| Jiagongtai gold deposit is a large gold deposit recently found in the south of Qilian Moutain of Danghenanshan area,Gansu Province,of which the reserves have proven to be more than 30t.Due to lack of intensive deposit genesis study,the understanding of the gold prospecting prospect in the area is severely restricted.Locating the problems of ore-forming material source and genesis of Jiagongtai gold deposit,and after the field investigation and petrology observation,petrogeochemistry analysis,fliuid inclusion analysis,microanalysis of pyrite and sulfur isotope geochemistry,the following results are obtained and presented in this paper:1)Jiagongtai gold deposit is found in the Lower Ordovician upper group.The mining area is outcropped by Plagiogranite,which shows adakite geochemical characteristics with low content of MgO,Cr and Ni,and does not display the process of metasomatism between the initial adakitic magma and the mantle peridotite,indicating that it may be the product of partial melting lower crust as a result of asthenosphere upwelling or basaltic magma underplating heating crust in late-orogenic phase.2)Gold ore bodies are mainly in the plagiogranite,a few in the sandstone of Lower Ordovician upper group,and are strictly controlled by the secondary NW-structure.They are vein or lentoid.The wall rock alteration includes silicification,pyritization,sericitization and chloritization,where silicification is closely related to the mineralization.Ores have gold-bearing quartz type and altered rock type,and have granular structure and disseminated-vein structure.Major metal mineral in the ores is pyrite and the rest is small amount of galena,chalcopyrite,molybdenite,native gold and telluride.Major gangue mineral in the ores are quartz and the other is small amount of chlorite,sericite,calcite,ankerite,etc.3)Gold in the ores exists as native gold or Au-Ag tellurides.It is the first time that calaverite,petzite,altaite and tellurbismuth are found in them,implying that Jiagongtai gold deposit may be more exactly Au-Ag-Te deposit,indicating the ores are shallow denuded and have tremendous potential of deep prospecting.4)Trace elements in pyrite show that gold-bearing pyrite Co/Ni ratio is much greater than 1,which implies that the gold deposit belongs to magmatic hydrothermal type;adding that sulfur isotope results show the sulfur in the ore-forming fluid has the characteristics of source-mixed crust and mantle,that the plagiogranite has the nature of adakite and that Au-Ag tellurides,altaite,and tellurbismuth are found in the ores,the initial ores is indicated as coming from mantle or heating lower crust,and the gold deposit is thus formed when the initial ore-bearing magma extracts and further enriches metallogenic material during upwelling in the crust,and at the end rises to near-surface.5)The ore-forming fluid inclusions in quartz indicate that H20-rich inclusion is the main type,while there is lack of C02-bearing type inclusion.The metallogenic temperatures are 180?-250?,metallogenic salinitie are 2.57%-12.96%Nacl eqv,metallogenic pressures are 46.0^-68.2 MPa,and the corresponding depths for gold mineralization are 5.39?6.68 km.According to the gold level of fineness,shape,size,and the relationship between Au-Ag contents and temperatures,it infers that the gold mineralization location(>3km)and metallogenic temperatures(100??300?).These figures indicate that Jiagongtai gold deposit has the characteristics of shallow metallogenic depth,low mineralization temperature,low salinity and poor CO2.6)The formation of Jiagongtai gold deposit is closely related to adakitic plagiogranite,showing that the gold deposit formed at the stage of the partial melting of the thickened crust.Plagiogranite pluton provides metallogenic material and ore-forming fluid for the formation of gold deposit.The gold deposit containing telluride indicates that the pluton or the ore-forming fluid is mixed with mantle material.The composition of inclusions also shows that the ore-forming fluid contains meteoric water. |
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