青藏高原东北缘古元古代铍钽锂矿化伟晶岩脉群的发现及其成矿意义  

Discovery of the Paleoproterozoic rare metal pegmatite swarm in the Shaliuquan-Baxingte area in the northeastern margin of the Tibetan Plateau and its implications for Be-Ta-Li mineralization.

作　　者：韩杰[1,2] 王涛 李玉龙 王明[1,2] 王秉璋 曹红亮[1] 马炳春 HAN Jie;WANG Tao;LI YuLong;WANG Ming;WANG BingZhang;CAO HongLiang;MA BingChun(Geological Survey Institute of Qinghai Province,Xining 810012,China;Key Laboratory of the Northern Qinghai-Tibet Plateau Geological Processes and Mineral Resources,Xining 810012,China;Bureau of Geology and Mineral Exploration and Development of Qinghai Province,Xining 810001,China)

机构地区：[1]青海省地质调查院,西宁810012 [2]青藏高原北部地质过程与矿产资源重点实验室,西宁810012 [3]青海省地质矿产勘查开发局,西宁810001

出　　处：《岩石学报》2025年第3期884-904,共21页Acta Petrologica Sinica

基　　金：第二次青藏高原综合科学考察研究项目(2019QZKK0806);青海省地质矿产开发局项目(青地矿科[2023]33号);西部之光青年学者项目联合资助.

摘　　要：全吉地块是青藏高原北部一个具有克拉通基底性质的古陆块残片。笔者最近在沙柳泉-巴兴特地区发现30条具有工业价值的稀有金属矿体(BeO平均品位0.08%,Ta_(2)O_(5)品位0.008%~0.030%,Li_(2)O品位为0.10%~0.75%)。通过对区内具有典型意义的ρ28号伟晶岩的详细研究,发现脉体自下盘至上盘具有不对称的矿物分带和结构分带,分别为文象结构带(Ⅰ)、块状微斜长石带(Ⅱ)、糖粒状钠长石带(Ⅲ)、石英云母带(Ⅳ)。Ⅱ带和Ⅲ带全岩组成具有明显的“M”稀土四分组效应,具有高SiO_(2)、Al_(2)O_(3)以及富Na_(2)O、K_(2)O的特征,稀土元素总量非常低,轻稀土元素略微富集,Eu具负异常。Ⅳ带全岩同样具有高SiO_(2)、Al_(2)O_(3)富Na_(2)O、K_(2)O的特征,但是其稀土总量更低,Eu具正异常,呈“W”型曲线特征。所有特征均显示伟晶岩经历了高度的分异演化。II、III、IV带内铌铁矿族矿物背散射图像和化学成分指示存在2个阶段的铌钽矿化:早阶段为岩浆结晶分异主导的铌钽矿化以背散射图像中较暗,Ta/Nb比值均小于1的富铌为特征;晚阶段为流体出溶主导的岩浆-热液过渡阶段铌钽矿化以背散射图像较亮,Ta/Nb比值均大于1的富钽为特征。铌铁矿族矿物组合显示矿体内部具有复杂的相转变,其演化系列表现为CGM1a(岩浆阶段形成的铌铁矿-铌锰矿)→CGM2a(岩浆阶段交代成因的铌铁矿-铌锰矿)→CGM2b(不完全交代阶段成因的钽锰矿)→CGM1b(热液阶段完全交代成因的钽锰矿)。该演化系列对应的Ta/(Nb+Ta)平均值由0.244→0.283→0.586→0.650,即Ta元素随着交代作用进行呈现逐渐增加的过程;此外,铌铁矿族矿物也呈现铌铁矿→铌锰矿→钽锰矿的变化趋势,Nb-Ta分馏特征显著。Ⅳ、Ⅲ、Ⅱ带伟晶岩铌钽铁矿U-Pb测年确定其成岩成矿年龄为1845Ma左右,表明全吉地块内存在古元古代稀有金属成矿事件。II和III带伟晶岩全岩ε_(Hf)(t)为值为The Quanji block,located in the northeastern margin of the Tibetan Plateau,represents a fragment of an ancient continent.Recently,30 rare metal ore bodies with industrial values have been discovered in the Shaliuquan-Baxingte area in this block(with an average BeO grade of 0.08%,Ta_(2)O_(5) grade ranging from 0.008%to 0.030%,and Li_(2)O grade ranging from 0.10%to 0.75%)by the authors so far.Take the No.ρ28 pegmatite as the research object,this study exhibits that it has asymmetric mineral and textural zoning from the footwall to the hanging wall,including the graphic texture zone(Ⅰ),blocky microcline zone(Ⅱ),saccharoidal albite zone(Ⅲ),and quartz-mica zone(Ⅳ).Whole rock compositions of ZoneⅡandⅢare characterized by obvious“M”-type rare earth tetrad effects,and exhibit high contents of SiO_(2) and Al_(2)O_(3),enrichment in Na_(2)O and K_(2)O,extremely low total rare earth elements(REEs)content,slight enrichment in light REE,and negative Eu anomalies.The whole rock compositions of ZoneⅣalso exhibit similar major contents of SiO_(2),Al_(2)O_(3),Na_(2)O and K_(2)O with zones II and III,but with lower total REE content,positive Eu anomalies,and“W”-shaped REE patterns,indicating a higher differentiation of the pegmatite.The bright backscattered electron(BSE)internal images and compositions of columbite-group minerals(CGMs)from Zone II,III and IV all indicate a two stage niobium-tantalum mineralization:the early stage dominated by magmatic crystallization differentiation,and characterized by dark BSE imaging and low Ta/Nb ratios(<1);while the late stage dominated by fluid exsolution and characterized by bright BSE imaging and high Ta/Nb ratios(>1).The compositions of CGMs show complex phase transitions from CGM1a(columbite-Fe and columbite-Mn formed during the magmatic stage)to CGM2a(columbite-Fe and columbite-Mn formed by alteration during magmatic stage)to CGM2b(tantalite-Mn formed by partial alteration)to CGM1b(tantalite-Mn formed by completely alteration during hydrothermal stage),with the cor

关 键 词：全吉地块 古元古代 铍钽锂矿化伟晶岩 铌钽铁矿 

分 类 号：P588.131[天文地球—岩石学] P597.3[天文地球—地质学] P618.7