一种特殊类型的云英岩:湘南香花岭地区癞子岭云英岩成岩成矿特征

The Petrogenesis and Mineralization of the Laiziling Greisen,Xianghualing District, Hunan Province, South China

癞子岭岩体具有极好的垂向分带性,从下部到顶部包括了花岗岩、云英岩和伟晶岩,其中云英岩以其厚度巨大,云母类型属于铁锂云母,黄玉含量高,W-Sn-Nb-Ta含量高,而区别于其他地区云英岩。通过对癞子岭云英岩进行岩石学、地球化学和矿物学的研究,本文得出:癞子岭云英岩是高硅的强过铝质岩石类型,全碱含量低(3~4.3 wt%),富集挥发组分,全岩Zr/Hf(~8)和Nb/Ta(~1.7)比值低。造岩矿物铁锂云母中Nb(~74×10^(-6))、Ta(~66×10^(-6))、W(~23×10^(-6))、Sn(~75×10^(-6))等成矿元素含量较高。副矿物锆石自形且成分均一,含有HfO_2约10 wt%,Zr/Hf比值最低为5,与云英岩下部的癞子岭钠长花岗岩中的锆石成分有连续过渡的关系。这些特征与南岭地区高演化稀有金属花岗岩或伟晶岩相当,体现了相近的演化程度。癞子岭云英岩中有明显的Nb-Ta-W-Sn成矿作用发生,主要形成铌铁矿族矿物、锡石和黑钨矿,成分和结构均具有岩浆成因特征。花岗质熔体中含有大量挥发组分Li和F,结晶出黄玉和Li-F云母,F在稀有金属的成矿作用和云英岩的成岩过程中发挥了非常重要的作用,成矿作用发生在岩浆演化的晚期并伴随有流体作用。因此,云英岩可能是钠长花岗岩高度分异演化之后的特殊产物,这为研究花岗岩岩浆-热液体系成岩成矿过程提供了新的窗口。

The Laiziling granitic pluton comprises three rock phases with a pronounced vertical lithological zonation,granite,greisen and pegmatite from base to top.Greisen from the Laiziling granitic pluton is characterized by its huge thickness,micas belonging to the zinnwaldite,high abundance of topaz and high W-Sn-Nb-Ta concentration.Petrological,geochemical and mineralogical studies have been carried out on the Laiziling greisen.It is concluded that the Laiziling greisen is a peraluminous rock with high silicon content,low content of total alkali(3-4.3 wt%),Zr/Hf(~8)and Nb/Ta(~1.7)ratios,and enrichment of the volatiles.Micas in the Laiziling greisen are classified into zinnwaldite,and LA-ICP-MS analysis results indicate they have high content of Nb(~74×10^-6),Ta(~66×10^-6),W(~23×10^-6)and Sn(~75×10^-6)concentrations.Zircons in the Laiziling greisen are commonly euhedral and homogeneous and contain high HfO2(~10 wt%)content,and Zr/Hf ratio as low as 5.The compositions of zircon display a successive change compared with those from the Laiziling albite granite.These chemical characteristics of the greisen are similar to those of highly fractionated rare-metal granites and pegmatites in the Nanling Range with similar fractionation degree.Columbite-group minerals,cassiterite and wolframite are found in the Laiziling greisen,suggesting the obvious W-Sn-Nb-Ta mineralization.Both mineral textures and chemical compositions indicate these oxide minerals formed in the magmatic stage.Granitic magma contains high concentrations of volatiles(Li and F),which play a very important role in the petrogenesis and the rare-metal mineralization of greisen.Li and F were incorporated into the Li-F mica and topaz and rare metal mineralization formed in the late stage of magmatism enriched in large amount of fluid.Consequently,greisen could probably be the product by the strong fractionation and fluid activity after the albite granite formed in the same granitic chamber.The results should open a new window for understanding the petrogenesis and rare-metal mineralization in the granitic magmatic-hydrothermal system.