西藏列廷冈-勒青拉矽卡岩型Fe-Cu-Pb-Zn多金属矿床白云母^(40)Ar/^(39)Ar年龄及其地质意义

Ar-Ar age of muscovite from the Lietinggang-Leqingla Fe-Cu-Pb-Zn deposit in Tibet and its geological significance

以列廷冈-勒青拉Fe-Cu-Pb-Zn多金属矿床为研究对象,通过对其Fe矿化阶段白云母的定年研究,从成矿流体演化持续时间的角度对该问题进行了新的探索。系统的野外踏勘、显微镜下观察和电子探针工作表明,列廷冈-勒青拉矿床中发育与磁铁矿共生的白云母。对白云母进行的Ar-Ar同位素测年工作表明,白云母Ar-Ar坪年龄为51.00±0.38 Ma,等时线年龄为50.45±0.62 Ma,认为其代表了列廷冈-勒青拉矿床氧化物阶段铁矿化过程中开始结晶的白云母达到Ar-Ar体系封闭后的年龄。对比前人得出的辉钼矿年龄(61.96±0.58 Ma),提出二者年龄的差异可能由于不同同位素体系在不同矿物中封闭温度的不同所致,认为该矿床热液演化经历了较长时间,演化过程导致的温度梯度的出现加剧了岩浆热液中带来的多金属物质(如Pb、Zn、Cu、Fe)的分异,为矿区多种金属矿物的共存提供了重要条件;该矿床所属的冈底斯北成矿亚带相比较于其它两个成矿亚带具有更长的岩浆演化时间和区域矿化持续时间,与其丰富的矿种组合发育相一致。

From north to south, there are three metallogenic belts in the Gangdise metallogenic region of the Tibetan Plateau, in which the northern metallogenic belt has a more abundant mineral composition（Fe-Cu-Pb-Zn-Mo） than the other two belts. The developmental mechanism of polymetallic coexistence has become the key of regional metallogenetic regularity research. In this paper, the authors studied the Lietinggang-Leqingla Fe-Cu-Pb-Zn deposit. Based on the detailed study of muscovite in Fe ore-forming stage, the authors conducted a new exploration of this problem from the duration of ore-forming fluid evolution. The systematic field work, microscopic observation and electron microprobe analyses show that the magnetite-associated muscovite is developed in the Lietinggang-Leqingla deposit. Ar-Ar isotopic dating shows that the -（40）Ar/-（39）Ar plateau age of muscovite is 51. 00 ± 0. 38 Ma and the isochron age is 50. 45 ± 0. 62 Ma. Systematic analysis suggests that this age represents the age of the muscovite crustallization during the oxide stage of the Lietinggang-Leqingla deposit after the closure of the Ar-Ar system. Compared with the molybdenite age（61. 96 ± 0. 58Ma）, the authors hold that the difference of ages may be attributed to the different closure temperatures of different isotopes in different minerals. It is believed that the hydrothermal evolution process lasted a long time. The occurrence of temperature gradients caused by this long hydrothermal evolution process might have aggravated the differentiation of polymetallic materials（such as Pb,Zn, Cu and Fe） in magmatic hydrothermal fluid, and provided an important condition for the coexistence of various metal minerals. From the perspective of the metallogenic belt, the Gangdise north mineralization belt had a longer magmatic evolution time and mineralization duration compared with the other belts, which is consistent with its rich mineral assemblage.