内蒙古白音汉石英脉型黑钨矿床正长花岗岩成岩时代、地球化学特征及其地质意义

Chronology,geochemistry and geological significance of syenogranite from Baiyinhan quartz vein wolframite deposit in Inner Mongolia

白音汉矿床是位于大兴安岭南段成矿带的石英脉型黑钨矿床。正长花岗岩是该矿床的主要围岩及成矿岩体,文章对该岩体进行了详细的锆石U-Pb年代学、主量元素、微量元素以及Hf同位素研究。LA-ICP-MS锆石U-Pb定年法测得白音汉花岗岩的成岩年龄为(139.1±2.5)Ma,与辉钼矿的Re-Os等时线年龄相同(139.6±7.6)Ma,矿床的成岩成矿时代一致。岩相学和岩石地球化学研究表明花岗岩属于高分异A型花岗岩,具有高硅(w(SiO2)=75.70%~76.71%)、富碱(w(Na_(2)O+K_(2)O)=8.32%~8.92%)、弱过铝质特征(A/CNK为1.00~1.05);稀土元素总量中等(ΣREE为214×10^(-6)~347×10^(-6)),富集轻稀土元素(ΣLREE为129×10^(-6)~219×10^(-6)),Eu强烈亏损(δEu=0.03~0.06),稀土元素标准化配分曲线呈右倾;微量元素以富集Rb、Th、K,而亏损Nb、Ta、P、Ti元素为特征。正长花岗岩中锆石的微量元素特征与岩浆锆石的典型特征一致,表现为轻稀土元素强烈亏损(58×10^(-6)~99×10^(-6)),重稀土元素十分富集(859×10^(-6)~2457×10^(-6))的特征,轻、重稀土元素比值LREE/HREE为0.03~0.07,平均值为0.05;具有明显Ce正异常(δCe=3.6~41)和弱Eu负异常(δEu=0.3~0.7)。锆石Hf的原位同位素分析表明,ε_(Hf)(t)值集中在为2.4∼5.0,均为正值,变化范围很小。岩石的Hf同位素两阶段模式年龄也较为集中,T_(DM2)为871~1042 Ma。白音汉钨矿与正长花岗岩的形成是早白垩世古太平洋板块后撤的环境中,区域伸展构造背景下软流圈上涌的响应。

The Baiyinhan deposit is a quartz vein wolframite deposit situated in the southern segment of the Great Xing'an Range.This research presents an in-depth analysis of the syenogranite,encompassing zircon U-Pb chronology,a detailed examination of major and trace elements,and Hf isotope studies.Utilizing LA-ICP-MS zircon U-Pb dating,it has been determined that the Baiyinhan syenogranite dates to(139.1±2.5)Ma.This discovery is corroborated by the Re-Os isochron age of molybdenite,which is(139.6±7.6)Ma,indicating a likely close relationship between the syenogranite and the mineralization events.Petrographic and geochemical analyses have characterized the syenogranite as a highly fractionated A-type granite.It is marked by significant enrichment in silica,with a content of SiO2 ranging from 75.70%to 76.71%,and in alkali elements,where the sum of Na_(2)O and K_(2)O varies from 8.32%to 8.92%.The syenogranite also exhibits a slightly peraluminous nature,with A/CNK values fluctuating between 1.00 to 1.05.The chondrite-normalized rare earth element(REE)patterns show concentrations varying from 214×10^(-6)to 347×10^(-6),with a pronounced enrichment in light rare earth elements(ΣLREE from 129×10^(-6)to 219×10^(-6))and strong negative europium anomalies(δEu ranging from 0.03 to 0.06).Trace element analysis indicates an enrichment in Rb,Th,and K,while depletion is noted for Nb,Ta,P,and Ti.The zircon trace element signature is consistent with that of magmatic zircon,showing a substantial depletion in light rare earth elements(58×10^(-6)to 99×10^(-6))and a high enrichment in heavy rare earth elements(ΣHREE from 859×10^(-6)to 2457×10^(-6)).The LREE/HREE ratios are notably low,ranging from 0.03 to 0.07,with an average of 0.05.Additionally,the zircon displays significant positive cerium anomalies(δCe from 3.6 to 41)and weak negative europium anomalies(δEu from 0.3 to 0.7).Theε_(Hf)(t)values are tightly clustered between 2.4 and 5.0,all positive.The two-stage Hf isotope model age(T_(DM2))of the zircon is also relatively uniform,ranging from 871 Ma to 1042 Ma.Previous studies have predominantly attributed the formation of tungsten deposits to concealed granitoids.However,this study posits that the metallogenetic granitoids associated with the Baiyinhan deposit are specifically the highly fractionated syenogranite.Both the Baiyinhan deposit and the syenogranite were coeval,forming during the Early Cretaceous period.This formation occurred in the backdrop of the retreat of the Paleo-Pacific plate and regional extensional tectonics,which is interpreted as a response to asthenospheric upwelling.