金川铜镍硫化物矿床的Re-Os“年龄”及其意义

Re-Os "ages" of Jinchuan copper-nickel sulfide deposit and their significance

分析了金川铜镍硫化物矿床的一组浸染状矿石和一组块状矿石的Re、Os含量和Os同位素组成。浸染状矿石和块状矿石分别给出Re-Os表观等时线年龄为(1126±96)Ma和(840±79)Ma,表观初始^(187)Os/^(188)Os比值为0.119±0.018和0.242±0.028。块状矿石的表观年龄与前人报道的同类矿石年龄在误差范围内一致,证明了其分析的可靠性。此结果也与前人报道的锆石和斜锆石的U-Pb年龄在误差范围内一致,说明了金川的新元古代成矿时代。而我们的和前人的工作表明,海绵陨铁状矿石和浸染状矿石给出的Re-Os表观年龄有相当大的变化范围,且都比块状矿石的年龄老。提出了两种模式解释这种现象:在约8亿年前金川矿床形成时地壳混染导致浸染状矿石和海绵陨铁状矿石的Os同位素组成不均一,从而给出偏老的表观年龄;相反,Os同位素在块状矿石中快速扩散而均一化,因此给出成岩成矿年龄。但是,还不能完全排除两期成岩成矿的可能性,即晚期岩浆活动引起了早期岩浆活动中封闭的Re-Os同位素体系的扰动。目前倾向于第一种解释,但后一种可能性还需要检验。

Re and Os concentrations and Os isotopic ratios for disseminated and massive ores from the Jinchuan Cu-Ni sulfide deposit are analyzed. The disseminated ores give an apparent Re- Os isochron age of （1 126 ± 96） Ma with an initial ^187Os/^188Os ratio of 0. 119 ± 0. 018 while the massive ores give an apparent age of （840± 79） Ma and an initial ^187Os/^188Os ratio of 0. 242 ± 0. 028. The apparent age of massive ores agrees within analytical uncertainties with that of the same kind of ores reported previously, which indicates that our data are reliable. The result is indistinguishable, within analytical uncertainties, from U-Pb ages of baddeleyite and zircons obtained by previous workers, suggesting that arock- and ore-forming event did occur in Neoproterozoic in Jinchuan. However, it is found that the apparent ages of disseminated ores obtained by this work and that of net-structured ores obtained by precious workers are different. They are all older than those of massive ores. Two models are proposed to explain the age trend shown by different kinds of ores. The Jinchuan deposit was formed at about 800 Ma ago. Because of heterogeneity in initial ^187Os/^188Os ratios of disseminated and net-structured ores caused by crust contamination, the apparent dates older than the true age were obtained. Meanwhile, the massive ores give the rock- and ore-forming age due to homogenization of initial Os isotopic ratio caused by fast diffusion of Os between closely connected sulfides in ores. However, the possibility of two stages of rock- and ore-forming processes cannot be absolutely ruled out. The Re-Os system closed during the earlier rock- and ore-forming event might be disturbed by the later one.We prefer the first model so far. Nevertheless, further work is needed to test the second hypothesis.