不同类型铍矿床中绿柱石的地球化学特征及其地质意义

绿柱石是稀有金属铍最重要的载体矿物, 开展绿柱石系统的矿物学和地球化学研究对查明铍成矿条件和机制从而指导找矿具有重要意义。本文利用电子探针、LA-ICP-MS等技术手段, 对我国不同地区不同类型铍矿床中的绿柱石开展了地球化学研究, 结果表明, 伟晶岩型、岩浆热液型铍矿床中绿柱石的主要成分BeO、Al₂O₃、SiO₂含量基本相当, 还含碱金属元素Na、Li、Cs、Rb和铁镁质元素Fe、Mg、Ca等, 两种绿柱石中MgO和Cr₂O₃含量差异较大。伟晶岩型绿柱石的Al位类质同象置换程度低于岩浆热液型绿柱石, 其组分差异与成矿温度、压力和结晶介质等因素有关, 伟晶岩型绿柱石结晶于黏度高的熔体, 高温高压使Al₂O₃活度增强, 岩浆的固相线温度降低, 使得Be以络合物的稳定形式迁移并沉淀, 不易携带外来元素; 浆热液型绿柱石结晶于粘度低、流动性更好的热液流体中, 温度和压力较低不足以形成稳定的络合物, 导致体系中元素分散, 且热液对成矿元素的溶解度更高, 易于与围岩发生强烈的交代反应产生元素置换。绿柱石中离子类质同象置换产生的Fe、Cr、V和Mg导致绿柱石呈现不同色调和亮度的绿色, 其中Mg可能是致翠绿色的原因之一; 伟晶岩型铍矿床中更可能形成富铬型绿柱石, 岩浆热液型铍矿床中更可能形成富钒型绿柱石, 也可能形成富铬型绿柱石, 推测是由于岩浆热液易于与围岩发生蚀变反应, 使基性岩-超基性岩中的Cr, 或变沉积岩中的V进入绿柱石结晶介质而形成祖母绿。本文提供了两种区分伟晶岩型和岩浆热液型绿柱石的指标, 即绿柱石Na/Li值一定程度上呈现出伟晶岩型绿柱石小于岩浆热液型的特征; Rb-Cs图解中由两条趋势线(岩浆热液型K₁=1.257, 伟晶岩型K₂=0.015)限定的I区(K≥1.257)多为岩浆热液型绿柱石, Ⅲ区(K≤0.015)多为伟晶岩型绿柱石。绿柱石的Mg-Cr组分多受到碳酸盐组的影响, 翠绿色绿柱石多形成于碳酸盐岩赋矿或含碳酸盐矿物的矿脉中。本次研究的伟晶岩绿柱石样品所处矿床多赋存于变泥质岩中, 形成环境缺乏碳酸盐类矿物, 今后应注意在赋存于碳酸盐岩中的伟晶岩脉中寻找祖母绿。

Geochemical characteristics and geological significance of beryl in different types of beryllium deposits.

Beryl is the most important carrier mineral of the rare metal beryllium. Beryl of excellent quality and color, such as emerald and aquamarine, has a certain ornamental and collection value. It is of great significance to study mineralogy and geochemistry of beryl system for identifying metallogenic conditions and mechanism and guiding prospecting. In this paper, the geochemistry studies on beryl in different beryllium deposits in different regions of our country have been investigated by using electron probe and LA-ICP-MS and other techniques. The results show that the contents of BeO, Al2O3 and SiO2 of beryl in pegmatite and magmatic hydrothermal beryllium deposits are similar. There is certain alkali metal elements Na, Li, Cs, Rb and iron and magnesium elements Fe, Mg, Ca, etc., in Beryl. The contents of MgO and Cr2O3 in the two beryl are significantly different. The degree of isotropic displacement of pegmatitic beryl is lower than that of magmatic hydrothermal beryl. The difference of beryl components is related to the temperature, pressure and crystallization medium of mineralization. Pegmatitic beryl crystallizes in a melt with high viscosity. High temperature and high pressure enhance the activity of Al2O3 and reduce the solid-phase line temperature of magma, so Be migrates and precipitates in a stable form of complex and is not easy to carry foreign elements. Magmatic hydrotherma beryl crystallizes in the hydrothermal fluid with low viscosity and better fluidity, and the low temperature and pressure are not enough to form a stable complex, resulting in the dispersion of elements in the system. Moreover, the hydrothermal solution has a higher solubility of ore-forming elements, and it is easy to have a strong metasomatic reaction with the surrounding rock to produce element replacement. Fe, Cr, V and Mg produced by isotropic substitution of ions in beryl lead to different shades and luminance of green, and Mg may be one of the causes of emerald green. Chromium-rich beryl is more likely to form in pegmatitic beryllium deposits. Vanadium-rich beryl and chrome-rich beryl are more likely to be formed in magmatic hydrothermal beryllium deposits. Vanadium-rich beryl is more likely to form in magmatic hydrothermal beryllium deposits, as is chrome-rich beryl. It is speculated that the magmatic hydrothermal fluid is easy to undergo alteration reaction with the surrounding rock, so that Cr in basic-ultrabasic rock or V in metamorphic argillaceous rock enters the beryl crystalline medium and forms emerald. In this paper, two indexes are provided to distinguish pegmatite type from magmatic hydrothermal type beryl, namely, the Na/Li value of beryl shows that pegmatite type beryl is less than magmatic hydrothermal type to a certain extent. In Rb-Cs diagram, zone I (K≥1.257), defined by two trend lines (magmatic hydrothermal type K1=1.257, pegmatitic type K2=0.015), is mostly magmatic hydrothermal type beryl, Zone Ⅲ (K≤0.015) is mostly pegmatitic type beryl. The Mg-Cr component of beryl is mainly affected by the carbonate formation, and the emerald green beryl is mostly formed in the carbonate ore-bearing rocks or the veins containing carbonate minerals. The deposits of pegmatite beryl samples in this study are mostly in metamorphic argillaceous rocks, and the formation environment is short of carbonate minerals. In the future, attention should be paid to finding emeralds in pegmatite dike occurring in carbonate rocks.