The Metallogenicand Prospecting Modelof Paleozoic Golddeposits In The Dunbasitaoand Adjacent Area, East Junggar, Xinjiang Province

The eastern Junggar area,located in the southernmargin of the Altay orogenic belt and the northeastern margin of the Junggar basin,is animportantgold polymetallic metallogenic area in the northern Xinjiang.As far as the geotectonic position,the eastern Junggar area is situated at the joint region of the Siberia plate and the Kazakhstan-Junggar plate.With the relative subduction and closure of the Paleo Asian oceans,this region is abounded in the Paleozoic magmatic rocks and gold deposits.In this dissertation,taking the Dunbasitao and adjacent areas deposites and the relevant magmatic rocks as the research object,combined with the previous regional research acievements,through comprehensive studies of field geology,geological features of the magmatic rocks and the ore deposits,petrography,zircon U-Pb geochronology,whole-rock geochemistry,Sr-Nd isotopic geochemistry,pyrite Rb-Sr dating,fluid inclusions analysis and C-H-O-He-Ar-S-Pb isotopic tracing,I make clear the petrogenesis of intermediate-acid intrusions,the mineralization characteristics of gold deposites,and the metallogenic tectonic settings.Furthermore,through the comparative study of regional metallogenic characteristics,a new model for the Palaeozoic tectonic-magmatic evolution and regional metallogenic in the East Junggar area for the gold deposites were built.The main cognitions and conclusions are obtained as follows:1.The quartz diorite porphyrite and diorite porphyry,with the LA-ICP-MS zircon U-Pb dating results for 378±2Ma and 381±2Ma,respectively,were the products of the middle late Devonian magmatic activity.The intrusive rocks show relatively low SiO₂content?52.44%⁶8.64%?,rich Na₂O with an average?5.14%?,Na₂O/K₂O?1.19².72?,and A/CNK is from 0.81 to 1.08,which belong to metaluminous calc-alkaline and high-K calc-alkaline series.Both of them show the characteristics of the I-type granites.The rocks are enriched in large ion lithophile elements LILE?Rb,Ba,Th?and light rare earth elements LREE,depleted in high field strength elements HFSE?Nb?Ta?P?Ti?.The rare earth element REE exhibit right-dipping patterns having significant LREE and HREE differentiation without Eu anomalies.All the samples have low(⁸⁷Sr/⁸⁶Sr)_i?0.703783⁰.703938?,positive?_Nd?t??+6.2⁺7.0?,as well as young Nd isotopic model ages(T_DM ranging from 546⁶08 Ma),indicating a deplete mantle source.These geochemical characteristics demonstrate that the rocks are arc magmatic rocks,with the characteristics of adakite.As the products of the subduction of the paleo-Asian ocean between the Siberia block and the Kazakhstan-Junggar,these rocks were mainly derived from decompression melting of the mantle wedge metasomatized by the the subducting fluid or melt in the local extensional environment.2.LA-ICP-MS zircon U-Pb dating shows the the monzogranite's ²⁰⁶Pb/²³⁸Pb ages are 328±3Ma and 329±3Ma,which are the products of the early Carboniferous magmatic activity.The rocks show relatively rich inSiO₂?68.68%⁷0.30%?,Al₂O₃?14.52¹5.32%?and?K₂O+Na₂O??8.76⁹.27%?,with Na₂O>K₂O and A/CNK<1.1,belonging to high-K calc alkaline series.Bue these rocks are characterized by low Fe₂O₃^T?2.13².39%?,MgO?0.6⁰.7%?,CaO?0.76¹.69%?,TiO₂?0.38⁰.43%?and P₂O₅?0.11⁰.13%?,the biotite and amphibole being main dark mineral,indicating characteristics of I-type granite.These intrusive rocks are enriched in LILE?Rb,Ba,Th,K?and depleted in HFSE?Nb,Ta,P,Ti?,but the depletion of Zr and Hf is not obvious.The REE exhibit right-dipping patterns having significant LREE and HREE differentiation without Eu anomalies.All the samples show a homogeneous low(⁸⁷Sr/⁸⁶Sr)_i?0.703556⁰.703888?,but high and positive?_Nd?t?values?+4.1⁺5.6?,with young Nd mode age(T_DM ranging from 566⁶75 Ma),indicating the origin of the juvenile crust.Major and trace elements and Sr-Nd isotopic data suggest that they may be derived from the partial melting of a juvenile crust source,which is probably a mixed lithology formed by pre-existing lower crust intruded or underplated by mantle-derived basaltic magma.Both of them were mainly formed in the tectonic setting transform collision to post collision,which manifests that the ocean basin of Erqisi has been closed before the Early Carboniferous.3.LA-ICP-MS zircon U-Pb dating results show that two late stage granitesformed at 265±1Ma and 268±2Ma,which are the products of the early Permian magmatic activity.Petrography and chemical compositions show that they belong to high-K calc-alkaline series and are characterized by high silicon?SiO₂=70.81⁷5.60%?,enrichment in kalium??K₂O+Na₂O?