湘中龙山大型金锑矿床成矿时代研究——黄铁矿Re-Os和锆石U-Th/He定年

龙山金锑矿床是湘中锑-金矿集区最重要的矿床之一,因缺少适合传统放射性同位素定年的矿物,其成矿时代以往未得到很好的限定,制约了对矿床成因的认识。由于分析测试技术的进步,Re-Os同位素定年技术得到了发展,可对热液矿床中形成的低Re、Os含量的硫化物进行较准确可靠的年龄测定,从而可为低温热液矿床的形成时代提供有效制约。锆石U-Th/He同位素定年,也是近年发展和成熟起来的定年技术,对低温热事件极其敏感,同样是约束低温成矿年龄的重要手段之一。本文采用矿床中黄铁矿Re-Os同位素和蚀变围岩中受成矿热事件影响的锆石U-Th/He同位素定年技术,对龙山金锑矿床的成矿时代进行了研究。定年结果显示:热液成因黄铁矿的Re-Os等时线年龄为195±36Ma,对应于印支晚期;锆石U-Th/He年龄为51.2~133.3Ma,经Ft校正后,U-Th/He年龄分布于93.78~258.29Ma之间,平均值为160.7±7.3Ma,对应于燕山早期。该矿床可能发生了200Ma和160Ma的两次成矿作用;或者矿床形成于200Ma左右,但是受到了160Ma左右岩浆热事件的改造,黄铁矿Re-Os年龄代表成矿年龄,而锆石U-Th/He年龄则代表第二期热事件发生的时间。无论是200Ma左右一次成矿,还是另有160Ma左右的成矿作用叠加,这两个年龄分别与区内两期岩浆活动的时间相当,这表明岩浆事件对驱动矿床的形成发挥了重要的作用。

Chronology of the Longshan Au-Sb deposit in central Hunan Province: Constraints from pyrite Re-Os and zircon U-Th/He isotopic dating

The Longshan deposit is one of the most important Au-Sb deposits in central Hunan Province. However, the mineralization age of this deposit is still not well constrained due to lacking of suitable minerals for conventional radiometric dating, limiting the recognition of its genesis. Pyrite Re-Os isotopic dating has been proved to be a reliable technique to determine the age of hydrothermal sulfides with low contents of Re and Os, which can be thus used to constrain the age of epithermal mineralization. The zircon U-Th/He isotopic dating is a newly developed technique, which can provide a potential constraint on the ages of low-temperature metallogenic events. Re-Os isotopic dating of the hydrothermal pyrites and U-Th/He isotopic dating of the zircons recording metallogenic event were carried out in this paper, aiming to constrain the mineralization age of the Longshan Au-Sb deposit. Dating results show that the Re-Os isochron age of hydrothermal pyrites is 195±36Ma, corresponding to Late Indosinian, whereas the U-Th/He ages of zircons from altered wallrock are 51.2~133.3Ma. After correction by Ft, the U-Th/He ages are distributing in the range of 93.78 to 258.29Ma with an average of 160.7±7.3Ma, corresponding to Early Yanshanian. It can be thus concluded that the Longshan Au-Sb deposit may have experienced two episodes of mineralization (i.e., ~200Ma and ~160Ma) or the Sb-Au mineralization may have occurred at ~200Ma which was remobilized by magmatic event at ~160Ma. Pyrite Re-Os age represented the timing of Au-Sb mineralization, whereas the zircon U-Th/He age represented the timing of the second thermal event. Thus, whether the Longshan Au-Sb deposit formed due to the single mineralization event at ~200Ma, or also experienced an overlapping of the mineralization event at ~160Ma, the two ages were roughly consistent with those of two magmatic events in the district, indicating that magmatism may have played a vital role in the formation of the Longshan Sb-Au deposit.