四川木落稀土矿床方解石铂族元素地球化学特征

PGE geochemistry of calcite in Muluo REE deposit,Sichuan,China

方解石作为木落稀土矿床常见的脉石矿物,其中的铂族元素(简称PGE)地球化学特征有可能记录了地质流体的性质。采用ICP-MS分析木落方解石中PGE的含量,并对铂族元素的分布、相关性、成因进行了探讨。木落方解石可以分为两类:I型方解石和Ⅱ型方解石。I型方解石中∑PGE(不含Os)0.62～1.33ng/g,具相对低的Pd/Pt、Pd/Rh、Pd/Ru、Pd/Ir比值,不太显著的Pt-Pd分配模式,为岩浆成因方解石,与成矿作用密切相关;Ⅱ型方解石中∑PGE(不含Os)1.85～2.97ng/g,具相对高的Pd/Pt、Pd/Rh、Pd/Ru、Pd/Ir比值和显著的Pt-Pd分配模式,为热液成因方解石,代表了成矿作用后的一期地质流体作用,与成矿关系不大,仅局部地区存在改造前次流体作用形成的稀土矿体。富CO2热液具有携带PGE的能力,并能够导致PGE的分异,与富CO2岩浆相比,富CO2热液对铂族元素具有一定程度的富集作用。

Calcite is the familiar vein mineral in the Muluo rare earth elements deposits; the characteristics of platinum group elements （PGE） geochemistry of the calcite may record the nature of geofluid. The contents of PGE of calcite were determined by ICP-MS and the distribution, correlation and origin of PGE of calcite were analyzed. The calcite may be classified into two types as typeI and type Ⅱ. The type Ⅰ calcite has the content of ∑PGE （excluding Os） from 0. 62 to 1.33 ng/g with indistinct Pt-Pd distribution model and lower ratios of Pd/Pt, Pd/Rh, Pd/Ru and Pd/Ir. Therefore, it is of magmatic origin and close to metallogenesis. The type Ⅱ calcite has the content of ∑PGE （excluding Os） from 1.85 to 2. 97 ng/g with distinct Pt-Pd distribution model and relatively high ratios of Pd/Pt, Pd/Rh, Pd/Ru and Pd/Ir. Therefore, it is the hydrothermal calcite representing the action of geofluid, and is not close to metallogenesis but related to the reconstruction of the rare earth elements ore body in the local area. The thermal fluid may carry and differentiate the PGE. In contrast with carbonatite magma, the thermal fluid enriched carbon dioxide may concentrate the PGE.