锂铍络合物稳定性与花岗伟晶岩中锂铍“差异跃迁”耦合关联

Coupling relationship between the stability of Li/Be complexes and Li/Be differential enrichment in granitic pegmatites—an experimental study

伟晶岩型锂铍矿床是国家紧缺的战略性金属锂铍的重要供给矿床类型。但伟晶岩成因中深熔作用产生熔体量少、萃取锂铍效率低;岩浆结晶分异能否高度富集和高效萃取锂铍也存在争议;随着富锂铍的硅酸盐熔体、熔体-热液不混溶作用的发现,岩浆不混溶作用可能也是新的成因机制。近年来,针对伟晶岩型锂铍矿床的熔体至热液阶段成矿过程的研究主要集中在:花岗伟晶岩全岩地球化学特征解析;造岩矿物(云母、石英、长石等)与矿石矿物(绿柱石等)的精细微区元素地球化学变化规律的总结;造岩矿物、矿石矿物及副矿物(石榴石等)中熔体-热液包裹体矿物学-地球化学特征解析。但伟晶岩矿床中熔体、流体包裹体类型复杂,且与矿床形成时代也没有明显关联性。锂铍在熔体-热液相间的分配行为和分配过程,以及运移锂铍的络合物稳定性差异是深入认识锂铍超常富集机制的关键。然而,对锂铍在熔体-热液相间分配行为的研究仍然相对薄弱,针对运移锂铍的络合物的稳定性也未开展研究。我们设计了不同矿化溶液的pH值、不同钙和铝含量影响下锂铍络合物结晶锂铍的实验,发现锂铍元素在以上3种不同条件下存在明显的差异性结晶沉淀行为:(1)pH对铍络合物的稳定性控制比锂络合物更明显;(2)在pH值不变的条件下,铝的加入促进了铍的沉淀,却影响锂的沉淀;(3)钙的加入对锂沉淀的影响没有对铍沉淀的影响大。后续将从实验地球化学角度(高温高压实验模拟)剖析锂铍各自络合物的类型、稳定性受控因素,以期建立“锂、铍络合物失稳-成矿”新机制。

Pegmatite lithium(Li)-beryllium(Be)deposits are an important type of strategic Li-Be deposit.However,anatexis,on one hand,only produces a small amount of pegmatite-forming melts while Li/Be extraction efficiency for anatectic pegmatites is low;on the other hand,it is still controversial whether magmatic fractional crystallization results in high Li/Be enrichment in pegmatites and increased extraction efficiency.With the observation of fluid immiscibility in Li/Be-rich silica melt,it suggested that melt-fluid immiscibility may also lead to pegmatite formation.Current studies on the pegmatite Li-Be mineralization processes during the melt-fluid phase mainly focus on the whole-rock geochemical characteristics of granitic pegmatites,the in-situ geochemical changes of the rock-forming minerals(mica,quartz,feldspar,etc.)and ore minerals(beryl,etc.),and the mineralogical and geochemical characteristics of melt/fluid inclusions in petrogenic minerals,ore minerals,and accessory minerals(garnet,etc.).So far no obvious correlation has been found between the depositional age and the types of melt/fluid inclusions.To understand the mechanism of abnormal Li/Be enrichments in pegmatites it is key to study the distribution and stability of Li/Be complexes during the melt-fluid phase,however,the former study is scarce and the latter non-existent.In this study we investigated the effects of pH and calcium/aluminum additions on the stability of Li/Be complexes.We found that(1)the stability of Be complex was more affected by pH compared to Li complex;(2)under constant pH,the addition of aluminum promoted Be but hindered Li precipitations;and(3)the addition of calcium had less effect on Li than on Be precipitations.Future high-temperature,high-pressure experimental simulation studies should further enhance our understanding of the Li/Be enrichment processes and provide a geochemical basis for new Li/Be mineralization models based on the stability of Li/Be complexes.