ABSTRACT

The Dahongliutan rare-metal pegmatite deposit, associated with the pegmatite dikes hosted by Dahongliutan pluton and metasedimentary rocks, is a new discovered Li–Be deposit in the the West Kunlun orogen, Northwest China. The Dahongliutan pegmatite is a classic and typical example of a highly fractionated Li-Ce-Ta (LCT) type pegmatite, with distinct regional zonation composed of well-developed garnet-tourmaline-microcline (GTM), beryl-tourmaline-muscovite (BTM) and spodumene-albite (SA) pegmatites from the barren core inward to outer zone. Detailed field studies, together with B-isotope studies of tourmaline and the chemistry of feldspar, muscovite and tourmaline, were undertaken to investigate the differentiation processes in a pegmatite magma and the different mineralizing episodes of the Dahongliutan rare-metal pegmatite deposit. Alkali metal fractionation trends (Rb, Cs and K/Rb) in the pegmatitic K-feldspar and muscovite define a primitive to evolved magmatic evolution. From low to high degrees of evolution, the mineralization stages of Dahongliutan pegmatite field are metal barren in centre, Be-rich in the intermediate and Li-rich in the outer zone. Furthermore, besides the magmatic tourmaline in the GTM pegmatite dikes, the fine-grained euhedral magmatic BTM-I type and subhedral to anhedral coarse-grained magmatic-hydrothermal BTM-II type in the BTM pegmatite dikes have been identified. The average δ¹¹B values of the magmatic tourmalines in the GTM-type is −8.82, BTM-I type is −8.53‰, and BTM-II type is −7.85‰. Combined with petrography and chemical data, we suggest that the Dahongliutan pegmatite dikes were derived from metasedimentary source within the continental crust and the BTM-II type tourmaline is correlated with separation of an immiscible B-rich fluid during the magmatic-hydrothermal evolution. Consequently, highly fractional crystallization, albite alteration and liquid immiscibility were the main factors controlling the spatial and temporal decoupling of rare element mineralization in the Dahongliutan pegmatite deposit.

摘要

大红流滩稀有金属伟晶岩矿床与大红流滩岩体和变沉积岩发育的伟晶岩脉共生，是西昆仑造山带新发现的锂-铍矿床。大红柳滩伟晶岩是典型的高分馏锂-铈-钽（LCT）型伟晶岩，具有明显的区域分带，由发育良好的石榴石-碧玺-微斜岩（GTM）、绿柱石-碧玺-白云母（BTM）组成和锂辉石钠长石 (SA) 伟晶岩从贫瘠的核心向内到外层。进行了详细的实地研究，连同电气石的 B 同位素研究以及长石、白云母和电气石的化学性质，以研究伟晶岩岩浆的分异过程和大红柳滩稀有金属伟晶岩矿床的不同成矿期。伟晶岩钾长石和白云母中的碱金属分馏趋势（Rb、Cs 和 K/Rb）定义了从原始到进化的岩浆演化。大红柳滩伟晶岩田从低到高演化程度为中部贫金属、中部富Be、外围富锂的成矿阶段。此外，除了 GTM 伟晶岩脉中的岩浆碧玺外，还发现了 BTM 伟晶岩脉中的细粒自形岩浆 BTM-I 型和亚自体至反面体粗粒岩浆热液 BTM-II 型。平均δ 中带富Be，外带富Li。此外，除了 GTM 伟晶岩脉中的岩浆碧玺外，还发现了 BTM 伟晶岩脉中的细粒自形岩浆 BTM-I 型和亚自体至反面体粗粒岩浆热液 BTM-II 型。平均δ 中带富Be，外带富Li。此外，除了 GTM 伟晶岩脉中的岩浆碧玺外，还发现了 BTM 伟晶岩脉中的细粒自形岩浆 BTM-I 型和亚自体至反面体粗粒岩浆热液 BTM-II 型。平均δGTM型岩浆碧玺的^{11 B值为-8.82，BTM-I型为-8.53‰，BTM-II型为-7.85‰。}结合岩相学和化学数据，我们认为大红柳滩伟晶岩脉来源于大陆地壳内的变质沉积源，BTM-II 型碧玺与岩浆-热液演化过程中不混溶的富 B 流体的分离有关。因此，高度分离结晶、钠长石蚀变和液体不混溶性是控制大红柳滩伟晶岩矿床稀有元素矿化时空解耦的主要因素。