Study On Genetic Types And Metallogenic Environment Of Iron Deposits In The Taxkorgan Area

The Taxkorgan area is located on the Taxkorgan block,and separated from the West Kunlun block by Mazha-Kangxi suture zone to the southeast,and separated from the Mingtiegai block by Qiaoer Tianshan-Qiaochalukou suture zone to the northwest.The Taxkorgan area is not only an important tectonic unit which is closely related to the expansion of the Tethys Ocean,but also has a good metallogenic environment due to the complex geological activities experienced.So the study on the geodynamics history and metallogenic environment of Taxkorgan area is of great significance.In recent years,a series of iron deposits have been discovered in the Taxkorgan area,such as the Zankan-Mokaer magnetite deposit and the Qieliekeqi siderite deposit.The sedimentation and metamorphism of the deposit are closely related to the evolution of the Tethys Ocean.During the expansion of the Tethys Ocean,the volcanic activity of the oceanic crust was extensively developed,the ore-forming fluid was deposited in different oceanic environments.During the reduction of the Tethys,the subduction of the oceanic crust formed voluminous magmas which intrude the block at tensile conditions,then reconstructed the original ore body.Therefore,detailed geochemical characteristics of the ore and surrounding rocks of typical iron deposits are necessary,for revealing the ore-forming environment,mineralization age and so on,which can do lots of help on understanding the history of ocean-continental transformation and summarizing the regional geodynamic history and ore-forming laws of the region.Two typical iron deposits were selected in the study area,they are Zankan-Mokaer V m agnetite deposit and the Qieliekeqi siderite deposit.Sudies on ore,minerals and surrounding rocks have been carried out to summarize the regional tectonic setting.The ore of the Zangkan-Mokaer magnetite deposit is mainly composed of Fe2O3 T and Si O2,and has rare Al2O3(average 1.93%)and Ti O2(average 0.35%)which represented the terrigenous clastic components.The trace element w(Sr)/w(Ba)and w(Ni)/w(Co)value indicate that the ore-forming source is closely related to the high-temperature hydrothermal fluid in the deep sea,and has the characteristics of marine volcanic sedimentation.The rare earth element PAAS distribution diagram is relatively flat,which has obvious differences from the typical banded iorn formation,and it has the imprint of the formation environment of high temperature hydrothermal fluid in deep sea.The laser ablation probe is used to test the magnet mineral,the analysis results indicate that it has the genetic tendency of the skarn type deposit which is the imprint of the Indosinian metamorphism.The characteristics of oxygen isotope also indicate volcanic origin.The surrounding rock of the Zankan-Mokaer deposit is the debatable Bulunkuole group.The zircon U-Pb isotope and Hf isotope study are carried out on the samples from surrounding rock of the deposit.The test results show that the magmatic genesis zircon has the early Paleozoic age forming age of 492 Ma~556 Ma,Hf isotope characteristics indicate a small amount of mantle-derived magma participated in the diagenesis process.Therefore,the Zankan-Mokaer deposit should be formed on the Paleo-Tethys ocean crust of the Early Paleozoic.The content of Ti O2 and Al2O3 in the ore of the Qieliekeqi siderite deposit is very low,indicating the lack of terrigenous clastic materials in the ore-forming process.The ratio of major elements like Al/(Al+Fe+Mn)indicates the mineralization source is mainly derived from hydrothermal fluid which is generated by submarine volcanic activity.Trace element enrichment factor(EFi)and Fe-Mn-(Cu+Co+Ni)×10 and other meaningful elemental ratio also indicate the submarine volcanic genesis.The rare earth element PAAS distribution mode has a slight "left tilt" with strong positive Eu anomaly and Ce anomaly,and indicate the submarine hydrothermal fluid resouces.The fluid inclusion characteristics show a metallogenic environment of 220°C to 250°C and depth of 2.76 km to 1.66 km under the sea.The carbon and oxygen isotopes also indicate that the ore's genesis are closely related to the submarine volcano and hydrothermal activity.The surrounding rock which is in contact with the ore body was once considered to be the Silurian,but recent studies have shown that the pluton which intrude the strata has the age of Early Paleozoic.Combined with the history of regional evolution,it is believed that the age of the deposit formed at early Paleozoic,which is close to the metallogenic age of the Zankan-Mokar magnetite deposit,and is also a product of the evolution of the ancient Tethys.The U-Pb isotope and Hf isotope analysis of the magmatic rock samples in the later intrusive pluton,show the Triassic age of 204.63Ma~214.54 Ma,the Hf isotope characteristics do not show the mantle-derived components.So the age of the Late Triassic is considered to represent the time when Paleo-Tethys Ocean closed in the end.And the intrusive pluton tested is the product of magmatism in the stage of orogenic collapse.In summary,the Qieliekeqi siderite deposit is formed on the bottom of the Paleo-Tethys ocean in the early Paleozoic,and metamorphosed by magmatic hydrothermal fluids during the Triassic.The Bulunkuole group is widely distributed in the Taxkorgan area,and its age has not been conclusive.There are two views on it,one is the age from Neoarchean to Paleoproterozoic(2700Ma~2016Ma),the o ther is the age from late Neoproterozoic to early Paleozoic(1845Ma to 480Ma).In this study,the direct surrounding rock samples of the Zankan-Mokar deposit,ie the samples of the Blenkur rock group,were picked out zircon and tested for U-Pb age.The Th/U ratio shows a magma genesis and has a diagenetic age of 492 Ma to 556 Ma.Few zircons of ancient age were found,and they exist in the form of captured zircon.The Hf isotope characteristics of zircon show the crust-source magma is dominant,and mantle-derived zircon can also be found.Therefore,the Bulunkuole group around the Zankan-Mokaer magnetite deposit,is believed to be formed in the early Paleozoic era.The ore-forming,ore-forming environment and the later hydrothermal reformation of the typical iron deposits selected in this study are closely related to the evolution history of the Paleo-Tethys.Comprehensive consideration of mineralization information of deposits and surrounding stratum and pluton,we divide the regional evolution process into four stages:(1)Late Neoproterozoic ~556 Ma,the Paleo-Tethys started to split while the Proto-Tethys subducted southward.(2)556Ma~492Ma,the volcanic activities under the Paleo-Tethys were strongly developed,mineralization fluid sprayed out of the ocean floor and finally formed the mineral sediment.The Proto-Tethys subducted southward.(3)492Ma~408Ma: The Paleo-Tethys subducted northward,diagenesis and mineralization ongoing.The Proto-Tethys closed at this stage.(4)408Ma~204Ma,the Paleo-Tethys was closed.Combined with the four stages of regional evolution history,the regional metallogenic model is summarized into four corresponding stages:(1)Late Proterozoic to Early Paleozoic,the Paleo-Tethys started to split,diagenetic material start to accumulate.(2)Early Paleozoic,the ore body begins to form.(3)Hercynian-Indosinian metamorphic stage of the ore body.(4)Supergenetic oxidation stage.