内蒙古东乌旗达亚纳钨-钼矿成岩成矿时代及其岩体地球化学研究

Geochronology and Geochemical Characteristic of Biotite Granite of the Dayana W-Mo Deposit in Dong Ujimqin Banner,Inner Mongolia,and Its Geological Significance

内蒙古东乌旗达亚纳钨-钼矿是近年来正在勘查的一个规模有望达中型的新矿床,矿床位于兴蒙造山带中西部,矿床类型为典型的石英脉型黑钨-钼矿,黑钨矿石英脉以近于直立(75°)的平直脉体产出在黑云母花岗岩内或岩体与地层的外接触带,并向下尖灭于岩体内部,主要的矿化蚀变类型有云英岩化、钾长石化、钠长石化、硅化等。伴随黑钨矿化共生大量的辉钼矿,辉钼矿化多产出在黑钨矿石英脉两侧或石英脉内。本文在系统研究矿床地质特征的基础上,对成矿岩体——黑云母花岗岩进行了高精度LA-ICP-MS锆石U-Pb测年及地球化学分析,U-Pb测年结果显示,两件黑云母花岗岩样品(DYN-19、DYN-30)有着一致的U-Pb谐和年龄,分别为134±1Ma(MSWD=0.41)、135±1Ma(MSWD=0.52)。地球化学特征显示,岩体富硅、富碱,SiO_2平均74.43%,Na_2O+K_2O平均8.33%,Na_2O/K_2O平均0.73,明显富钾,属高钾钙碱性系列;Al_2O_3平均为12.72%,铝饱和指数A/CNK全部近1.0,为准铝质到弱过铝质花岗岩类,相对贫Mg;稀土总量较低,ΣREE变化于63.68×10^(-6)~91.11×10^(-6),表现为右倾稀土元素配分模式,弱富集轻稀土,并具有明显的负铕异常,高场强元素Th、U、Nd、Hf明显富集,而Ba、Sr、P、Ti、Nb显著亏损;为壳源重熔型花岗岩的地球化学特征。锆石Hf同位素分析显示,^(176)Hf/^(177)Hf值0.28295~0.28302,ε_(Hf)(t)值+6.4^+8.8,二阶段模式年龄值408~540Ma,年轻的二阶段模式年龄和不大的ε_(Hf)(t)正值,指示了其岩浆源区可能为新生下地壳。同时对矿区7件辉钼矿样品进行了铼-锇同位素分析测试,获得等时线年龄为133±3Ma,MSWD值为2.2。可见成岩年龄和成矿年龄在误差范围内具有一致性,为同一期地质作用的产物,即成岩成矿时代为早白垩世,属燕山期构造岩浆活动的产物。

The Dayana W-Mo deposit in Dong Ujimqin Banner of Inner Mongolia is a quartz-vein type deposit newly discovered in the mid-western part of Mongolia-Hinggan Orogen. It is typical of quartz vein-type wolframite-molybdenum ore deposit, with wolframite-bearing quartz veins occurring as nearly vertical, parallel quartz veins （75°） within biotite granite or in outer contact zone between rock mass and strata. Main mineralization and alteration includes greisenization, potash feldspathization, albitization, and silicification, etc. A large amount of molybdenite formed accompanied with wolframite and occurs on both sides of wolframite-bearing quartz veins or in quartz veins. Based on detailed analysis of deposit geological feature, this study carried out LA-ICP-MS zircon U-Pb dating and geochemistry analysis for ore-forming rock mass-biotite granite. The U-Pb dating yielded two similar ages （134＋1 Ma （MSWD=0.41） and 135 ＋1 Ma （MSWD=0. 52）） for tow biotite granite samples. Major and trace clement geochemistry shows that the biotite granite is characterized by high SiO2 and K20 contents, a ＂right-inclined＂ shape of the chondrite normalized REE patterns, enrichment of large ion lithophi high field strength elements （HFSEs） such as Nb, P, Ba. The blot e elements （LILEs）, and depletion of te granite is high-K calc-alkaline, has a strong negative Eu anomaly （Eu/Eu＊ =0.22-0.45）, low P2Os content, A/CNK value close to 1. These characteristics define the Dayana biotite granite as a highly fractionated granite. The in situ zircon Hf isotopic data show that the 176 Hf/177 Hf isotopic ratio of the zircons from the biotite granite is 0. 28295 0.28302, with εHf（t） values ranging from （＋6. 4） to （＋8. 8）. It indicates that the source region of the biotite granite magma was derived from the juvenile Lower crust. The Re-Os isotopic analysis of seven molybdenite separates from the deposit yielded an isochron age of 133 4- 3 Ma （MSWD=2.2）, suggesting that the biotite granite and mineralization have the identical age within error range, are probably genetically related, and closely related to the cretaceous metallogenic events in North China.