Re-os Dating Method: A Case Study Of The Molybdenite From Tungsten Deposit In Xihuashan

With the advances in both chemical separation procedures and chemical clean room,the Re-Os isotopic system is applied in a wide variety of samples,and also is playing an important role in dating of metallic ore deposits,which is helpful to the study on the source of ore-forming materials.On the basis of the seniors'research,the article tried to establish a simple and quick method to determine the age of molybdenite Re-Os.This method simplified the chemical separation of Re and Os using concentrated HNO₃ for digestion of molybdenite.The supernatant of the solution was then diluted to an appropriate volume.M187/M185 ratio of the resulting solution was determined directly by Inductively Coupled Plasma-Mass Spectrometry?ICP-MS?.According to the principle of radioactive decay,it can be inferred that the mass ratio is positively correlated with the age.The Re-Os age of the sample can be calculated by comparing with the M187/M185 ratio of the molybdenite standard material?JDC and HLP?.The method was validated using national certified molydenite standads,and the results were well consistent with the certified values.Using this method for determination of different sizes of molybdenite sampled from Xihushan Mo deposit.Based on the analyses of experimental method,the characteristics of molybdenite and the theory of Re-Os isotope dating,new development is achieved in the following three aspects:?1?A simple and quick method to determined the age of molybdenite Re-Os.Molybdenite samples were digested with HNO₃.M187/M185 ratio of the ratio of the resulting solution was determined directly by the Inductively Coupled Plasma-Mass Spectrometry?ICP-MS?.National reference materials and molybdenite samples from Tungsten Deposit in Xihuashan were analyzed.The resulting ages were identical to the reference ages within 10%.The results show that the data is reliable.?2?This method,without spike addition,can avoid the trouble of sealing and opening the Carius tube and the process of separating and purifying Re and Os,which significantly improves the experimental efficiency.This simple and quick method is highly suitable for the preliminary analysis of molybdenite Re-Os ageing.?3?Test data instability may have been caused by the decoupled within molybdenite grains.For some coarse-grained molybdenites,the analysis of small sample aliquants may fail to yield accurate or reproducible Re-Os ages,whereas the analysis of larger aliquants from the same mineral separate does yield reproducible Re-Os ages.Such an observation can be best explained if Re and ¹⁸⁷Os are internally decoupled within molybdenite grains.The degree of decoupling between Re and ¹⁸⁷Os seems to increase as a function of both increasing grain size and increasing age of molybdenite.In practive,it is suggested to use enough amounts of molbdenite,which shoud be homogenized fine-grain in order to obtain representative Re-Os age.