| The East Kunlun orogen?EKO?is located in the east end of the Tethys domain and the north margin of Qinghai Tibet Plateau.It records the evolution process of the proto-Tethys ocean in Early Paleozoic and the Paleo-Tethys ocean in late Paleozoic to Mesozoic.It also contains rich gold resources,and thus attracting extensive attention in recent years to study the relationship between Tethys evolution and gold mineralization.However,at present,the following issues are of few studies or controversial:the transformation timing from the Proto-Tethys ocean subduction to continental collision in the early Paleozoic,the timing of late Paleozoic to Mesozoic back arc extension and closure of the Paleo-Tethys ocean,the relationship between multi-stages tectonic-magmatism and gold mineralization,and the mechanism of gold mineralization during multi-stage fluid activities.The Gouli goldfield,located in the east of the EKO,is one of the main gold-rich areas in the EKO.Its known gold resources are more than 110t,including many large and medium-scale vein gold deposits such as Goluolongwa and Walega gold deposit.In this paper,zircon U-Pb dating and Hf isotopic,whole rocks elemental and Sr-Nd isotopic data are used to determine the petrogenesis of regional Paleozoic to Mesozoic basic rocks and further to constrain the tectonic setting.We also try to establish the evolution process of the Proto-Tethys ocean and Paleo-Tethys ocean in combination with regional structural,magmatic,and sedimentary data.In addition,detailed field investigation and mineralogical identification,fluid inclusions,He-Ar isotopes analyses for pyrite,in-situ trace elements analyses for pyrite and arsenopyrite were carried out for Guoluolongwa and Walega gold deposits to ascertain the characteristics of structural ore-control and the orogenesis.The primary conclusions of the dissertation are as below:Early Paleozoic Langmuri olivine gabbro and Kehete gabbro in Gouli area are located in the north of the Central Kunlun fault zone,their emplacement ages are 450Ma and 430 Ma,respectively.The elements and Sr-Nd isotopic composition of the Langmuri olivine gabbro show that it is characterized by low K and high Mg and Fe contents,and the rock type is low-k tholeiite,with variable Ba/La value and consistent Th/Yb values.The Kehete gabbro has higher K content and lower Mg and Fe contents and the rock type is medium-K calc-alkaline.Both the Ba/La and Th/Yb values of the samples from it are inconsistent.The two rocks are rich in light rare earth elements and large ion lithophile elements and depleted of high field strength elements.The Sr-Nd isotopic compositions of the rocks are consistent and gradually enriched from Ordovician to Silurian.These results show that the Langmuri olivine gabbro and Kehete gabbro are not obviously contaminated by the crustal materials,and the magma originated from the melting of the mantle wedge metasomatized by materials from subduction slab.In Ordovician,the mantle wedge was mainly metasomatized by the slab.In Silurian,the subducted slab began to melt due to the deepening of subduction,and the mantle wedge was metasomatized by the slab melts.These changes of the nature and petrogenesis of the basic rocks from Ordovician to Silurian also means the change from the immature arc in Ordovician to the mature arc in Silurian.both of the Langmuri olivine gabbro and Kehete gabbro were formed in the northern subduction process of the Proto-Tethys ocean,while the Kehete?430Ma?gabbro represents the late stage of oceanic subduction.The subsequent continental collision also occurred around 430 Ma.The Late Paleozoic to Mesozoic Kengdenongshe gabbro and Annage hornblende gabbro intruded at ca.266Ma and 240 Ma,respectively.The Kengdenongshe gabbro is characterized by high and variable Mg O and Fe2O3contents,as well as low K2O and Ti O2contents,typical of low-K tholeiitic series.The rock shows enrichment in large-ion lithophile elements,depletion in high field strength elements,with rare earth element signature similar to that of a back-arc basin basalt.The rock displays relatively low and uniform(87Sr/86Sr)i ratios and high?Nd?t?values.These geochemical and isotopic features are interpreted to represent magma derivation from the partial melting of an enriched asthenospheric source.Our model envisages that the initial melting started in the garnet-facies??4%?and continued in the spinel-facies??6%?.The geochemical and isotopic systematics of the gabbro is consistent with magma generation in a back-arc basin setting triggered by the subduction of the Paleo–Tethys Ocean.The Annage hornblende gabbro is