, Quartz Epidote-Albite-K-feldspar-Sulfide stage.The ore minerals are dominated by magnetite, a small amount of pyrite and chalcopyrite; electron probe analysis showed that Magnetite overall has low Ti, Cr, V, content, trace group content is similar with IOCG, and skarn type iron magnetite features, Ti/V ratio is about 1.02, indicated genesis of hydrothermal replacement, and Ni content is very low, not magmatic and sedimentary characteristics; trace elements of Pyrite reflect S/F ratio was greater than the theoretical values, indicating low oxygen and sulfur fugacity of high temperature hydrothermal environment, the Co/Ni ratio>1, reflecting mineralization have relationship to volcanic-magmatic hydrothermal function.Early skarn is mainly calcium skarn: Garnet is Calcium-Iron Garnet-Calcium Aluminum Garnet series, mainly is Andradite; Pyroxene is mainly Diopside-Hedenbergite series, mainly Diopside, and hedenbergite; Amphibole mainly Actinolite, Ferro-Actinolite, a small amount of iron Amphibole; Chlorite is mainly Iron-Magnesium Chlorite, Iron Chlorite; Epidote develop from retrograde alteration stage to late stage, early stage Epidote relatively rich in iron, Fe/(Fe+Al) ratio of relatively high. The sodium-potassium develop in wall rock alteration of periphery and quartz sulfide stage, Albite(Ab58.06-99.23An0.72-40.91Or0.04-1.04), K-feldspar(Or95.56-98.44Ab1.56-4.36An0.00-0.07). The wall rock alteration is widely developed, from the early stages to the late stage reflect alteration zoning feature, and the skarn and iron-forming is closely related.Quartz monzodiorite, Granodiorite, Biotite monzonitic granite and K-feldspar granite, developed in Hongyuntan iron deposit, major elements show metaluminous calcium-alkalinity granite, REE and trace element characteristics suggests that are the volcanic arc granites characteristics. LA-ICP-MS zircon U-Pb dating of Quartz monzodiorite is 351.5±1.2Ma, Biotite monzonitic granite is 326.76±0.78Ma; K-feldspar granite is 297.36±0.51 Ma. Hf isotopic compositions εHf(t)=4.5~14.4, and second-order mode age tDM2 were older than the zircon U-Pb age, indicating that magma may be the partial melting of lower crustal material from the plate subduction caused. Yamansu group volcanic rock(Quartz keratophyre) is medium-basic series, chemical composition belongs to calc-alkaline series, REE and trace elements display continental margin arc volcanic rocks’ geochemical charecteristics; LA-ICP-MS zircon U-Pb dating of Quartz keratophyr is 324.1±3.1Ma. Hf isotopic composition εHf(t)=0.1~6.1, close to chondritic line, source region is, oceanic crust subduction environment, young mantle and lower crust partial melting of materials formed by magma, volcanic rocks and intrusive rocks form from coupling tectonic environment.Fluid inclusion temperature measurement shows that garnet of skarn stage uniform temperature is more than 550°C, salinity is 9.21~15.95wt%; Epidote of degradation erosion stage and quartz uniform temperature peak is 240°C~330°C. The calcite of the late quartz sulfide stage uniform temperature peak is 200°C~220°C. The temperature and salinity of the fluid reflected decreasing at the process.Hydrogen, oxygen and sulfur isotope show that garnet δ18O(6.5~9.7‰), shows ore-forming fluid is mainly magmatic water; Epidote δ18O(3.9~5.4‰), K-feldspar δ18O(3.45~3.65‰), quartz δ18O(-2.3~0.2‰) shows late ore-forming stage added meteoric water. δ18O of magnetite is 1.2~5‰, average of 3‰, showing that iron source is from magma. δ34S of pyrite concentrate on-3.8%~4.3%(average-0.7%), with the characteristics of mantle derived sulfur. In view of other people’s research before, The ore-forming materials are mainly derived from the magma sources, and the ore-forming fluid mainly comes from the magmatic hydrothermal fluid, and there is the air or seawater in the late period.Taking the previous results of regional rock diagenesis and mineralization chronology research into consideration, according to the orebody roof rock of quartz porphyry zircon U-Pb age of 3.241±3.1Ma, late intrusive rocks K-feldspar granite 297.36±0.51 Ma, we limit Hongyuntan iron deposit formation between the 324~300Ma, Late Carboniferous iron mineralization.Combined our research work and previous achievements, Taking consideration to the tectonic setting, and comparison to regional typical volcanic type iron deposits with, We draw the conclusion that Hongyuntan iron deposit is formed in late stage of Late Carboniferous volcanic-magmatism, reflecting the rich iron fluid alter-fill Yamansu group volcanic rocks and have multiple stage characteristics, should belong to the marine volcanic hydrothermal type iron deposit.