阿尔泰大喀拉苏稀有金属矿床绿柱石矿物化学特征及成矿意义

Geochemistry of beryl from Dakalasu rare metal deposit,Altai,and its implications for mineralization

稀有金属铍是国民经济发展急需的战略性金属资源。为深入了解阿尔泰造山带稀有金属铍的富集机制,本文选取大喀拉苏花岗伟晶岩型稀有金属矿床的绿柱石为研究对象。针对伟晶岩不同部位的绿柱石进行岩相学观察和原位微区分析,具体产状包括1号伟晶岩脉的粗粒边缘带(Brl-Ⅰ)、块体中间带(Brl-Ⅱ)和细粒带及晶洞(Brl-Ⅲ)。野外及岩相学观察发现粗粒边缘带及块体中间带绿柱石晶体大小约20~50 cm,其中Brl-Ⅰ绿柱石与黑色电气石、碱性长石、钠长石、石榴子石和磷灰石等共生;Brl-Ⅱ绿柱石与黑色富钽矿物、钠长石、白云母、热液锆石等共生,边缘部位以细粒绿柱石(<1 cm)为主;Brl-Ⅲ绿柱石与钠长石、白云母等共生。绿柱石主微量元素特征显示这3种绿柱石均为Na-Li绿柱石种属,元素替代机制显示Brl-Ⅰ、Ⅱ为Na(Fe^(2+),Mg)□_(-1)Al_(-1),Brl-Ⅲ为NaLi□_(-1)Be_(-1)。Brl-Ⅰ的Na/Cs和Mg/Fe均大于20.46和0.31,而Brl-Ⅱ、Ⅲ的Na/Cs和Mg/Fe值分别小于20.44和0.20。矿物共生组合和元素比值变化说明大喀拉苏伟晶岩母岩浆早期富含大量的挥发分B元素和碱性长石等矿物,晚期岩浆以富集钠长石为特征。结合实验岩石学数据及瑞利分馏模式得出,结晶分异程度达95%时可造成熔体Be饱和,继而晶出绿柱石等稀有金属矿物。本文及前人数据显示大喀拉苏矿床稀有金属铍的富集主要受控于结晶分异和熔体-熔体-流体不混溶作用。绿柱石与碱性黑电气石的共存可能可以作为寻找铍矿床的标志。

Beryllium as an important strategic metal is widely applied in many fields.This paper focuses on the enrichment of beryllium in the Chinese Altai orogenic belt,the beryl of the Dakalasu rare metal-bearing granitic pegmatite has been studied.The beryls were sampled from the coarse-grained wall zone(Brl-Ⅰ),intermediate blocky zone(Brl-Ⅱ),and fine-grained and miarolitic zone(Brl-Ⅲ).Petrographic observations and in situ analyses of the major(EPMA)and trace elements(LA-ICP-MS)were carried out.The grain size of beryl from the coarse-grained and intermediate zone are 20-50 cm,whereas the beryls from the fine-grained zone are smaller than 1cm.Brl-I coexists with black tourmaline,alkaline feldspar,albite,garnet and apatite,Brl-Ⅱis associated with dark Ta-bearing minerals,albite,muscovite and hydrothermal zircon,and Brl-Ⅲgrows with albite and muscovite.Elemental compositions of beryl show that these beryl are Na-Li beryl species,and the element substitution mechanisms are Na(Fe^(2+),Mg)□_(-1)Al_(-1)for both Brl-ⅠandⅡ,NaLi□_(-1)Be_(-1)for Brl-Ⅲ.On the one hand,the Na/Cs and Mg/Fe ratios of Brl-Ⅰare greater than 20.46 and 0.31,respectively.On the other hand,Brl-ⅡandⅢshow apparently low values,i.e.,Na/Cs and Mg/Fe ratios are lower than 20.44 and 0.20,respectively.In line with these evidences,the parental magma of the Dakalasu pegmatite is rich in volatile(B)and alkaline feldspar at the early stage,and subsequently it becomes enriched in albite due to progressive fractional crystallization.Together with the experimental petrology and the Rayleigh fractionation model,it can be concluded that fractional crystallization up to 95%can lead to Be saturation in the melt,followed by the crystallization of rare metal minerals such as beryl.This paper and previous research show that the enrichment of beryllium in the Dakalasu deposit is mainly due to crystallization fractionation and melt-melt-fluid immiscibility.The coexistence of beryl and black alkaline tourmaline can likely be used to beryllium exploration.