阿拉善东七一山大型萤石矿床萤石钐-钕同位素年龄及地质意义

Sm-Nd Isotopic Dating of Fluorite Seperates from Dongqiyishan Fluorite Deposit, Alxa, Western Inner Mongolia

为了查明东七一山矿床的形成过程 ,对主矿体中的 5件萤石样品进行了钐_钕同位素测定 ,所获得的等时线年龄为 (5 11± 5 )Ma和MSWD值为 0 .10。此外 ,14 3 Nd/ 14 4Nd初始比值为 0 .5 1176 0± 5 (2σ)和 εNd(t)为 - 4.2±0 .5 (2σ)。依据上述同位素分析数据 ,同时结合容矿围岩和矿体的地质特征 ,可以推测 ,东七一山萤石矿床是加里东期构造_岩浆活动的产物。成矿热液来源与壳源岩浆结晶分异作用有关。

Located at the easternmost part of the Kazakhstanian-Beishan plate, the Dongqiyishan deposit is the largest fluorite deposit with the highest ore-grade in Northwest China. A great number of quartz-fluorite and fluorite veins, veinlets and stockworks occur within the Dongqiyishan porphyric granitoid stock and its metamorphic wall rocks. With a Rb-Sr isochron of (649±56) Ma, the granitoid stock has been considered as a product of Caledonian igneous activity. Five fluorite samples from one of the major orebodies of the Dongqiyishan deposit give a Sm-Nd isochron age of (511±5) Ma with ε Nd (t) value of (-4.2±0.5) (2σ). As the Sm-Nd fluorite age is in agreement with field geological evidence, it is thought to be the ore-forming age of the Dongqiyishan deposit. The ε Nd (t) values of the fluorite separates indicate that the Dongqiyishan deposit might have been formed through the following stages: (1) re-melting of the old continental block, (2) differentiation and crystallization of crust-derived magma and (3) emplacement and precipitation of hydrothermal fluids. Moreover, the Sm-Nd age data and ε Nd (t) values of the fluorite separates also indicate that the major ore-bearing granitoid intrusions and their related ore deposits were formed at a transitional continental margin during Caledonian orogeny.