| The North China Craton(NCC)is one of the oldest cratons in the world,which underwent a dramatic lithospheric thinning/destruction in the Mesozoic.However,the timing and mechanism of the lithospheric thinning/destruction remain controversial.The Liaodong Peninsula is located in the eastern margin of the NCC,and has experienced dramatic lithospheric thinning with numerous related magmatic rocks developed.In addition,there are many super-large and large gold deposits in the Liaodong Peninsula,which is an important gold district in the NCC.The Wulong goldfield is typical in the Liaodong Peninsula.The coupling relationship between the deep evolution of magma and gold mineralization in the Wulong goldfield is not clear,which restricts the genetic research and prospecting work of gold deposits in the area.Thus,the dissertation takes the Mesozoic magmatic rocks and gold deposits in the Wulong goldfield as the research object.Based on detailed field geological surveys and indoor petrology/mineral facies studies,zircon U-Pb chronology,whole-rock major and trace element geochemistry and whole-rock/in-situ Sr-Nd-Pb-Hf isotopes of the dykes and intrusions were performed to detect their petrogenesis and tectonic setting,and to provide evidence for the thinning/destruction of the NCC.Moreover,for the Wulong gold deposit,the fluid inclusion,H-O isotopes,sulfide LA-ICP-MS trace elements and in-situ S isotopic analysis,combined with previous studies,were carried out to decipher the origin of ore-forming fluids and materials,and to discuss the genetic relationship between tectonic magmatism and gold mineralization in the Wulong goldfield.The main results and understandings achieved are as follows:(1)The Wulong super-large gold deposit contains>80 t of gold at an average grade of 5.35 g/t.The orebodies occur in the form of coarse gold-bearing quartz veins hosted in the Early Cretaceous diorite dike and the Late Jurassic gneissic biotite granite.The direction of gold ore bodies in the area is mainly controlled by the NNE-and NW-trending tension-torsion fractures.The ores are mainly composed of pyrite,pyrrhotite,chalcopyrite,galena,sphalerite,and quartz.The common wall-rock alterations are silicification,sericitization,pyrite mineralization,carbonation,and chlorite.Four stages of hydrothermal mineralization are recognized,including sulfide-poor quartz stage,quartz-pyrrhotite-pyrite stage,quartz-polymetallic sulfide stage,and quartz-carbonate stage.(2)Numerous intermediate-mafic dikes are widespread in the Wulong gold ore field,which are closely related to the gold deposit.They are chemically subdivided into the ca.126 Ma diorite porphyry and coeval(ca.119 Ma)low-Ti(Ti O2<1.20 wt.%,Ti/Y<375)and high-Ti(Ti O2>1.90 wt.%,Ti/Y>580)diabase dikes.The diorite porphyries exhibit a moderate silica(56.71–61.13 wt.%)and high-K calc-alkalic signature,with low Ti O2and total Fe2O3 and high Cr and Ni compositions.They are enriched in large ion lithophile elements(LILEs)(e.g.,Rb,K,Pb,and Sr)and depleted in high field strength elements(HFSEs)(e.g.,Nb,Ta,Ti,and P).They have high radiogenic Sr isotopes(whole-rock(87Sr/86Sr)i=0.711553–0.714284,in-situ(87Sr/86Sr)i of plagioclase=0.71392),low?Nd(t)values(-19.9 to-14.5),and old Nd model ages(1.81–2.25 Ga).Meanwhile,some plagioclase phenocrysts have reverse zonings with Na-rich cores(An:35–48)and Ca-rich rims(An:48–57).These features imply that the diorite porphyries were generated by mixing between magmas from the Archean–Paleoproterozoic lower crust and the lithospheric mantle.The low-Ti diabase dikes are characterized by medium to high-K calc-alkaline series with high Al2O3 but low Ti O2 contents.They show arc-like trace element signatures,such as enrichment in LILEs(Rb,K,Pb,and Sr)and depletion in HFSEs(Nb,Ta,Zr,Hf,Ti,and P).They also have relatively lower initial 87Sr/86Sr ratios(0.706331–0.708315 and 0.70837 for the whole-rock and in-situ Sr isotopes,respectively)compared to the porphyries and negative?Nd(t)values(-13.6 to-5.0),indicating that they were derived from partial melting of an enriched lithospheric mantle.Their variable Th/Yb and Th/Nb ratios with constant Sr/Nd and U/Th values further suggest that the mantle source