=8.73¹0.22%?and iron?Fe₂O₃^T=1.30².25%?but depletion in calcium?CaO=0.33².24%?,and magnesium?Mg O=0.11⁰.89%?.The A/CNK values vary from 0.92 to 1.07,show a metaluminous to weakly peraluminous affinity.The granitic rocks are enriched in high field strength elements?HFSE??e.g.Zr and Hf?and large ion lithophile element?LILE??e.g.Rb,U and Th?but depleted in Ba,Sr,P,and Ti.REEs are characterized by negative Eu anomalies?Eu/Eu^*=0.62⁰.85?and exhibit right-inclined patterns with LREE enrichment but HREE depletion.Mineralogy and geochemistry show an affinity to A-type granite.The rock show a homogeneous low(⁸⁷Sr/⁸⁶Sr)_i?0.703199⁰.703610?,positive?_Nd?t?values?+3.1⁺4.0?,with young Nd mode age(T_DM ranging from671⁸15Ma).Major and trace elements and Sr-Nd isotopic data suggest that they may be derived from the partial melting of a juvenile crust source,which is probably a mixed lithology formed by pre-existing lower crust intruded or underplated by mantle-derived basaltic magma.These granites generated in a post-collisional extensional tectonic setting.4.Gold deposits in the Dunbasitaoand the East Junggar area have similar metallogenic geological characteristics.Three NW trending regional fault structure is closely related to ore-forming.Ore-controlling structure is the NWW trending ductile-brittle shear zone.Regional ductile shear zone controls gold deposits distribution.Brittle structure controls the gold deposits.The main orebodies are hosted in the first member of the Lower Carboniferous Jiangbasitao group,which comprises low-grade metamorphic clastic rocks.The deposit is well controlled by the NWW trending structures.The dominant alterations related to gold mineralization are arsenopyrite-pyritization,silicification and sericitization.The hydrothermal ore-forming processes can be divided into four stages accoding to the mineral assemblages and the crosscutting relationships of the mineral as follow:quartz vein;coarse-grained pyrite-quartz vein;fine-grained pyrite-arsenopyrite-quartz vein;and quartz-carbonate vein.The third stage is the chief ore-forming stage,which was formed in the extensional environment.5.Rb-Sr isotopic dating of 9 pyrites separated from the Dunbasitao gold deposit yielded an isochron age of 268.3±2.8Ma?MSWD=0.7?,this result is in good agreement with the early Permian?265 Ma²68Ma?granite in the periphery of ore district,indicating that Gold mineralization was related to granite crystallization.Meanwhile,combining with the high-precision studies about metallogenic chronology of diagenetic and mineralization in eastern Junggar in recent years,we point out that Early Permian from 270Ma to 260Ma may be an important time range of gold mineralization in eastern Junggar,Xinjiang.According to the precise Rb-Sr ages we have reported and previous studies,we interpret the Dunbasitao gold deposit formed in an extensional setting related to activity of tectonic,magma and fluid during the Early Permian in eastern Junggar.6.The fluid inclusions from Dunbasitao gold deposit have final homogenization temperatures of 140¹80?,180²60?and 320³60?,respectively.Most of the inclusions are distributed in the temperature range from 180?to 260?,showing the characteristics of middle to low temperature mineralization,which is similar to the magmatic hydrothermal deposits derived from the magmatic activities of the study area.The?¹⁸O and?D values range from+1.1 to+3.9‰and from–115.5 to–120.6‰,respectively.The H-O isotopes signature indicates that the ore-forming fluid was dominated by magmatic water with the participation of some meteoric.The?¹³C values of the ore-forming fluid range from-24.7‰to-25.4‰,indicating that carbon of ore-forming fluid is a mixture of inorganic carbon from deep source and organic carbon from strata at varing proportions.It is obviously that gold mineralization has been related to deep-source fluid and the organic carbon derived from sedimentsry strata in this area plays an important role on transportation and unloading of deep-source gold.Fluid inclusions in pyrite He-Ar isotopic ratio of ³He/⁴He?0.52¹.58Ra?is between the crust-derived fluid?0.01⁰.05Ra?and mantle-derived fluid?6⁹Ra?.Metallogenic fluid contain crust-derived He and mantle-derived He,with mantle-derived He proportion accounted for 6.4 to 19.6%.The ³He/⁴He ratio of ore-forming fluid was lower than that of saturated meteoric water,which showed the characteristics of modified saturated meteoric water.The He and Ar isotopes also display a magma origion,which indicate that the ore fluids origion of the Dunbasitao deposit characterized by mantle,crust and modified air-saturated water.These data suggests that the ore-forming fluids are mainly from a magmatic water,and the mineralization of hydrothermal superimposition disply a more possibility.The?