摘要
桂北苗儿山-越城岭岩基是一个多时代复式岩体,出露有新元古代、加里东期、印支期和燕山期花岗岩,与花岗岩相关有多个加里东期和印支期的大型W矿床,如牛塘界钨(锡)、界牌钨铜、云头界钨钼矿床等,另外越城岭的戈洞坪地区还发现了燕山期铌钽成矿作用。豆乍山岩体位于苗儿山岩体中部,主要由二云母花岗岩组成,本次研究在豆乍山钻孔样品中发现富含Nb-Ta-W氧化物的(含电气石)白云母花岗岩,为进一步全面认识苗儿山-越城岭地区的多时代成矿信息,查明不同类型成矿过程,对钻孔内花岗岩及其成矿作用开展了详细的研究。钻孔样品主要包含二云母花岗岩和细粒的(含电气石)白云母花岗岩,其中细粒的(含电气石)白云母花岗岩穿插了主岩体。详细的矿物学工作显示(含电气石)白云母花岗岩中含有多样的稀有金属副矿物,其中白云母花岗岩中有铌铁矿族矿物、铌铁金红石、锡石等氧化物,而含电气石白云母花岗岩则包含了复杂的Nb-Ta-W的氧化物矿物集合体,主要为铌铁矿族矿物、钨铌铁矿、骑田岭矿和黑钨矿等;另外还在白云母花岗岩中发现一些富含白钨矿的电气石细脉。白云母花岗岩中岩浆锡石U-Pb定年结果为219±4 Ma,属于印支期成矿作用。结合岩相学观察结果和云母的微量元素结果显示,含电气石的白云母花岗岩中钨富集程度更明显,形成更多铌钽钨的氧化物,后期随着含B流体的进一步聚集,在白云母花岗岩晚期穿插的电气石细脉中形成了大量的W的独立矿物,黑钨矿和白钨矿。相对于二云母花岗岩,(含电气石)白云母花岗岩全岩成分具有较高的Na和P含量和较低的K、Ti和Fe含量,较低的稀土总量,且四分组效应更明显,它们还含有更高的稀有金属元素Nb、Ta、W、Sn等(最高分别可达56×10-6、40×10-6、547×10-6和89×10-6)和较低的Nb/Ta和Zr/Hf比值(分别低至1.4和11.2)。这些全岩地球化学参数显示了(含电气石)白云母花岗岩更高的演化特征。通过与苗儿山-越城岭岩基中多个印支期二云母花岗岩、白云母花岗岩进行全岩地球化学对比,云头界含电气石白云母花岗岩具有与豆乍山(含电气石)白云母花岗岩相似的特征,推测该W矿床也可能具有Nb-Ta富集的潜力。
Miao’ershan-Yuechengling batholith,located in the north Guangxi,is a multi-aged composite granite pluton,including Neoproterozoic,Caledonian,Indosinian and Yanshanian granites.Granites in this region are related to several large tungsten(W)-depositsformed in the Caledonian and the Indosinian,such as the Niutangjie W-(Sn)deposit,the Jiepai W-Cu deposit and the Yuntoujie W-Mo deposit.Additionally,Yanshanian niobium-tantalum mineralization is found in the Gedongping district in the Yuechengling.Douzhashan pluton located in the middle of the Miao’ershan,is mainly composed of two-mica granites.In this study,we also found(tourmaline-bearing)muscovite granites which contain abundant Nb-Ta-W oxides in the Douzhashan drill-holes.We studied these samples in detail to further understand the multi-aged mineralization in the Miao’ershan-Yuechengling batholith and investigate the ore-forming processes of different types of deposits.The granitic samples from drill-holes include fine-grained(tourmaline-bearing)muscovite granites and two-mica granites.The(tourmaline-bearing)muscovite granite intrudes into two-mica granite which occurs as the main granite body.Detailed mineralogical studies show that the(tourmaline-bearing)muscovite granite contain various rare-metal minerals,such as columbite-group minerals,ilmenorutile,and cassiterite found in the muscovite granite,and complex Nb-Ta-W oxide mineral aggregates,including columbite-group minerals,wolframixiolite,qitianlingite,and wolframite found in the tourmaline-bearing muscovite granite.Morever,we also found some tourmaline-bearing stockworks which contain abundant scheelite grains in muscovite granite.U-Pb data of magmatic cassiterites(219±4 Ma)suggest that the mineralization occurred during the Indosinian.Combined with petrological observations and trace element composition of muscovite,it is found that the tourmaline-bearing muscovite granite is enriched in tungsten and thus contains higher abundance of the Nb-Ta-W oxide minerals.With the progressive enrichment of boron-rich fluid,scheelite and wolframite precipitated in the late-stage tourmaline-bearing stockwork.Compared to the two-mica granites,the(tourmaline-bearing)muscovite granites contain higher Na and P,but lower K,Ti,Fe,and rare earth element concentrations with more obvious tetrad effect.Additionally,they also have higher rare-metal elements such as Nb,Ta,W,and Sn concentrations(up to 56×10-6,40×10-6,547×10-6and 89×10-6,respectively)and lower Nb/Ta and Zr/Hf ratios(as low as 1.4 and 11.2,respectively).These characteristics of whole-rock geochemistry show the(tourmaline-bearing)muscovite granites are more evolved than those two-mica granites.In addition,compared to selected Indosinian two-mica granite and muscovite granite related to the W mineralization in Miao’ershan-Yuechengling batholith,the tourmaline-bearing muscovite granite in the Yuntoujie W deposit share similar characteristics with the Douzhashan(tourmaline-bearing)muscovite granite.It is inferred that the potential Nb-Ta mineralization locally could be found in the Yuntoujie W deposit.
作者
田恩农
谢磊
王汝成
张文兰
陈琪
胡欢
TIAN Ennong;XIE Lei;WANG Rucheng;ZHANG Wenlan;CHEN Qi;HU Huan(State Key Laboratory for Mineral Deposits Research,School of Earth Sciences and Engineering,Frontiers Science Center for Critical Earth Material Cycling,Nanjing University,Nanjing,Jiangsu 210023,China;Changsha Uranium Geology Research Institute,CNNC,Changsha,Hunan 410007,China)
出处
《地质学报》
EI
CAS
CSCD
北大核心
2023年第2期376-395,共20页
Acta Geologica Sinica
基金
国家自然科学基金项目(编号42072062)
关键地球物质循环前沿科学中心科研基金(编号DLTD2104)资助成果。
