| Haoyaoerhudong gold deposit in Inner Mongolia is the largest open-pit gold deposit in North China,with the gold resource reserves of 249 t and an average grade of 0.62g/t.The ore body occured in the low metamorphic black rock series of the Bayan Obo Group in the Middle Proterozoic.A large number of studies have been carried out by previous researchers on the geological characteristics of the deposit.However,the source of ore-forming materials and metallogenic processes are still not clear.On the basis of mineralogy,this research redefines the mineralization periods and stages and carries out in-situ microanalysis of sulfides for different generations.Therefore,the composition of major,trace elements and sulfur isotopes of sulfides can be obtained,so as to constraint the activation and migration behaviours of elements,trace the source of ore-forming materials,and discuss the enrichment mechanism of gold and mineralization process.The main achievements and understandings are as follows:The gold mineralization process of Haoyaoerhudong gold deposit experienced three periods:sedimentary diagenetic period,regional metamorphic period,and brittle-ductile shear deformation and hydrothermal period.The latter is the main mineralization period.From the early period to the late period,the ore mineral composition shows from simple to complex.The ore minerals during the sedimentary diagenetic period are mainly fine-grained disseminated pyrite and a small amount of arsenopyrite.The ore minerals during regional metamorphic period are mainly pyrrhotite,pyrite and chalcopyrite.The mineral assemblage in the first stage of the main mineralization period is pyrite+pyrrhotite±arsenopyrite±loellingite,and the mineral assemblage in the second stage is pyrite+arsenopyrite±loellingite±native gold±pyrrhotite±chalcopyrite.The results of electron microprobe analysis show that the average contents of Fe and S in pyrite and pyrrhotite of different generations are very close to the theoretical value.Pyrrhotite belongs to sulfur rich and iron deficient pyrrhotite.The main element composition(Fe,As and S)of arsenopyrite in different generations is stable.It is slightly poor in arsenic and sulfur.The average content of main elements(As and Fe)in loellingite is 66.56% and 26.01%.It is deficient in arsenic and slightly deficient in iron.Arsenopyrite and loellingite both contain a small amount of Co(0.24%?4.98%)and Ni(0.14%?6.37%).The results of LA-ICP-MS trace element analysis show that pyrite in sedimentary diagenetic period is enriched in Au,As and other trace elements,while pyrite and pyrrhotite in regional metamorphic period does not contain Au and As.Au,Te,Bi,Pb and Ag are enriched in the porous arsenopyrite in the first stage of the main metallogenic period.Au is enriched in pyrite in the second stage of the main metallogenic period,and Au,Sb,Te,Bi,Pb,Ag,Cu and Zn are enriched in loellingite.Au exists in the form of “invisible gold” in pyrite during the sedimentary diagenetic stage and arsenopyrite during the first stage of the main metallogenic stage.In the second stage of the main metallogenic stage,Au mainly occurs in the form of native gold in the interior of loellingite and its contact interface with arsenopyrite.Loellingite is the main gold-bearing mineral.During the second stage of the main metallogenic period,the fluctuation of temperature and sulfur fugacity and the reaction between hydrothermal solution and surrounding rock are the enrichment mechanisms driving the activation and re-migration of gold.In-situ sulfur isotope research results show that sulfur has a relatively homogeneous source in haoyaoerhudong gold deposit.Sulfur is the product of seawater sulfate reduction,and may come from the Bilute formation of Bayan Obo Group.The study of Pb isotope shows that the ore Pb comes from Bayan Obo Group.The study of carbon-oxygen isotope shows that the carbon mainly comes from the decarboxylation of organic matter in sedimentary rocks.Haoyaoerhudong gold deposit experienced the formation of source bed in the sedimentary diagenetic stage of Mesoproterozoic,the activation and migration of gold and other ore-forming materials in the regional metamorphic stage of Early Carboniferous,and the precipitation and enrichment with fluid migration in the brittle ductile shear-hydrothermal stage of Late Permian,and finally formed the gold deposit. |
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