might have been previously modified by the subduction-related melt.In contrast,the high-Ti diabase dikes belong to the high-K calc-alkaline to shoshonitic series,with higher Ti O2 and total Fe2O3 but lower Al2O3 contents.They have OIB-like trace elements(e.g.,slight enrichments of Nb and Ta with no HFSE depletion,high Nb/U values(42±5))and Sr–Nd isotopic compositions(e.g.,positive whole-rock?Nd(t)values(+3.1 to+4.3)),suggesting that they were generated from an asthenospheric mantle source.The coexisting Wulong low-Ti and high-Ti mafic dikes with similar formation ages(ca.119 Ma)may indicate a transition from lithospheric to asthenospheric mantle source.In combination with previous studies on the Mesozoic–Cenozoic mafic rocks in the Eastern Block of the NCC,we propose that the lithospheric thinning beneath the Liaodong Peninsula initiated at ca.119 Ma and this process beneath the NCC lasted for a narrow duration(ca.13 Ma).The unsteady mantle flows due to the rapid subduction and rollback of the Paleo-Pacific plate might trigger the thickened,modified lithosphere to lose its gravitational stability,eventually leading to lithospheric delamination and thinning of the NCC.(3)Zircon U-Pb chronology and geochemistry were performed on the widespread granitoid in the Wulong goldfield,including the Dandong pluton,the Sanguliu pluton and enclaves,and the Wulongbei pluton.Zircon LA-ICP-MS U-Pb dating suggests that the Dandong pluton was formed in the Late Jurassic(161±1 Ma),and the other plutons and enclaves were formed in the Early Cretaceous(126±1 Ma).Sr-Nd-Pb-Hf isotopes,as well as major and trace elements,show that the Sanguliu and Wulongbei intrusions/enclaves have the comparable trace elements and isotopic compositions with those of the coinstantaneous diorite porphyries,indicating that they have similar magma sources,that is,they are derived from partial melting of the ancient lower crust with magma mixing of the lithospheric mantle.The widespread Early Cretaceous intrusions that formed by crust-mantle mixed magmas have similar petrogenesis and tectonic backgrounds to the coeval Qianshan A-type granite and Gudaoling I-type granite in the Liaodong Peninsula,indicating a large-scale extensional tectonic setting in the Early Cretaceous.(4)Combined with the diagenetic ages of dikes formed before and after the mineralization and previous studies on the sericite Ar-Ar,molybdenite Re-Os,and hydrothermal monazite U-Pb dating,the mineralization age of the Wulong gold deposit is suggested to be 126–123 Ma.The metallogenic age is highly similar to the aforementioned diagenetic ages of the diorite porphyry dikes,Sanguliu intrusions/enclaves,and Wulongbei intrusions,indicating that the magmatism and mineralization of the Wulong gold ore field show a strong coupling relationship.(5)Studies on fluid inclusions imply that the primary fluid inclusions in the Wulong gold deposit mainly include pure CO2 inclusions(type I),CO2-H2O inclusions(type II),and two-phase aqueous inclusions(type III).The type II and type III inclusions have different gas-liquid ratios.However,the coexisting fluid inclusions with different types and gas-liquid ratios in the same mineralization stage show similar homogenization temperatures,indicating the fluid immiscibility.The ore-forming fluid is characterized by a medium-high temperature and medium-low salinity,and belongs to the H2O-CO2-Na Cl ternary system.As the evolution of the ore-forming fluid,both the temperature and salinity show a decreasing trend.The H-O isotopic data of the Wulong gold deposit are mainly plotted in the area representative of the magmatic water and tend to drift towards the meteoric water in the late stage,suggesting that the ore-forming fluids were originally derived from magmatic water and were mixed with meteoric water in the later period.In addition,the He-Ar isotopes are mainly located between the crustal fluid and the mantle fluid,showing a mixing characteristic between them.