| The study area is located in the eastern region of QingHai province and stretch over 4 secondary tectonic units:Mid-Qilian Block?Shule Nanshan-Lajishan early Paleozoic suture zone,Southern Qilian Block and Zongwulong-Qinghai Nanshan late Paleozoic-larly Mesozoic chasmictrough.Stratum in the study area are simple,mainly including Tuolai group of Paleoproterozoic,Huashishan Group of Jixianian Period,Huangzhong Group of Changchengian Period,Liudaogou formation of Cambrian,Longwuhe formation and Elashan formation of Triassic,Xining formation of Paleogene,Linxia formation of Neogene and some Quaternary stratum.The magmatic activity in this area is very strong and frequent,and it mainly is the intrusion of magma,including Precambrian period to Mesozoic.Meanwhile,the structure in this area are well developed and the tectonic framework and deposits in this area were controlled by the NW-SE direction faults.We find out the geochemical characteristics of 22 elements,including Au,As,Sb,Hg,Ag,Sn,Cu,Pb,Zn,Cr,Ti,V,P,Y,Nb,Co,Ni,Mo,W,Bi,La,Li by studying the data of the 1:5 0000 stream sediment geochemical survey.So three geochemical partitions have been devided,namely Lajishan,Luohantang and Yeniushan.Also,we consider that the main ore-forming elements are Cu,As,Au,Mo,Sb,W,Sn and Ag by following the coefficient of variation of the original data and background data.Finally,combined with the analysis of geological background,we hold that Longwuhe formation,Elashan formation Liudaogou formation,Beimenxia formation and Qingshipo formation are the main ore-forming stratum in the study area.In this paper,the thresholds of anomaly in the study area are determined by the calculation of different geochemical partition and references of regional geochemical exploration results,which avoid the interference of caused by geological background effectively.So,anomalies in this area were divided into 79 comprehensive anomalies,incluting 7 anomalies of class A,40 anomalies of class B and 32 anomalies of class C.Anomalies of class A in this area all are anomalies related to minerallization,and Damajigou Au deposit,Guobode Cu-Sn deposit,Kerou Cu deposit,Sancha Au-Cu-Ni deposit and Qunaihai Pb deposit are found in this anomalies.The Damajigou Au deposit,located in the Shule Nanshan-Lajishan early Paleozoic suture zone,are hosted in the basalt of Liudaogou formation and its orebody were strictly controlled by the NW-SE direction faults.Ores in this deposit mainly are altered basalt with sulfides,such as pyrite,chalcopyrite,arsenopyrite,pyrrhotine and sphalerite.Wall rock alteration in strong and dominated by silicification,phyllic alteration,chloritization and carbonation,and silicification is generally related closely to Au minerallization.The mainly textures of the ore are granular,metasomatized and intersertal texture and it is generally massive,disseminated,and vein-or stockwork-hosted.The minerallization in this deposit are divided into three distinct stages,including quartz-pyrite-arsenopyrite stage,quartz-polymetallic sulfide stage and quartz–calcite stage.Studies on fluid inclusions of quartz show that the aqueous inclusions have final homogenization to liquid at temperatures between 170.8 and 375.2°C,and melting of ice in the range of –10.5 to –3.2°C.The melting temperatures correspond to salinities from 5.24 to 14.52 wt.% NaCl equivalent.These inclusions indicate fluid at densities from 0.70 to 0.97 g/cm3,all of which are indicative of high-to medium-temperature,,low-to medium-salinity,low-to medium density fluids with a wide range of compositions.Metallogenic pressure of this deposit is 16.88~39.12 Mpa,and the mineralization depth is 1.69~ 3.91 km.The fluids in stage ? that were of high temperature,low–moderate density and high pressure evolved into stage ?moderate to low temperature,moderate density and low pressure.We conclude,therefore,the Damajigou Au deposit is defined as the medium to high temperature hydrothermal vein-type gold deposit.The Guobode Cu-Sn deposit,located in the Zongwulong-Qinghai Nanshan late Paleozoic-larly Mesozoic chasmictrough,are hosted in the skarn belt related to the Indosinian granite and carbonate rocks and its minerallization are controlled by the skarn belt.The skarn minerals in this deposit mainly are garnet and diopside,and pyrite,chalcopyrite,pyrrhotine are the main sulfides within this deposit.Therefore,we consider Guobode as the skarn tpye Cu-Sn deposit.Based on the 1:50000 geochemical anomalies of stream sediments and combined with the metallogenic geological conditions and minerallization,two ore prospect areas are defined of Southwestern Luohantang Cu-Sn polymetallic and Huashishan-Nanxia Au-Cu ore prospect areas,and six prospecting targets with.Moreover,we consider that mesozonal hydrothermal vein-type Au deposit and skarn type Cu-Sn deposit are the haloes for the mineral deposits,which provides the direction of further prospecting. |
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