Geological Characteristics And The Deep Metallogenic Forecast Of The Dishuishan Gold Deposit,Gansu Province

Located in the western section of Calidonian fold belt in the northern Qilian Mountain,southeast of the Arjin Fault,Dishuishan gold deposit,which is related to ductile-brittle shear zone of altered rock-host,was discovered in recent years.Through systematic study of the geological characteristics,geochemical characteristics and the deep metallogenic regularity of the Dishuishan gold deposit,conclusions concerning the mineral composition,the occurance of granodiorite and pyroclastic rock,the shear zone characteristics and deep metallogenic forecast of this deposit have been drawn as follows:?1?On the basis of comprehensive data collection,detailed field geological survey and systematic laboratory study,the mineral composition and the occurance of gold has been identified.The metalliferous minerals of Dishuishan glod deposit are mainly consist of pytite,galena,chalcopyrite,arsenopyrite,sphalerite,chalcocite,phrrhotite,covellite and so on.In addition,gold mainly exists in the form of electrum.?2?The LA-ICP-MS U-Pb age study on the zircon of the granodiorite in Dishuishan gold deposit shows 426.9±6.7 Ma.The chemical composition of the rock reveals high SiO₂?68.13%⁷3.73%?,averagely 70.94%,alkali?7.22%⁷.58%of K₂O+Na₂O,averagely 7.38%?,K₂O/Na₂O ratio 0.49⁰.64,averagely0.57,aluminum in-dex?0.46⁰.57 of??K₂O+Na₂O?/Al2O3??,Rittmann index?1.78².07?,belonging toK calc-alkaline series.The trace elements show enrichment in K,Rb,Th,U and relative deficiency of Nb,Sr,Ba.The rare earth elements distribution curves tend right.The light rare earth elements are relative enrichment and Eu loss medium.Combining with structure formation background,it is considered that the granodiorite were formed in syn-collisional stage,belonging to volcanic-arc setting.The LA-ICP-MS U-Pb age also study on the zircon of the pyroclastic rock in Dishuishan gold deposit shows 479.7±6.9 Ma.The trace elements show enrichment inK,Rb,Th,and relative deficiency of Nb,Ta,Ti.The rare earth elements distribution curves gentlely tend right.The light rare earth elements are relative enrichment and Eu anomaly is weak.Combined with the trace elements and the REE characteristics and the previous research in this region,it infers that the pyroclastic rock were formed in island-arc setting.?3?There are three phase deformation in this shear zone,the first phase deformation is the dextral ductile-brittle shear deformation which in the process of oblique thrust from north to south,the second phase deformation is brittle deformation,and the third deformation is the the deformation of gravity which happened after mineralization.According to the occurrence characteristics of the nearly ore rock,the enrichment degree of gold is gradually increased from lithic sandstone facie zone?crystal tuff facie zone?tuffaceous slate facie zone?altered mylonite facie zone?schisteous crystal tuff facie zone?altered cataclastic rock facie zone?Gold mineralization silicified cataclastic rock facie zone,and significantly enriched in quartz vein in altered cataclastic rock facie zone/schisteous crystal tuff facie zone of 80-110°.?4?With using geological and mathematical analysis methods,the paper carries through the analysis on the spatial distribution characteristics,the primary halo feature and the elemental association characterisitics of trace elements which located in Au2 orebody.The results showed that:1)On the basis of Grigoryan's zonationindexes,the primary halo axial zoning sequence from the top to the deep part in No.141 and 175 geologic section are?Pb-As?-Au-Cu-Zn-Bi-Hg-Sb-Ag-Mo and?Hg-As-Au-Sb-Pb-Cu?-Zn-Mo-Ag-Bi respectively,which exists the mutualsuperposi-tion of the front halo elements,proximal elements and tail halo elements;2)the mineralization is prospective in the deep portion of the Au2 orebody.