西藏拉隆穹窿地质特征和Be-Nb-Ta稀有金属矿化的厘定及其战略意义

Geological Characteristics and Metallogenic Types of Be-Nb-Ta Rare Metals in the Lalong Dome,Southern Tibet,China

拉隆穹窿位于北喜马拉雅片麻岩穹窿带东段,介于康马穹窿和错那洞穹窿中间。在野外地质调查和精细剖面测量基础上,通过详细的构造变形、野外岩石学和矿物学综合研究,发现拉隆穹窿由内向外由三部分组成,依次是核部、滑脱系和盖层。核部主要是一套新生代高分异的淡色花岗岩组合,未见老基底出露;滑脱系主要由一套中高级变质的云母类片岩、和夹于片岩中的大理岩、矽卡岩和变形花岗岩组成,整体属于一条规模较大的韧性剪切带;盖层主要是一套浅变质或未变质的千枚岩、千枚状板岩和粉砂质板岩组成。滑脱系与盖层之间以上拆离断层为界,下拆离断裂在穹窿中还未完全剥露出来,整体以鞘褶皱发育为典型特征。且拉隆穹窿分别经历了由南向北逆冲挤压构造、由北向南伸展剪切构造、成穹构造和近E-W向伸展构造等四期构造变形;其中第二期构造变形在穹窿中发育和保存最好。在变质作用上,拉隆穹窿保存了两类变质作用:一类是围绕穹窿核部呈环带分布的典型巴洛式变质作用,由外向内依次是硬绿泥石‒堇青石变质矿物带、石榴石‒黑云母变质矿物带、十字石变质矿物带、蓝晶石变质矿物带;另一类是受岩体热烘烤所形成的热接触变质作用,典型的变质矿物有红柱石。穹窿核部花岗岩呈良好的垂向分带特征,从下向上依次是二云母花岗岩、白云母花岗岩、伟晶质花岗岩、钠长石花岗岩和伟晶岩壳/石英壳。在稀有金属成矿方面,拉隆穹窿中共识别出三种矿化类型:矽卡岩型Be-Nb-Ta稀有金属矿、钠长石花岗岩型Be-Nb-Ta稀有金属矿和构造‒热液型Cu-Pb-Zn多金属矿,其中拉隆穹窿Be-Nb-Ta稀有金属矿床,特别是钠长石花岗岩型Be-Nb-Ta稀有金属矿的发现,极大地丰富和扩展了北喜马拉雅带稀有金属的成矿类型和成矿潜力;同时在摸清我国关键矿产资源家底、提高关键矿产资源战略统筹能力、发挥其作为大国博弈的重要利器作用等方面具有十分重要的战略意义。

The Lalong dome is located at the eastern part of the North Himalayan Gneiss Domes(NHGD),sandwiched in-between the well-studied Kangmar dome and the Cuonadong dome.The Lalong dome consists of three lithologic-tectonic units from core to rim:the lower,middle and upper units.The lower unit mainly consists of the Cenozoic leucogranites including two mica granite,muscovite granite,pegmatitic granite,and albite granite.The middle unit predominately consists of the strongly deformed mica schist,marble,skarn,granite,pegmatite,amphibolite and quartzite sequence,which is a meso-scale ductile shear zone.The upper unit consists of the Jurassic metasedimentary and sedimentary rocks that belong to the low-grade THS,and mainly composed of phyllite,phyllite slate,silty mudstone and silt-slate.The typical metamorphic minerals in this unit are predominantly cordierite,chloritoid,and andalusite.The upper detachment fault is the boundary between the middle unit and the upper unit,while the lower detachment fault is not well exposed in the Lalong dome,which is characterized by the sheath fold around the core of the Lalong dome.Rocks in the Lalong dome recorded four major deformational events:D1,top-to-S thrust resulting in N-S shortening and vertical thickening;D2,top-to-N extension characterized by vertical thinning and N-S extensional deformation;D3,doming deformation associated with intrusion of leucogranites;and D4,late extensional faults with N-S,E-W and NE trending which also cut three units of the dome.Two types of metamorphism have been identified in the Lalong dome:1)the typical Barrovian metamorphism and 2)the lower thermal contact metamorphism.Based on textures and mineral assemblages of the granites,five varieties of granite could be distinguished vertically:two mica granite,muscovite granite,pegmatitic granite,albite granite,and pegmatite shell and or quartz shell.Three types of mineralization have been first discovered in the Lalong dome:skarn-hosted rare-metal mineralization(Be,Nb,Ta,W,Sn),albite granite hosted rare-metal mineralization(Be,Nb,Ta,W,Sn,Li,Rb,Cs,Mo),and tectonic hydrothermal Cu-Pb-Zn mineralization.The discovery of the Lalong Be-Nb-Ta rare metal deposit,especially the albite granite type rare metal mineralization,not only enriched the metallogenic types and the potential of the rare metals in the northern Himalaya,but also played an important strategic role in understanding key mineral resources in China,and improved the strategic coordination capacity for key mineral resources.