同位素年代学研究对江西会昌密坑山锡矿田成矿作用的启示

Isotopic dating of the Mikengshan tin ore-field in Huichang county,Jiangxi province,and its implications to metallogenesis.

密坑山锡矿田是南岭地区与火山-次火山作用有关锡成矿的典型代表。本文运用锆石 U-Pb、全岩 Rb-Sr 和辉钼矿Re一()s同位素之年方法,对该区成岩和成矿年龄进行了系统测定,并据此讨论了它们的成矿意义:研究结果表明,密坑山岩体锆石 U-Pb 年龄为(136.0±1.7)Ma,全岩 Rb-Sr 等时线年龄(124.5±0.7)Ma,二个计时体系之间存在明显的时差,计算的冷却速率为8.7℃/Ma,指示岩体在成岩早期冷却相当缓慢。矿田范围内2件不同矿化类型辉钼矿的 Re-Os 模式年龄变化于(114.0±1.6)Ma 和(120.2±5 3)Ma 之间,它们均明显晚于成岩年龄,指示区内具有长时间的热液循环系统,成矿时间延续也较长,并可能经历了多期次的成矿作用区域动力地质背景和岩体地球化学特征的综合分析表明,密坑山矿田深部存在地幔物质的上涌,从而维系了高的地热场环境。这一高热场环境一方面使岩体缓慢冷却,同时也维系了长时间的热液循环系统,进而引发了大规模的锡成矿作用。

The Mikengshan tin ore-field is a typical example of tin mineralization related to volcanic-subvolcanic activity in the Nanling region of South China. A multi-radiometric dating using zircon U-Pb, whole-rock Rb-Sr and molybdenite Re-Os techniques has been carried out in systematically determining the magmatic and mineralization ages, and the metallogenic significances of the systematic differences among the ages dated by different isotopic systems are hereby discussed. Radiometric dating for the Mikengshan pluton yields a zircon U-Pb age of （ 136. 0 ± 1. 7） Ma and a whole-rock Rb-Sr age of （ 124. 5 ±0. 7） Ma, which are quite different from each other. According to the respective closure temperatures, a cooling rate of 8.7℃/Ma is thus obtained, indicating that the Mikengshan pluton was cooled quite slowly during the early rock-forming stage. Re-Os model ages for two molybdenite samples from ores of different type mineralization in the Mikengshan tin ore-field vary from （ 114. 0 ± 1.6） Ma to （ 120. 2 ± 5.3） Ma, which are all significantly later than the age of magma crystallization, implying that there was a long-time duration of hydrothermal circulation and Sn mineralization, or perhaps existed mutli-stage ore-forming events in this district. Integrated geological and geochemical data suggested that there was an upwelling of mantle materials beneath the Mikengshan ore-field and thus led a high geothermal field. The high geothermal environment on one hand kept the pluton cooling slowly, and on the other hand also led a long-term hydrothermal circulation system, thus resulted in a large-scale tin mineralization.