Typomorphic Characteristics And Significance Of Pyrite In The Gold Metallogenic Belt Of Dadu River

The gold metallogenic belt ofDadu River is located at the northern end of the north-south tectonic belt of the Kangdian axis,the special tectonic site formed by the intersection of the trigeminal rift which consists of Xianshui River,Longmen Mountains and Panxi Rift.In recent years,the geological exploration work shows that the research area has a "three-storey" structure,and deposits in the area also have a "three-storey" pattern of distribution characteristics.The deposits are divided into ones in the basement,ones in the interface and ones in the cover.The gold-bearing mineral of gold deposits in different storey is mainly pyrite,which indicates that pyrite has important significance for the genesis of gold deposits in Dadu River Basin.In this paper,I mainly do research to the typomorphic characteristics of pyrite in the gold deposits of Dadu River gold metallogenic belt.With the combination of field geological survey,indoor tests and analysis,and previous research results,I have come to the following conclusion.The mineral assemblage in the gold deposits of different storey in the Dadu River gold metallogenic belt is similar.Metal minerals are mainly pyrite,followed by galena,chalcopyrite and so on.The main gold-bearing minerals are pyrite and the gangue minerals are quartz.The country rocks of gold deposits in the cover is carbonatite,while those in the basement is granite.The crystal shapes of pyrite in gold deposits of different storey are not the same.In the cover,the shapes are mainly cube and are rarely pentagonal dodecahedron.In the interface,the crystal is mainly in the combination form and is rarely in the simplex form of cube.In the basement,the shapes are mainly pentagonal dodecahedron and cube.This shows that the ore-forming temperatures of the three kinds of gold deposits are different.The simplex form of pentagonal dodecahedron appears most in the basement,this shows that the ore-forming temperature of the basement is the highest and is suitable for the formation of pentagonal dodecahedron crystal.While in the cover,the ore-forming temperature is lower and is suitable for the formation of cube crystal.There are significant differences in the thermoelectricity of pyrite from gold deposits of different storey.Conductive type of pyrite is P-type in the cover,N-type in the basement and mainly P-type with less N-type in the interface.The conductive type can indicate the ore-forming temperature of pyrite and we conclude that from cover to basement,the temperature becomes higher.While the temperatures are within the scope of the ore-forming temperature of mesothermal and epithermal deposits,the gold deposits are such kind of deposits.The trace element characteristics of pyrite in all deposits in research area are enriched in U,and this explains that the three kinds of gold deposits have the same material source which is deep hydrothermal.Influenced more by the fluid flow in magma source,the enrichment of U in the basement is highest.Sr in the cover is depleted more because of crustal contamination.Characteristics of rare earth elements in pyrite also show the source of ore-forming materials.In the basement,the total amount of REE??REE?in pyrite is 8.60×10^-6,which is similar to the ?REE=7.30×10^-6 of upper mantle.In the cover,?REE=25.29×10^-6,which is similar to the ?REE of upper crust.This indicates that some ore-forming materials of deposits in the cover may from the crust.The pyrite of different position in the Dadu River gold metallogenic belt shows Eu anomaly and no Ce anomaly.That indicates the ore-forming fluid is a reductive fluid in the gold metallogenic belt of Dadu River.The pyrite in the cover,basement and interface shows the right deviation of LREE.That indicates they have the same material source which is deep mantle source.