| Mineral resource is the base for the development of the national economy.To carry out the geological exploration campaign,then to satisfy the need of rapid development of the national economy for mineral resources,is an important task for geology and mineral survey at present days.Maqu area in Gansu Province is located in Dashui Au mineralization area(?),Xiqinshan-Bailongjiang mineralization belt(?),South Qinling mineralization zone(?)of Hercynian-Yanshan period.The area is the important Au concentrated area in China,in which the representative Au deposits are Geerke,Geertuo,Zhongqu,etc.With the long Au mining,the Au deposits in the area commonly faced the depletion of mineral resources,therefore,to carry out the deep mineral exploration in the given Au deposits and the prospecting potential of the periphery of Au deposits is of importance in searching the continuation of resources for prolonging the life of the existing Au deposits and for providing resources protection for sustainable economic and social development in the area.Stream sediment geochemical prospecting is a common method in regional geology and mineral resource survey,which has the superiority to other mineral exploration methods in direct,fast delineate the anomaly.The basic principle for the prospecting is to the distributions of elements in stream sediment through sampling and analyzing the stream sediment samples,then to delineate the geochemical anomaly,and to interpret and evaluate the anomaly,and finally to delineate the targets of geochemical exploration in combination with characteristics of geology and mineral resource.Stream sediment geochemical prospecting as a mature exploration method has been widely used in the geological exploration at the home and aboard,which has gained a good exploration result.The key techniques for stream sediment geochemical prospecting are the fraction size of stream sediment samples and the determination of the reasonable anomaly thresholds of elements.The study area is located in Maqu County,Gannan Tibetan autonomous Prefecture,Gansu Province,belonging to Gansu alpine grassland landscape.The water drainage of the area is developed well,which is benefit for stream sediment geochemical prospecting.The soils in the area are mainly the meadow soil,peat soil,bog soil,ect.,and the soil horizons are clearly layered.The paper is based on the ore-prospecting program of the Maqu-Hezuo ore concentrated area in Gansu Province funded by China geological survey,and stream sediment geochemical prospecting campaign is carried out in the area,and fraction size for stream sediment sampling is-10-+60 meshes.In the paper,stream sediment samples are sampled,and Ag,As,Au,Bi,Cu,Hg,Mo,Pb,Sb,and Zn in the samples are analyzed.The content levels are studied,and the co-relations between elements and elements associations are investigated by multi-statistical analysis.The anomaly thresholds of the elements are determined by the gradually eliminate method,the cumulative frequency method,and the fractal method,respectively,with which the individual element anomalies,and the composite anomalies are delineated.The comprehensive anomalies are delineated using the anomaly threshold determined by the cumulative frequency method.The comprehensive anomalies are ordered by the combinations of total NAP values,the numbers of elements,the area of comprehensive anomalies,and geological settings of the anomalies;the comprehensive anomalies are classified in light of the related geochemical prospecting specification.The comprehensive anomalies are interpreted and evaluated through the combinations of geology and mineral resource features,then the prospecting targets are delineated.The main conclusions are as follows:(1)the variation coefficients of elements in stream sediments decreased in the order of Au,Hg,Mo,Sb,Pb,Cu,As,Ag,Bi,and Zn.Among elements Au has the maximum variation coefficients(13.87),Hg ranks the second,with variation coefficients of 4.15,and other elements has variation coefficients ranging from 1.36 to 3.95.Above mentioned ten elements in stream sediments can be divided into three groups,the first one is composed of Cu,Zn,Bi,and Pb,which is related to Pb and Zn mineralization;the second one is consisted in Ag,As,Mo,and Sb,which is related to Mo and Ag mineralization;the third one comprises Au and Hg,which is related to Au mineralization.(2)The thresholds of Ag,Au,Hg,and Zn set by the cumulative frequency method are in between those by the gradually eliminate method and the fractal method,the thresholds of As,Mo,and Sb determined by the cumulative frequency method are higher than those by the gradually eliminate method and the fractal method,and the thresholds of Bi,Cu and Pb determined by the cumulative frequency method are lower than those by the gradually eliminate method and the fractal method.Through comparisons of the individual element anomalies,composite anomalies and element multiplicative index it can be found that the cumulative frequency method is superior to other to methods in identifying composite anomalies of Bi,Cu,Pb,and Zn,therefore,the cumulative frequency method is taken as the method for delineating the comprehensive anomalies.(3)Sixteen comprehensive anomalies are delineated,among them two comprehensive anomalies are of Class one,and another two ones are of Class two,and left others are of Class three.Three exploration targets are delineated.The results of the paper are of significance for the next geological arrangement in the study area. |
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