与碱性岩-碳酸岩杂岩共生的铌-稀土成矿作用——兼论东昆仑大格勒铌-稀土矿床中的碱性岩-碳酸岩杂岩成因

Nb-REE Mineralization Associated with Alkaline Rock-carbonatite Complex——On the Genesis of the Alkaline Rock-carbonatite Complex in the Dagele Nb-REE Deposit,East Kunlun

铌(Nb)和稀土(REE)分别是我国重要的紧缺、优势战略矿产资源,其中Nb对外依存度接近100%。Nb和REE矿床在全世界(包括中国)都有分布,这类矿床中的矿石矿物种类繁多,但目前工业提取REE的矿石矿物主要为富轻稀土的氟碳铈矿、独居石,富重稀土、钇的磷钇矿,风化壳淋积型黏土矿等,而工业提取Nb元素的矿石矿物主要为烧绿石和铌铁矿等。与碱性岩-碳酸岩杂岩有关的Nb、REE矿床等是铌与稀土(尤其是轻稀土)资源最重要的来源。自然界中的Nb矿床分布较广,但绝大部分Nb资源集中在全球约十多个大型矿床中,且主要与碱性岩-碳酸岩及其相关的风化产物有关。碱性岩-碳酸岩杂岩主要来自地幔,且产出于一些特殊的构造背景(如裂谷或陆内伸展、碰撞后伸展、地幔柱、火山弧等),是研究深部地幔富集与熔融过程、物质(如碳)循环和深部动力学机制的重要“岩石探针”,是当前固体地球科学研究的前沿和热点领域之一。因此,研究碱性岩-碳酸岩杂岩的形成机制及相关的Nb-REE成矿作用,具有极其重要的地球动力学和关键金属成矿意义。在东昆仑新发现了与碱性岩-碳酸岩杂岩共生的大格勒Nb-REE矿,成矿岩石主要为碳酸岩、金云母岩和橄榄岩,包含多种含Nb、富REE和含P矿物,初步确定为碳酸岩型Nb-REE-P矿床。Nb矿体中最重要的含Nb矿石矿物是铌钙矿和铌铁矿,其Nb_(2)O_(5)含量最高分别达81.17%、76.06%~79.67%,粒径最大分别可达1.2 mm、~1 mm,粒径和赋存形式明显优于白云鄂博矿床(粒径<0.04mm)。初步的勘探工作圈定出5条铌矿体,Nb_(2)O_(5)平均品位0.092%~0.156%(最高1.28%),估算Nb_(2)O_(5)推断资源量超过10万吨,表明大格勒矿区Nb资源已经达到大型矿床规模。矿区杂岩体中镁铁质-超镁铁质岩和碳酸岩主要形成于早泥盆世早期(~418Ma),明显富集Nb、Ta、P和Sr等,具有相对稳定的ε_(Nd)(t)(-0.5~2.7)和变化的(^(87)Sr/^(86)Sr)i(0.7037~0.7063)。这些岩石主要起源于软流圈地幔,可能形成于碰撞后伸展背景中。但是大格勒矿区成岩与成矿的精确时代、岩浆来源与岩浆储库演化过程、岩石与矿床形成的深部动力学机制、金属超常富集与成矿作用、成矿元素的赋存状态与可利用性等还需要进一步深入研究。

Niobium(Nb)and rare earth(REE)are important strategic mineral resources in China,and the dependence of Nb on foreign countries is close to 100%.Nb and REE deposits are distributed all over the world(including China),and there are many types of ore minerals in these deposits.However,at present,the ore minerals for the industrial extraction of REE are mainly bastnaesite,monazite,xenotime,and ion-adsroption REE deposits in weathering crusts,whereas the ore minerals for the industrial extraction of Nb are mainly pyrochlore and niobite.Nb and REE deposits related to alkali-carbonatite complexes are the most important sources of Nb and REE(especially LREE)resources.Nb deposits are widely distributed,but most of the Nb resources are concentrated in more than ten large deposits worldwide that are mainly related to alkaline rocks,carbonatites,and related weathering products.Alkali-carbonatite complexes are mainly derived from the mantle and occur in special tectonic settings such as rift or intracontinental extension,post-collisional extension,mantle plume,and volcanic arc.They are important“rock probes”for the study of deep mantle enrichment and melting processes,material(e.g.carbon)cycle,and deep dynamic mechanisms,and are one of the frontier and hot fields of current solid earth science research.Therefore,the study of the formation mechanism of the alkali-carbonatite complex and the related Nb-REE mineralization is of great significance in geodynamics and key metal mineralization.The newly discovered Nb-REE deposit in Dagele,East Kunlun,is associated with an alkali-carbonatite complex.The ore-forming rocks are mainly carbonatites,phlogopites,and peridotites,which contain various Nb-bearing,REE-rich,and P-bearing minerals.It is preliminarily determined to be a carbonatite-type Nb-REE-P deposit.The most important Nb ore minerals in Nb ore bodies are ferroniobite and columbite,with the highest content of Nb_(2)O_(5) of up to 81.17%and 76.06%-79.67%,respectively,and the largest grain size of up to 1.2 mm and~1 mm,respectively.The grain size and occurrence form are obviously better than those of the Baiyun Obo deposit(grain size<0.04 mm).Preliminary exploration has delineated five niobium ore bodies,giving an average grade of Nb_(2)O_(5) of 0.092%-0.156%(the highest is 1.28%),and inferred resources of Nb_(2)O_(5) exceed 100,000 tons,indicating that the Nb resources in the Dagele deposit have reached a large-scale deposit.The mafic-ultramafic rocks and carbonatites in the complex in the ore district were mainly formed in the early Early Devonian(ca.418 Ma),which are obviously enriched in Nb,Ta,P,and Sr,and have relatively stableε_(Nd)(t)(-0.5 to 2.7)and varying(^(87)Sr/^(86)Sr)i(0.7037 to 0.7063).These rocks are primarily derived from the asthenospheric mantle and may have formed in a post-collisional extensional setting.However,the precise ages of the magmatism and ore mineralization,the magma source and the evolution process of the magma reservoir,the deep dynamic mechanism of rock and deposit formation,the abnormal enrichment and mineralization of metals,and the occurrence and availability of ore-forming elements in the Dagele Dagele deposit need further study.