水晶晶体中的球粒状与管状铁氧化物包裹体的形貌结构与成因

The Shape Texture Feature and Genesis of Pelletoidal and Tubular Iron Oxides Inclusion in Rock Crystals

产出在四川美姑峨眉山玄武岩中的水晶有两种类型,即产出在玄武岩气孔中第一成矿阶段紫色水晶和"三色"层水晶与玄武岩层间滑(错)动裂隙或晶洞中第二成矿阶段的浅色水晶。晶体中铁氧化物包裹体为镜铁矿、赤铁矿、纤铁矿。包裹体的形貌结构有两种类型:其一具三圈层构造球粒(0.15～0.5?),它由球核、球壳与表层三部分组成,球核与表层为红棕色,球壳为铁黑色,称"红夹黑"。扫描电镜(SEM)与能谱分析(EDS)表明,球壳由绕球核呈放射状排列的片状镜铁矿组成,表层为红色半透明鳞片状纤铁矿,球核由微晶赤铁矿定向聚合呈圆球形或向外伸出的圆柱形,球粒产出在"三色"层水晶的红棕色间歇层的夹层(0.4～0.7?)中;其二具两圈层构造的中空管,管的长轴方向垂直水晶的菱面体单形晶面方向。SEM及EDS测试表明,管中心为空管,内圈为铁黑色片状镜铁矿,外圈为厚度不一的红棕色赤铁矿,管的形貌有滴管状、钟乳状、倒葫芦状,晶出在第二成矿阶段第一世代条带状水晶的条带中。"三色"层及条带状水晶中的包裹体是强酸性氧逸度高、且热动力生长环境条件极不稳定的产物。水晶流体包裹体均一温度及拉曼光谱分析研究,水晶矿属低温热液(150～190℃)成矿,成矿流体的盐度较低,5.7%～13.2%NaCleq,其中紫晶盐度最高(13.2%NaCleq),条带状水晶盐度较低(6%NaCleq)。流体包裹体主要由H2O和SO2及微量CH4组成。这项研究对了解铁氧化物生长机理、生长速率、生长环境热动力条件变化及成矿阶段划分都具重要意义,同时对研究现今表生氧化环境所形成铁氧化物种类、机理也有现实指导意义。

There occur two types of rock crystals in Emeishan basalt from Meigu county, Sichuan province： amethyst and the ＂triehroie＂rock crystal of the first ore-forming stage in vesicles of basalt and the light color rock crystal of the second ore-forming stage in interlayer-gliding（faulted） fissures or vugs of basalt. The iron oxides in rock crystals are specularite, hematite and lepidocrocite. There are two iron oxides of two types of shape textures： The one is three-circle layered structure peloids occurring within partings（0.4~0.7 mm） of red--brown interstitial layers of ＂trichroic＂rock crystals ,with the diameter of 0. 15~0.5 ram, consisted of the spherulite, coccosphere and the surface, the spherulite and the surface are red--brown, but the coccosphere is ferri-black, so called ＂ maily red with black＂. Scanning electron microscope（SEM） and energy dispersive spectrometer（EDS） analysis indicated that the coccosphere is consisted of the sheet speeularite arranged radially around the nucleus. The surface is red semitransparent scale-likeo the form of circular spheres or epitaxial growth circular prisms. The other is the two-circle layered structure central-cavity tube, was crystallized in bandings of banded rock crystals of the first generation of the second ore-forming stage. The direction of the macro-axis of the tube is vertical to the direction of the simple-form crystal face of rhombohedron of the crystal. SEM and EDS analysis indicated that the center of the tube is hollow. The endocircle is ferri-black sheet specularite. The direction of the sheet is vertical to the macro axis of the tube. The exoeircle is different-thikness red--brown hematite. The tube is drip-like, stalactitic and inversion calabash-like. Both three-circle layered structure peloids and the two-circle layered structure central-cavity tube were formed on the condition of the strongly acidic and highly oxygen fugacity environment. By using the analysis of the homogenization temperature of the fluid inclusion and Raman spectrum of the crystals, the rock crystals were formed on the condition of epithermal （150-190℃） ore deposit genesis. The salinity of the ore-forming fluid is lower. 5.7%~13.2% NaCleq. The salinity of amethyst is highest（13.2% NaCleq）. The salinity of banded rock crystals is lower（6% NaCleq）. The fluid inclusion is consisted mainly of H20, SO2and a small amount of CH4. This study has important significance to understand the growth mechanism, the growth rate, the variation of therrmodynamies under the given condition in ferrous oxide and the differentiation of mineralization stage. It also provides a useful guide to studying the species and mineralization of ferrous oxide under the supergene oxidation condition.