滇东—黔西地区峨眉山玄武岩富Nb榍石矿物学特征

Mineralogy study of Nb-rich sphene generated from the Emeishan basalts in Eastern Yunnan-Western Guizhou area, China

滇东—黔西地区广泛发育一套Nb-Ga-REE多金属矿化富集层,其底板为峨眉山玄武岩。为了探讨底板玄武岩对该矿化层的贡献,本文系统开展了玄武岩的微区矿物学及地球化学研究,尤其利用电子探针对玄武岩中的榍石进行了深入的研究和系统的面扫分析。结果发现研究区玄武岩主要为高钛玄武岩,以富榍石为特征。高钛玄武岩中含有多种形态的榍石,主要有自形榍石、半自形榍石、信封状榍石、晶粒状榍石、云雾状榍石、片状榍石、十字形榍石、团块状榍石和蚀变榍石等。榍石中微量元素除富含Nb外,还有一定量的Zr、Y、Ce,结合该地区玄武岩地球化学特征,认为研究区榍石可能有2种成因:其一主要形成于岩浆活动晚期,可能是原始基性岩浆的产物;其二是由岩浆早、中期析出的钛铁矿、晶粒状榍石、含钛辉石在岩浆后期蚀变阶段的水化作用和氧化作用下形成。另外,高钛玄武岩中榍石的表生风化蚀变作用可能是研究区发生铌矿化的主要原因,榍石为上覆的矿层提供了Nb、Zr、Y和Ce等成矿元素。

A set of enriched Nb-Ga-REE polymetallic layer is widely developed in the eastern Yunnan-western Guizhou area,with its footwall of the Emeishan basalts.In this paper,microscopic mineralogy and geochemistry of the Emeishan basalts,especially the intensive analyses and systematic elemental scanning of sphene grains in the Emeishan basalts by using the electron probe microanalysis(EPMA),have been systematically studied to explore the contribution of footwall basalts to the Nb-Ga-REE polymetallic mineralization layer.The results show that most of basalts in the study area belong to the Ti-rich basalt type featured by the sphene enrichment.The Ti-rich basalt contains various morphological sphenes including euhedral,semi-euhedral,enveloped,grain shaped,cloudy,flaky,cross-shaped,crumby,and altered sphenes.Besides the enriched Nb,sphene contains a certain amount of Zr,Y and Ce.Combining with geochemical characteristics of the Emeishan basalts,sphene could be either as a product of primitive basic magma formed mainly in late stage of magmatism or generated from the hydration and oxidation of minerals,including ilmenite,grain sphene,and Ti-rich pyroxene,formed in the early and middle stages of magmatism.In addition,the surface weathering of sphane hosted in the Ti-rich basalt could be the main reason of Nb-mineralization in the research area as the weathered sphane could have provided ore-forming elements such as Nb,Zr,Y,and Ce for the overlying ore layer.