德兴朱砂红矿床含辉钼矿岩石样品Re_Os定年及地质意义

辉钼矿中Re、Os含量高且几乎不含普通Os,通常被认为是开展金属矿床定年的理想对象。然而,在一些矿床中辉钼矿常以浸染状产出,矿物颗粒微小,挑纯难度较大,本文尝试使用含辉钼矿的岩石样品进行测年实验。选择研究程度较高、年龄数据较多的德兴斑岩铜矿田朱砂红矿床的4件含辉钼矿岩石样品进行了Re_Os同位素定年,结果获得等时线年龄为172.6±2.6 Ma,与前人已获得的年龄数据基本一致,说明该年龄数据可靠,样品中的硅酸盐矿物对定年结果没有影响。德兴斑岩铜矿田中3个矿床的辉钼矿Re_Os年龄与花岗闪长斑岩的锆石U_Pb年龄相一致,表明其矿化与斑岩侵入体直接相关。而前人获得的Rb_Sr年龄及K_Ar年龄与近年得到的Re_Os、U_Pb年龄数据存在差别的原因可能是后期热液蚀变及热事件破坏或重置了Rb_Sr和K_Ar同位素体系。

Dating of molybdenite-bearing rock in the Zhushahong deposit of Dexing and its geological significance

Molybdenite occurring widely in metal deposits has been considered to be the most ideal target for directly dating deposits due to its high content of Re and Os and low content of common Os. However, it is very difficult to collect enough pure molybdenite grains for dating in many types of ore deposits due to its fine-grained mode of occurrence and low content. As we know, the Re and Os are chalcophile and siderophile elements, and they would be concentrated mainly in sulfide minerals rather than in silicate mineral crystals. So it is theoretically feasible to determine Re-Os age directly by using molybdenite-bearing whole rock samples. To examine this idea, the authors chose age well-constrained Dexing porphyry copper orefield as a case study. In the study, five molybdenite-bearing rock samples were collected from drill holes in the Zhushahong deposit for Re-Os isotopic analysis. As a result, the five samples yielded a high-quality isochron age of 172.6 ±2.6 Ma, which is the same with previously reported Re-Os age of the deposit obtained by using pure molybdenite samples. This shows that directly dating molybdenite-bearing wholerock samples is not only feasible but also reliable. Additionally, the Re-Os age of molybdenite the authors reported in the study is in accordance with the zircon U-Pb age of ore-forming granodiorite porphyry, implying that the mineralization and the porphyry intrusion are genetically related to each other. The previous Rb-Sr ages and K-Ar ages of the deposit do not coincide with the Re-Os age and the U-Pb ages, which might be caused by the late hydrothermal alteration and thermal events that damaged or reset the Rb-Sr and K-Ar isotopic systems.