Re-Os同位素定年对岩浆型Cu-Ni硫化物矿床成矿时代的制约

Re-Os同位素体系被认为是岩浆Cu-Ni硫化物矿床直接定年的有力工具。通过研究世界级典型岩浆Cu-Ni硫化物矿床的Re-Os等时线表面年龄,发现有时能得到成矿年龄,有时得到与成矿年龄不一致的表面年龄或根本得不到Re-Os等时线,还有时同一矿床的不同类型矿石给出不同的Re-Os等时线表面年龄。文章介绍了能够解释上述现象的理论模型。模型预测对与基性-超基性岩共生的Cu-Ni硫化物矿床,除一些特殊情况外,块状矿石可得到有地质意义的Re-Os等时线年龄,浸染状矿石常得到没有地质意义的假等时线;而浸染状矿石常得到有地质意义的Os同位素初始比值,而块状矿石给出的"Os同位素表面初始比值"往往没有地质意义。因此,建议在应用Re-Os同位素体系定年时要结合矿床类型、矿石类型以及成矿过程等因素综合考虑;对Re-Os数据的科学合理解释同样需要从矿床学和同位素地球化学的基本原理出发,结合矿床的实际情况,进行深入分析研究。

Re-Os dating: Constraints on mineralization age of magmatic Cu-Ni sulfide ore deposit

Re-Os isotopic geochronology has been proved to be a powerful tool for direct dating of magmatic Cu-Ni sulfide ore deposits. Through Re-Os isotopic studies of world-class ore deposits, the authors have found that the apparent Re-Os ages sometimes are consistent with or close to the mineralization age, but sometimes are inconsistent or even fail to be obtained. What is more, different Re-Os isochron ages can sometimes be obtained for different types of ores from the same ore deposit. In this paper, theoretical models were illustrated for solving the above problems. On the basis of these models, massive ores of mafic-ultramafic rock associated Cu-Ni sulfide deposits would yield geologically meaningful Re-Os age while pseudo-isochron would often be obtained for disseminated ores. Disseminated ores usually yield geologically meaningful initial Os isotopic ratio, whereas massive ores would yield geologically meaningless apparent initial ¹⁸⁷Os/¹⁸⁸Os ratio, except for a few cases. Therefore, in order to obtain geologically meaningful Re-Os age, we should take into account such factors as the type of the ore deposit, the type of the ore and the ore-forming process. Furthermore, in order to correctly interpret the geological meaning of the apparent Re-Os age, it is necessary to carry out a detailed study of the dated ore deposit based on the basic principles of economic geology and isotope geochemistry.