西藏努日白钨矿床微量和稀土元素地球化学特征——对成矿流体与矿床成因的指示

Geochemical characteristics of rare earth elements and trace elements in the Nuri scheelite deposit,Tibet,China——indications for ore-forming fluid and deposit genesis

对努日矿床白钨矿中稀土元素、微量元素的地球化学特征研究表明:白钨矿亏损V、Cs、Hf、Ta等元素,并具有异常低的Rb/Sr、Zr/Hf、Nb/Ta的比值,以及Hf/Sm、Nb/La、Th/La值也都远远小于1,指示原始富钨成矿流体来源于深部壳源岩浆结晶分异,并总体以富CL热液为主;采自不同位置的白钨矿样品其稀土元素组成基本一致,其球粒陨石配分曲线表现为明显轻稀土富集的右倾模式,具有明显的Ce正异常(δCe=1.066~1.107)和弱Eu负异常(δEu=0.768~0.910),表明白钨矿与流体之间稀土元素发生了不太明显的分异;白钨矿与夕卡岩化有密切关系的黑云母花岗岩的REE球粒陨石标准化配分曲线均为明显的右倾模式,ΣREE、δCe、δEu值基本一致,暗示岩体对努日白钨矿成矿物质的贡献。努日矿床初始热液流体为一中高温、中低盐度的流体,其沿着构造裂隙运移过程中,与比马组地层发生接触交代反应,体系中CO2和Ca含量的增加,流体的初始平衡体系被破坏,最终导致大量Ca2+与WO42-结合形成了巨量的白钨矿。

Scheelite is the main ore mineral in the Nuri Cu-Mo-W deposit,Tibet.Geochemical characteristics of rare earth elements(REE) and trace elements compositions in scheelite were studied.Results show that the scheelite is depleted in V,Cs,Hf and Ta,with extremely low ratios of Rb/Sr,Zr/Hf and Nb/Ta.Ratios of Hf/Sm,Nb/La and Th/La are far less than 1,indicating the feature of original W-rich ore-forming fluid source is from deep crustal magmatic crystallization and differentiation,which is Cl-rich and hydrothermal.Scheelite collected from different locations is characterized by enrichment in light REE and obvious positive Ce anomalies(δCe 1.066-1.107) and weak negative Eu anomalies(δEu 0.768-0.910),showing that REE between scheelite and fluid do not have obvious differentiation.Moreover,both scheelite and the biotite granite connected with skarn have similar distribution of ΣREE,δCe and δEu(basically the same in both) implying that the rock contributes to form scheelite.The original ore-forming fluid is at mid-high temperature with mid-low salinity migrating along structural fractures.It contacted and experienced a metasomatic reaction with the Bima formation layer.With increasing CO2 and Ca contents,the initial balance system was disrupted,and eventually led to the formation of massive scheelite on the basis of Ca2+ and WO42-in combination.