稀有金属矿物溶解度对花岗伟晶岩成矿作用的制约

Solubility of rare metals as a constraint on mineralization of granitic pegmatite

花岗伟晶岩型矿床是稀有金属矿床重要的类型之一。在花岗伟晶岩中,稀有金属元素Li、Be、Nb和Ta主要以独立矿物的形式存在,前人对稀有金属独立矿物在硅酸盐熔体中的溶解度及其影响因素展开了系统研究。本文综合分析了已有的实验数据,其结果表明,影响稀有金属独立矿物溶解度最为重要的2个参数是温度(T)和铝饱和指数(ASI)。因此本文建立了稀有金属独立矿物,尤其是铌锰矿和钽锰矿溶解度,与温度(T)和铝饱和指数(ASI)之间的定量关系:lg[w(Li)/10^(-6)]=-0.37×[1000/(T/K)]+4.56,R^(2)=0.44 lg[w(BeO)/10^(-6)]=-4.21×[1000/(T/K)]+6.86,R^(2)=0.91 lg[K_(sp)^(Nb)/(mg^(2)·kg^(-2))]=-(2.86±0.14)×ASI_(Mn+Li)-(4.95±0.31)×[1000/(T/K)]+(4.20+0.28),R^(2)=0.86 lg[K_(sp)^(Ta)/(mg^(2)·kg^(-2))]=-(2.46±0.11)×ASI_(Mn+Li)-(4.86±0.30)×[1000/(T/K)]+(4.00+0.30),R^(2)=0.80式中,温度T为热力学温度,ASI_(Mn+Li)(ASI=Al_(2)O_(3)/(CaO+Na_(2)O+K_(2)O+Li_(2)O+MnO),摩尔分数比)和T的适用范围分别为0.6~1.2和1073~1373 K的范围内。上述公式为估算硅酸盐熔体中稀有金属含量提供了便利,为量化花岗伟晶岩成矿模型提供了基础。稀有金属独立矿物溶解度随温度降低和铝饱和指数的增加而急剧降低,因此,在岩浆演化过程中,由岩浆侵位、分离结晶以及流体作用等因素引起的岩浆温度降低和铝饱和指数的增加,是导致稀有金属独立矿物结晶的主要机制。

Granitic pegmatite deposit is one of important types of rare metal deposits.In the granitic pegmatite,the rare metals Li,Be,Nb and Ta mainly occur in mineralogically distinct zones.Their solubilities in silicate melts have been studied systematically and shown to be mostly affected by temperature(T)and aluminum saturation index(ASI).Here we set out to establish quantitative relationships between T/ASI and the solubility of rare metal minerals(Nb and Ta)in silicate melts by multiple linear regression analysis of literature data.We found the following relationships:lgc(Li)(10^(-6))=-0.37×(1000/T)+4.56,R^(2)=0.44 lgc(BeO)(10^(-6))=-4.21×(1000/T)+6.86,R^(2)=0.91 lgK_(sp)(Nb)=-(2.86±0.14)×ASI_(Mn+Li)(4.95±0.31)×(1000/T)+(4.20+0.28),R^(2)=0.86 lgK_(sp)(Ta)=-(2.46±0.11)×ASI_(Mn+Li)(4.86±0.30)×(1000/T)+(4.00+0.30),R^(2)=0.80 Where c is percentage content;K_(sp) is solubility,in mg^(2)/kg^(2);ASI_(Mn+Li)is the molar ratio Al_(2)O_(3)/(CaO+Na_(2)O+K_(2)O+Li_(2)O+MnO),applicable between 0.61.2;and T is temperature of silicate melt in Kelvin,applicable between 10731373 K.These quantitative relationships can be used as a basis for quantifying the metallogenic model of granitic pegmatite.The sharply decrease of solubility with decreasing T and increasing ASI can be the main mechanism leading to the crystallization of rare metal minerals,because the same conditions are met during magma evolution,where magma emplacement,fractional crystallization and fluid-melt interaction cause increase of magma temperature and decrease of ASI.