| Gold(Au)and molybdenum(Mo)mineralization are pervasive in the southern margin of the North China Craton(NCC).This region contains four ore concentration areas:Xiaoqinling,Xiong'ershan,Xiaoshan,and Waifangshan.The southern margin of the NCC is the largest molybdenum metallogenic belt in the world and one of the most important gold metallogenic belts in China.The proven reserves of Mo and Au are 5million tons and 1000 tons,respectively.Gold deposits in the area are mostly formed between the Late Jurassic and Early Cretaceous(110?160Ma),which are contemporaneous with large-scale Mo deposits(120?157Ma)and plutons(124?158Ma).Moreover,they are also closely related in space.Although the studies of Au and/or Mo mineralization in the area have aroused extensive attention,the relationship of Mo and Au deposits in the area remains poorly understood.To explore the genetic relationship between Mo and Au in the area,this study selected the Qiyugou Au-deposit and the Leimengou porphyry Mo-deposit as the study subjects in the southern margin of the NCC.In this thesis,we carried out detailed analyses of geology,mineralogy,geochronology,and geochemistry for the Qiyugou Au-Deposit to explore the diagenetic,metallogenic,and enrichment mechanism of the gold deposit.Combining with previous studies of the Leimengou porphyry Mo-deposit,this thesis discussed the relationship of Mo and Au deposits in the area and further investigated the geodynamic background for the formation of molybdenum.The main conclusions are as follows:(1)the Qiyugou Au-deposit contains three types of ores,including porphyry-,breccia-and quartz-vein-types.Based on the mineral assemblages,the hydrothermal alteration,and the cross-cutting relationships,five stages of mineral precipitation are recognized:quartz-K-feldspar,quartz-pyrite(Py1),gold-pyrite(Py2),gold-polymetallic sulfide assemblage(Py3),and carbonate-quartz stages.The Au in the Qiyugou Au-deposit almost is present as visible(native gold and electrum),together with small amounts of nano-scale native gold that is associated with Pb-Bi-Ag-Cu mineral inclusions in pyrite.(2)numerous bismuth minerals have been recognized in the Qiyugou Au deposit.These Bi-minerals are mostly shown in Stage 3 and 4,which contain different assemblages of Bi-minerals assemblages show the difference.Stage 3 is dominated by coarse-grained pyrite with native-Bi,Ag-Pb-Bi(-Cu)sulfosalts(members of the lillianite-homologous series),and Pb-Bi sulfosalts inclusions(Fig.7).In contrast,Stage4 mainly comprises cbismuthinite and aikinite.The calculation of physicochemical conditions shows that gold was formed in conditions of f Te2=?10-11and f S2=?10-11 to10-12 for Py2,and f Te2=?10-9 to 10-11and f S2=?10-10 to 10-11 for Py3.The oxygen fugacity ranged from 10-26 to 10-23 at Stage 2,while the oxygen fugacity ranged from10-30to 10-36 at Stage 3 and Stage 4.The decreases in f O2 and temperature triggered the precipitation of liquid bismuth and drove the scavenging of Au from the hydrothermal fluids.Sulfidation driven by cooling or increasing sulfur contents results in the transformation of the Au-Bi liquid into a stable assemblage of native gold and bismuth minerals.Moreover,our new findings emphasize that the formation of the Qiyugou gold deposits involved Au scavenging by Bi melt(i.e.,Stages 3 and stage 4).(3)There are three types of medium-acid magmatic rocks in the Qiyugou golddeposit,including quartz porphyry,granite porphyry,and monzonite porphyry.The age of quartz porphyry(QP)is 159±6 Ma.The ore-bearing granite porphyry(GP),with a zircon U-Pb age of 131±1Ma and 130±1Ma,was intruded by the post-mineralization monzonite porphyry(MP),which was emplaced at 125±1Ma.The Si O2 contents of GP range from 61.03 to 68.75wt%,with 14.77 to 17.02 wt%for Al2O3,1.37 to 3.79 wt%for Ca O,2.69 to 4.65 wt%for Na2O,and 4.29?7.69 to 2.38 wt%for K2O.Most samples plot in the field of high-K calc-alkalic,and have A/CNK[Al2O3/(Na2O+K2O+Ca O)]values varying from 0.96 to 1.00,indicating metaluminous characteristics.The rocks are relatively enriched in LREE[(La/Yb)N?20.5?44.7,25.4 on average).It is of metaluminous to peraluminous highly fractionated I-type.The granite is characterized by negative?Hf(t)values from-12.9 to-25.0 with old t DM2ages of 2300?2600 Ma.ISrratios vary between 0.7082332 and 0.7086899,and negative?Nd(t)values range from-14.6 to-15.0 with old t DM2 ages of 2110?2140 Ma.All these results indicate that the rocks derive from the melting of the Taihua Group,accompanied by the addition of some young components(mantle or juvenile-crustal material).The dynamic background is an extension induced by the subduction of the Paleo-Pacific plate to North China Craton and subsequent retreat,and the heating of mantle source magma bottom,which promotes partial melting of the lower crust.(4)The GP has an average Eu/Eu*value of 0.70,and an average Ce4+/Ce3+value of238,whereas the MP has an average Eu/Eu*value of 0.57,and an average Ce4+/Ce3+value of 23,suggesting that the GP have high oxygen fugacity values(above the hematite-magnetite(HM)buffer),and the post-mineralization MP has lower oxygen fugacity values than the ore-bearing GP.The F,Cl,and S contents in apatite show that the sulfur content of apatites in GP is significantly higher than that of the apatites from MP,implying that the separation of sulfur-rich fluids is of great significance to the mineralization of Qiyugou gold deposit.The apatite in the ore-forming GP has a higher Ce/Pb ratio(157?916,320 on average),which indicates that the fluid activity of the ore-forming GP is stronger than that of the post-mineralization MP.These characteristics indicate that high oxygen fugacity,high sulfur content,and high fluid activity are of great significance for gold mineralization.(5)Combined with our results and previous studies,we proposed that the Qiyugou granitic porphyry and Leimengou granite porphyry have similar geochemical characteristics,genetic types,and material sources.The major and trace elements have a continuous evolution trend,indicating that they might result from the crystallization differentiation of the same parent magma.However,the magma crystallization of Leimenggou porphyry is more evolved,and the volatile is more riched in F but poor in Cl and S.The difference of the volatile composition and the degree of the crystallization evolution may be the critical factors for controlling the difference in ore-forming of the granite. |
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