滇西北衙金多金属矿床磁铁矿元素地球化学特征及其对成矿作用的制约

Geochemical characteristics of magnetite from Beiya gold polymetallic deposit in western Yunnan and its constraint on mineralization

北衙金多金属矿床处于扬子地台西缘,是与喜马拉雅期富碱斑岩具有紧密成因联系的典型矿床代表。文章在详实的野外地质工作和室内研究基础上,将北衙矿床中铁矿的成矿作用划分为早期气液交代阶段和晚期富铁质流体贯入充填阶段。电子探针和ICP-MS分析表明,磁铁矿主量元素具有富Si,贫Ti、V、Mn、Mg的特征;微量元素Th、Sr、Zr含量较低,Cs、U相对富集,一致的高场强元素(Zr、Hf、Nb、Ta)变化特征。研究表明,磁铁矿与区域内基性-超基性岩浆无关,其形成与矿区中酸性富碱岩浆密切相关。矿区中的铁矿床存在早期气液交代型和晚期贯入充填型2种矿化类型。早期交代成矿作用并不是北衙矿床中铁的唯一供给机制,还存在岩浆演化后期富铁流体贯入充填的成矿方式。

The Beiya gold-polymetallic deposit located in western Yangtze Block is a typical alkali-rich intrusion-related deposit formed during Himalayan epoch. Based on detailed field geological investigation and mineralogical and petrological study, the authors divided the ore-forming process into two stages： Early metasomatic mineralization stage and late melt-filling stage. Electronic microprobe and ICP-MS analyses of magnetite show that magnetite in the Beiya deposit is poor in Ti, V, Mn, Mg and rich in Si. Trace elements in magnetite are rich in Cs, U but depleted in Th, Sr, Zr. The values of high field strength elements（ Zr,Hf,Nd,Ta） within the magnetite of two mineralization stages are consistent. Geological characteristics and magnetite geochemistry indicate that the Beiya iron deposit is associated with the intermediate to acidic alkali-rich magma rather than with the basic rocks or ultra basic rocks, that metasomatism is not the only mechanism of iron deposition, and the intrusion of the iron-enriched melt derived from magma might also have had to do with mineralization.