西南天山巴什苏洪稀有稀土矿地质特征及控矿因素研究

西南天山巴什苏洪矿是新发现的REE矿床,本文以三年勘探工作为依托,开展了岩石学和岩石地球化学研究,总结了矿床的地质特征、控矿因素和找矿标志。结果表明:巴什苏洪杂岩体可识别出四期,其中第一期为中粒橄榄辉长岩体,第二期为中粗粒霓石正长岩体,第三期为火成碳酸岩体,第四期为中细粒碱性花岗岩体。杂岩体为过碱性,从碱性辉长岩类→正长岩类→碱性花岗岩类呈现出完整的演化趋势。各期岩体中橄榄辉长岩整体体现出轻稀土富集、分异不明显的特征;霓石正长岩体现出轻稀土富集,并且出现了负Eu异常,已经体现出部分结晶分异的特征;相对而言碱性花岗岩稀土总量更高,整体呈现出强烈的负Eu异常,但并未发生强烈的轻重稀土分馏,整体呈现出"海鸥式"配分特征,可能与岩浆晚期的伟晶岩以及流体活动有关。巴什苏洪矿床属碱性花岗岩和碱性伟晶岩复合型矿床,岩体有两种含矿形式:含稀有稀土的碱长花岗岩脉和侧向的霓长伟晶岩。主矿体为第四期中细粒碱性花岗岩,近东西向呈密集脉状,陡倾,分布稳定连续。估算铌钽资源量超过1万t,达中型规模,具有成为低品位、中厚度、大范围、复合型的大型稀有稀土金属矿床远景。构造、岩浆岩和岩浆分异作用共同控制矿体的形成。找矿标志包括:遥感影像灰白色条带,航磁弱正异常,化探Nb-Y-Zr-Zn-P组合异常,地表碱性花岗岩株产出于深大断裂附近,出现独居石、铌铁矿-铌钽铁矿等有用矿物以及霓石、钠长石等有利共生矿物组合等。

Research on geologic features and ore controlling factors of Bashisuhong REE deposit in Southwest Tianshan

The Bashisuhong deposit is a newly discovered REE deposit in southwest Tianshan Mountains. On the basis of three years of exploration, this work conducted petrology and petrogeochemistry study of this deposit, and summarized the geological characteristics, ore-controlling factors and prospecting indicators. The result shows that Bashisuhong complex can be divided into four stages: the first is medium-grained olivine gabbro; the second is medium- to coarse-grained aegirine syenite; the third is igneous carbonatite; and the fourth is of medium- to fine-grained alkaline granite. This complex is of peralkaline with a complete evolution trend of alkaline gabbro to syenite to alkaline granite. Among rock masses of each period, the olivine gabbro is characterized by light rare-earth enrichment without obvious differentiation as a whole; the aegirine syenite shows light rare-earth enrichment and negative Eu anomalies, partially featured by crystallization differentiation. In comparison, the alkaline granite has relatively higher total REE content, showing strong negative Eu anomalies on the whole but no strong light-heavy rare-earth fractionation, and is of seagull-type distribution characteristics, which is possibly related to the pegmatization and fluid activities in the late magmatic period. The Bashisuhong deposit belongs to alkaline granite and alkaline pegmatite composite deposits and contains two types of orebodies, i.e., alkali feldspar granite vein with rare rare-earth and aegirine pegmatite. The main orebody is the medium- to fine-grained alkaline granite of the fourth period, showing a shape of dense veins in nearly EW direction with steep dip, and is distributed stably and continuously. It has estimated Nb and Ta resources of over 10, 000 tons, reaching a medium scale with a prospect of becoming a composite large-scale REE deposit featured by a low grade, medium thickness and wide range. Structures, magmatic rocks and magmatic differentiation jointly controlled the formation of orebodies. Prospecting indicators include gray white strips on remote sensing images, weak positive aeromagnetic anomalies, combined Nb-Y-Zr-Zn-P anomalies from geochemical exploration, alkaline granite stocks near deep major faults, and the appearance of useful minerals such as monazite and niobate-niobate-tantalite, and favorable paragenetic mineral assemblages such as aegirine and albite.