located in the east of Eastern Kunlun Orogenic Belt,north of the Central Kunlun Fault.LA-ICP-MS zircon U-Pb dating yields a weighted mean age of 242±2Ma for the intrusion.It is characterized by low Si O2?46.00%?52.40%?,low Na2O+K2O?3.07%?3.79%?contents.In addition,it is enriched in Al2O3?17.35%?20.10%?,Mg O?4.65%?6.53%?and Fe O*?8.77%?11.07%?with high Mg#?68?75?,indicating that it belongs to the calc-alkaline series.This intrusion has low?La/Yb?N and Nb/Ta ratios,negative Eu anomalies,with enriched large-ion lithophile elements and depleted high field strength elements.All geochemical characteristics show that this intrusion is result of typical island arc magmatic activity.Additionally,the samples have relatively uniform(87Sr/86Sr)i ratios?0.70880?0.71036?and?Nd?t?values?-4.8?-3.4?,and relatively high?Hf?t?values?-4.9?-0.4?.According to the lithology,geochemical characteristics and isotopic compositions of the intrusion,we propose that the magma has not been significantly contaminated by the crust and it was derived from the low-degree partial melting?2%?10%?of the mantle wedge,which had undergone metasomatism induced by the fluid from the subducted slab.The mantle source was characterized by the spinel phase-phlogopite lherzolite.The Guoluolongwa gold deposit occurs in the phyllonite of the Ordovician Nachitai Group.The structure of the mining area is dominated by faults,and can be further divided into four stages.The metal minerals in the ore mainly include natural gold,pyrite,chalcopyrite,sphalerite,galena,limonite,malachite The wall rock alterations include silicification,potassification,sericitization and epidotization,which occur alternately on both sides of the ore body.The ore-forming period can be divided into hydrothermal ore-forming period and epigenetic leaching enrichment period.The hydrothermal ore-forming period can be further divided into three ore-forming stages including the quartz with little pyrite stage,quartz-pyrite stage,and polymetallic sulfide stage.Pyrite in the ore from goluolongwa deposit can be divided into 5 different periods or types?py1-py5?.The contents of Au in py1 and PY3 are relatively higher?>1ppm?,while the contents of Au in py2,PY4 and py5 are all less than 1ppm.Au is positively correlated with Pb and Bi,but showing no correlations with As contents.The comprehensive analysis shows that gold migrates in the form of sulfur hydrogen complex in the fluid,and reacts with Fe minerals?chlorite?in the country rocks during the migration process.Au occurs in pyrite?py1and PY3?in the form of small natural gold.Due to the decrease of pressure but the increase of fluid temperature in the later stage,py1 partially recrystallizes to form py2and release gold at the same time.In the evolution process from quartz pyrite stage?PY3?to polymetallic sulfide stage?py4-py5?,fluid immiscibility occurs due to the decrease of fluid pressure,and a large number of brittle deformations developed in py4 provides a channel for fluid infiltration.Gold and some sulfur-friendly sulfide precipitated together in PY4 fracture.The host rock of walega gold ore is mainly Ordovician biotite granite.The structure of the mining area is also dominated by fault structure,and its structural sequence can be divided into four stages.The metal minerals in the ore include natural gold,pyrite,arsenopyrite,galena,sphalerite,pyrrhotite,tetrahedrite,and non-metallic minerals are mainly quartz with a small amount of Sericite.The wall rock alteration near the mine includes silicification,epidotization and sericitization,and the alteration is mainly developed in the footwall of the ore body.The metallogenic stages can be divided into hydrothermal metallogenic stage and epigenetic stage,in which the hydrothermal metallogenic stage can be further divided into three stages:Quartz stage,polymetallic sulfide stage and quartz arsenopyrite stage.Pyrite in walega gold deposit can be divided into five types.Arsenopyrite can also be divided into three types?Apy1-Apy3?.In pyrite,Au is positively correlated with as and Sb,while in arsenopyrite,Au is negatively correlated with Sb.In addition,the core of apy2 has low gold,high Sb and Ni content,and the edge has high Au,low Sb and Ni content.Our results show that Au replaced the Fe in pyrite and arsenopyrite because As replaced the S in pyrite and arsenopyrite,which is the main occurrences of invisible gold.Nature gold is mainly distributed in Apy3 or at the boundary between Apy3 and py4-5,which is mainly due to the remelting of arsenopyrite caused by later chloride fluid activity. |
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