³⁴S values of the pyrite and arsenopyrite form the Dunbasitao ores range from 3.0‰to5.8‰and from 4.2 to 5.6‰,respectively,which displays that the derivation of ore-forming materials sulfur mainly comes from the magmatic sulfur with the participation of deep-source sulfur.The ²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values of the pyrite form the Dunbasitao ores range from 17.875 to 18.102,from 15.483 to15.625,and from 37.699 to 38.148.Pb isotope is coincident and reveals that lead mainly comes from the mantle,which is a mixture of mantle-source and upper crust-source lead.Based on the H,O,C,He,Ar,S and Pb isotope geochemistry signatures of the mineral systems,combined with the comprehensive studies on geological characteristics and metallogenic tectonic setting of ore deposit,we argue that Dunbasitao gold deposit is a structure-controlled magmatic hydrothermal type and altered rock type deposit,and formed in an extensional setting related to activity of tectonic,magma and fluid during the Early Permian in eastern Junggar.7.Based on our results from geochronology and geochemistry studies of theintermediate-acid intrusive rocks,combinedwith the systematic summaries of the previous geological research,published data and newly comprehensive geological evidence,we proposed that the East Junggar area could have experienced a complex tectonic-magmatic evolution history as follows:Armantai oceanic exist idirectional subduction?north and south?in the early stage of Ordovician.The tectonic setting transformation from ocean to continentinthe early Paleozoic through arc-arc collision in the early stage of Early Silurian.The Erqisi ocean start a new subduction process before the early Devonian.Before the late stage of the EarlyCarbonifeous?330Ma?,the ocean basin could finally closed and the tectonic setting begin to transformation from arc-arc collision or arc-continent collision to post-collision.The East Junggar terrane witnessed a tectonic transition from compression to post-collisional extension between the Early Carboniferous and the Late Carboniferous.The East Junggar terrane begin into a post-collisional extension stage from the Late Carboniferous to the Early Permian?265³30Ma?.In the geodynamic setting of arc-arc or arc-continent collision,the Early Permian gold deposits in the Dunbasitao and adjacent areas first form the ductile shear tectonic system in the extruded tectonic setting in the Early-Middle Carboniferous,and then form all kinds of brittle cracks along the ductile shear belt of the early stage in the post-collisional extension tectonic setting in the Late Carboniferous-Early Permian.The tectonic deformation control on gold mineralization is expressed by geological discontinuity between the ductile-brittle andbrittle deformation.The gold mineralization kept up with the brittle deformationand had a significant superposition relationship with the ductile deformation.Brittle structure controlled ore body unified in theright-lateral strike-slip shear and extensional system,with different deformation levels,deformation characteristics and mineralization characteristics.8.Considering to the evolution history of the Paleo-Asian ocean between the Siberia plate and Kazakhstan-Junggar plate,and its geodynamic implications for the perogenesis,mineralization,and metallogenic tectonic setting.Based on the relevant research on Dunbasitao and adjacent area of gold deposit,this dissertation summed up the metallogenic controlling factors and mineralization regularityof the Dunbasitao golddeposit,and carried out a comparative study of gold metallogenic tectonic background,metallogenic epoch,mineralization types,mineralization and ore-controlling tectonic mode ofregional contemporaneous gold deposits.This dissertationargue that these gold deposits have some similaritiesin metallogenic tectonic background,gold mineralization characteristics and other aspects,belonging to the middle-low temperature hydrothermal gold deposit.These gold deposits can be compared completely.Combined with the research of the typical gold deposits with similar geological setting in the East Junggar area,this dissertation construct a gold prospecting model of the East Junggar and adjacent areas in the Early Permian,which is universal.