(6)Mineralogy and sulfide LA-ICP-MS trace elements show that sulfides of the Wulong gold deposit can be divided into five generations of pyrite(Py1–Py5),two generations of chalcopyrite(Ccp1–Ccp2),and one generation of pyrite(Po)and sphalerite(Sp).The quartz-pyrrhotite-pyrite stage is mainly composed of Py1–Py3,Po and Ccp1.Py1 and Po are often symbiotically developed,and their trace element contents are extremely low.However,Py2 and Py3 have high contents of Au(average of 7.10 ppm and 1.67 ppm,respectively)and As(average of 1433 ppm and 949 ppm,respectively).In the Py2,Au existed in the form of lattice-bound gold and Au-Ag-Pb-Bi-Te inclusions.Py3 often grew as the porous edge of Py2 with Au existing in the form of lattice-bound solid solutions.Trace elements in the Ccp1 mainly include Ag,Zn,Pb,and Bi.Quartz polymetallic sulfide stage is mainly composed of Py4–Py5,Sb,and Ccp2.The Py4 also has porous structure,and shows similar trace elements to those of the Py3.Gold in the Py4 is given priority to lattice-bound solid solutions with a small amount of gold inclusions.The Py5 usually grew as the smooth edge of Py4,and possessed high Au(mean 6.11 ppm)and As contents(mean 8423 ppm).Gold in the Py5 output in the form of lattice-bound solid solutions and inclusions.Sb and Ccp2 have less content of gold,but contain some Ag,Pb,and Bi.In general,strong positive correlations can be observed between Au and As in pyrite at different stages,indicating the process in which As–substitute S–into the pyrite enhances incorporation of Au due to lattice distortion of the pyrite.The metallogenic physical and chemical conditions show that gold in the ore-forming fluid may be transported in the form of Au(HS)2–.When it was transported to the shallow brittle fault,the sudden release of pressure caused fluid boiling and fluid immiscibility,leading to a large amount of gold precipitation.In-situ S isotopic studies of sulfides from different generations in the Wulong gold deposit yield?34S values ranging from 1.0 to 6.3‰for pyrite,1.0 to 2.3‰for pyrrhotite,1.0 to 6.0‰for chalcopyrite,and3.1 to 4.3‰for sphalerite.The?34S values of all sulfides in the quartz-pyrrhotite-pyrite stage is relatively low and similar(focus on 1.5–2.5‰),showing the characteristics of magma or mantle sulfur.The?34S values of Py4 and Sp in the quartz polymetallic sulfide stage are comparable to those of sulfides in the quartz-pyrrhotite-pyrite stage.However,the Py5 has significantly elevated?34S values(average of 4.5‰),and the Ccp2 show lower values(mean of 1.5‰).These features were suggested as the result of isotope fractionation among sulfides and mixing of 34S-rich fluids.The 34S-rich fluid may originate from deep-source(mantle-derived)magma,which were mixed with deep-seated arsenic-rich sedimentary rocks.Based on the previous studies on S-Pb isotopes of sulfide and wall-rocks in Wulong gold deposit,this paper suggested that the ore-forming materials of gold deposits in the Wulong goldfield were mainly derived from crust-mantle mixed magmas.when the wall-rock is the Liaohe Group,some materials form the strata were involved in the ore-forming materials.(7)The gold deposits in the Wulong goldfield are closely related to magmatism in terms of time and space distribution,ore-forming fluid,and ore-forming materials.In terms of the time scale,the mineralization ages of gold deposits are highly consistent with those of intermediate-acid magmatic rocks and mafic dikes in the region,corresponding to the initial timing of lithospheric large-scale destruction/thinning of the NCC.In terms of the spatial scale,most gold deposits are controlled by the Early Cretaceous NNE brittle fractures,and are distributed around the Sanguliu intrusion.The Dandong and Sanguliu intrusions as well as various kinds of mafic dikes widely distributed in the area are important wall-rocks of the gold deposits.In terms of ore-forming fluids and ore-forming materials,they were all derived from crust-mantle mixed magmas,and the repeated injection of the mantle-derived magma and subsequent MASH process in the deep magma chamber provided most of the gold-rich fluids and materials for gold deposits in the Wulong goldfield.Therefore,a magma-tectonic-metallogenic synthesis model has been constructed,which is expected to provide theoretical guidance for further prospecting and exploration in the Wulong goldfield. |
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