| There is an important gold deposit concentration area in Jiaodong area of China,with more than 4000 tons gold reserves,which mainly consists of two types of deposits:vein type and disseminated type.Previous researches about gold deposits in Jiaodong area has been focused on their basic geological features,stable isotopic characteristics,fluid inclusions,geochronology,etc.However,it is still hotly disputed about the exsiting state of Au and the source of ore-forming materials and fluids.This dissertation chooses Linglong gold ore field which has both the two types as my research subject.By using in situ analysis methods,I aim to compare the two type about the origin,transportation and enrichment mechanism of Au,deepen the understanding of gold deposits in Jiaodong area and finally provide a guidance to the regional exploration of gold deposits.The Linglong gold ore field is located in the northern section of the Zhaoping fault zone.The ore body is hosted in the Linglong granite of fault footwall,which is controlled by the main fault to form the disseminated type and by the secondary fault to form the vein type.The U-Pb chronology of the hydrothermal monazite which formed from vein type and disseminated type mineralization shows that the two mineralization events are consistent,both concentrated at 121122Ma.The wall rock alteration of two types of mineralization is similar,mainly including Silicidation,sericinization and potassization,but in different scale:the disseminated type has wide alteration zone while the vein type ore body is narrow.The disseminated type is characterized by the thick pyrite-sericite-quartz alteration zone,and the ore-forming process can be divided into the potassium stage of early period,quartz-pyrite-sericite stage and quartz polymetallic sulfide stage of main period,and quatze-carbonate stage of later period.The vein type is characterized by the quartz-sulfide veins of varying thickness and can be divided into potassium-sericite-quartz stage,milky quartz stage,quartz-pyrite stage,quartz-polymetallic sulfide stage and quartz-carbonate stage.Both the two share the similar mineral composition.The metal minerals are pyrite,chalcopyrite,sphalerite,galena,native gold and telluride,etc.Au is mainly occurred as native gold,kustelite or petzite in the pyrite fracture or surrounded by pyrite.According to the mineralization stages,paragenetic association of minerals and BSE image features,the pyrite of vein type is divided into PyV-1 of quartz-pyrite stage,PyV-2 of quartz-polyetallic sulfide stage and metasomatic pyrite PyV-a;the pyrite of disseminated type is divided into PyD-1-1 of quartz-pyrite-sericitization stage,PyD-2 of quartz-pplymetallic sulfide stage and metasomatic pyrite PyD-a.The characteristics of PyV-1 and PyD-1 are similar,both of which are of the euhedral-subhedral granular texture,accompany with quartz,sericite but few other sulfides,besides the BSE images of PyV-1 and PyD-1 are uniform.The characteristics of PyV-2 and PyD-2 are similar,both of which are fine euhedral-subhedral granular texture,intergrow with chalcopyrite,sphalerite,galena and other polymetallic sulfides,with uniform BSE images.The characterisics of metasomatic pyrite are similar.We can see in specimen that quartz-polymetallic sulfide veins cut pyrites of early stage.With images under microscope and BSE,the metasomatic pyrite has the features of porous,corrosion structure,a large number of sulfide inclusions and dark color in BSE images,reflecting the later fluid replace and reconstreuction structure.Laser ablation inductively coupled plasma mass spectrometry?LA-ICP-MS?analyses show that the Au content of pyrite crystal lattice in all stages of the two type ore bodies is extremely low,and the average value of Au in 89 test points is 0.189 ppm,with median value of 0.049ppm.The content of Au in the pyrites from the early stage to the late stage hardly changes,with the contents of Ni,Bi,Te,Ag,Zn,Cu,Sb and Pb increasing.The Au content in metamorphic pyrites is lower than that of the original pyrites,while the content of Co,Ni,Bi,Te,Ag,Zn,Cu,Sb and Pb is higher,indicating that the late stage fluids make Au reactive from the pyrite lattice of early stage.Laser ablation multi-receiving inductively coupled plasma mass spectrometry?LA-MC-ICP-MS?pyrite sulfur-lead isotope analyses show that the pyrites of the vein type and the disseminated type have similar sulfur isotope variation range,the main range is 4.98.5‰,which is different from Jiaodong Group,TTG rock series,Fenzishan Group and pegmatite rock.The sulfur isotope change of pyrite from early stage to the late stage is small and has a weak trend of decreasing.The lead isotope results of the two types are similar.They are mainly located in the overlapping area of granite and intermediate-basic dyke,which is also quite different from Jiaodong Group,TTG rock series and Fenzishan Group.The sulfur-lead isotope results of pyrite indicate that the sulfur and lead of pyrite may mainly come from deep source magmatic rock.Based on the study of quartz in different stages of mineralization by SEM-CL and cold CL techniques,multiple generations of quartz in both mineralizations are found.The early-generation quartz in different stages of vein type and disseminated type is mainly in blue color under cold CL,and has a bright color under SEM-CL.The late generation is mainly dark red under cold CL,and dark at SEM-CL.In situ oxygen isotope analyses of quartz show that quartz oxygen isotope results are similar.Through calculation,the fluid oxygen isotope value is 7.310.1‰in the milky quartz stage of the vein type while in the quartz-pyrite stage is 3.28.1‰and in the quartz-polymetallic sulfide stage is 3.112.9‰;the oxygen isotope value in the early stage fluid of the disseminated type gold deposit is 3.28.9‰while in the late stage fluid is-3.38.9‰.Most of the data falls in the range of magmatic water,and value of the fluid oxygen isotope in the late stage is reduced,reflecting the mixing of atmospheric water.In summary,the vein type and disseminated type mineralizations of the Linglong gold ore field have many similarities including the pyrite mineralogy and trace element patterns,ore forming fluid evolution,sulfur and lead isotope of pyrite,reflecting that they are the products of the same metallogenic event,which have the same source of ore-forming materials and fluids.The difference of them is mainly due to the different tectonic setting during formation.In view of previous researches,I get the point that the genesis of gold deposits in Jiaodong area is different from that of typical orogenic gold deposits--it is produced under the background of craton destruction and strong extensional structure.Both the ore-forming materials and fluids are from the deep source-magma,and the gold deposit forms after the rapid upward movement of the fluid along the tectonic fault and the secondary tectonic activity. |
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