Rare Gas Isotopic Characteristics Of The Dadu River Gold Metallogenic Belt

The Dadu River gold metallogenic belt is located at the north end of the famous Kang Dian axis in China,the India plate,the Yangtze plate,the North China plate extrusion suture zone,the Longmen mountain fold belt and the Bayankara fold belt.The ore deposits in this area can be divided into three parts,which are basement,interface and cover.This paper compares the geological characteristics of gold deposits with different ore hosting strata in the Dadu River gold metallogenic belt.The abundance and composition of He and Ar isotopes in quartz and pyrite minerals in different ore hosting strata,basement,interface and cap rocks are determined.Based on the characteristics of sulfur and hydrogen and oxygen isotopes,the genesis of gold deposits in different ore bearing strata is discussed,and the origin and genesis of the ore-forming fluid in the metallogenic process of the Dadu River gold metallogenic belt are discussed.The main understanding is as follows:?1?The mineral assemblages in the gold deposits of different ore bearing beds in the Dadu River gold metallogenic belt are basically similar.The metal minerals are mainly pyrite,galena and chalcopyrite,and the non metallic minerals are mainly quartz.In addition to the fluorite bearing gold bearing minerals in the Piyan deposit,the main gold bearing minerals in different ore hosting strata are pyrite and quartz.From a single deposit,the upper part of the deposit is mainly pyrite quartz vein type gold ore,and the lower part of the deposit is mainly polymetallic sulfide quartz vein type gold ore,and there is a certain mineral vertical zoning phenomenon in the whole.The assemblage of mineral assemblages in this area is probably caused by the same mineralization of different gold deposits.?2?The value of ³He/⁴He in quartz minerals in the Dadu gold metallogenic belt is between 0.30⁰.64Ra,and the value of 3He/4He in pyrite minerals is between0.08².11Ra,which is lower than the isotopic composition of the atmosphere and mantle,higher than the value of ³He/⁴He in the crust fluid;the ratio of ⁴⁰Ar/³⁶Ar in quartz minerals is changed between 758.07¹2864.47 and ⁴⁰Ar in pyrite minerals.The ratio of ³⁶Ar varies from 1148.24¹1316.21 to ⁴⁰Ar/³⁶Ar,and the ratio of mineral to mineral is also between atmospheric and mantle values.?3?There is no significant correlation between the ⁴⁰Ar/³⁶Ar and ⁴⁰Ar in the pyrite and quartz minerals in the Dadu River gold metallogenic belt,indicating that the ⁴⁰Ar in the quartz minerals is very little influenced by the radioactive origin.In general,the value distribution of ³He/⁴He-⁴He is between MORB and crustal fluid evolution line.The composition of He and Ar isotopes is characterized by the mixture of mantle fluid and crustal fluid,and from the calculation formula of the two element mixed mode,the ore fluid of the Dadu River is 10.57%of the average proportion of the mantle He,indicating the source of the ore-forming fluid of the Dadu River gold deposit.The crustal fluid is dominant.The rare gas isotopic characteristics of the gold deposits in different ore bearing beds are not exactly the same,but the overall characteristics are similar,and the mineralization,ore genesis and the source of the ore-forming fluid in different ore bearing beds are consistent.?4?The He isotopic abundances of rare gases in the pyrite and quartz minerals of different ore bearing beds are very different,but the characteristics of the rare gas isotopes are basically similar,indicating that quartz and pyrite are of the same significance in the study of the genesis of the ore deposits and the source of the ore-forming fluid.?5?Through the analysis of the characteristics of sulfur isotopes,it is concluded that the pyrite in the cover and the pyrite in the basement are derived from the deep mantle,but the sulfur source in the basement is more single and only from the mantle,and the sulfur source in the caprock is more complex,which is similar to the source of sulfur in the basement,which is derived from the deep mantle.Sulfur has been accepted in the process of upward movement of mantle material.The delta ³⁴S value in the interface is higher,indicating that the source of sulphur in the ore-forming fluid is affected by deep magmatic hydrothermal activity.?6?In combination with previous studies on the hydrogen and oxygen isotopes of quartz in the Dadu River gold metallogenic belt,the source of water in the ore-forming fluid of the Dadu River gold metallogenic belt is more complex,which may be composed of two or more mixtures of atmospheric precipitation,metamorphic water and magmatic water.The evolution process of the ore-forming fluid in the Dadu River gold metallogenic belt may be that the precipitation is under the channel of the shear zone,and the fluid from the deep part of the ore-forming fluid is also rising as the channel,which is the main source of the water in the metallogenic medium,and has a certain genetic connection with the orogenic gold deposits.