蒙古国博洛大型金矿区花岗岩SHRIMP锆石U-Pb测年及地质意义

SHRIMP Zircon U-Pb Dating of Ore-bearing Granite in the Boroo Large-size Gold Deposit,Mongolia and Its Geological Significance

本文首次对博洛金矿床容矿围岩—博洛黑云母花岗岩进行锆石SHRIMPU-Pb年龄测定,获得两组年龄441.9±6.6Ma和452.2±3.9Ma,表明博洛花岗杂岩体应属于早古生代晚期岩浆活动的产物。其主量元素特征表现为高硅(SiO2=75.90×10-2~76.43×10-2)、富钾(K2O/Na2O=1.52~1.72)、富碱((K2O+Na2O)=8.03×10-2~8.22×10-2,碱度率AR=3.99~4.13)、低钙(CaO=0.77×10-2~0.97×10-2)、弱过铝质(A/CNK=1.02~1.05)的特点;稀土元素ΣREE=(160.56~91.28)×10-6,δEu=0.19~0.41,稀土元素分布型式为具有明显负铕异常"燕型"曲线。微量元素除P、Ti相对原始地幔略有亏损外,其它微量元素都表现出不同程度富集,蛛网图曲线总体为右倾的趋势。(87Sr/86Sr)i=0.70986~0.70891,说明其岩浆来源应主要是壳源,但受到幔源物质的混染;(143Nd/144Nd)i=0.512138~0.512115,平均0.512127,εNd(t)变化在+1.4^+1.2,平均+1.3,为正值,具有幔源的特点。这些特征表明博洛黑云母花岗岩为壳幔相互作用的产物,并有新生地幔物质加入,产于造山后环境。博洛金矿床的形成时代为早侏罗世,博洛花岗杂岩仅仅为金矿床的容矿围岩,为矿床提供容矿空间,与金矿床成因关系不密切。

The SHRIMP U-Pb age dating results of zircons obtained for the first time from biotite granite of Boroo complex in the Boroo large-size gold orefield suggest that this region experienced the late Paleozoic tectono-magmatic activity.Two groups of ages were obtained,i.e.,441.9 ± 6.6Ma and 452.2 ± 3.9 Ma,indicating Late Ordovician.The major elements of Boroo biotite granite are characterized by high silica（SiO2=76.43～75.90%）,rich potassium（K2O/Na2O=1.72～1.52）,rich alkaline（（K2O＋Na2O）=8.03～8.22%,alkali-degree rate AR=3.99～ 4.13））,low calcium（CaO=0.77%～0.97%）,low magnesium（MgO=0.15%～0.16%）,low phosphorus（P2O5= 0.02%～0.05%）,and quasi-aluminous to weak peraluminous nature（Al2O3=12.39%～12.74%,and A/CNK= 1.02～1.05）.Rittmann index σ=1.93～2.05,implying calc-alkaline nature.In comparison with the A-type granite,their average chemical compositions are similar to each other;nevertheless,the average chemical composition of Boroo biotite granite is obviously different from that of S-type granite and standard granite.In the K2O-SiO2 diagram,the rock samples fall into the range of high-K calc-alkaline series.Geochemical characteristics of rare earth elements are ΣREE =（160.56～91.28） × 10^-6,LREE/HREE = 7.72～ 5.01,（La /Yb） N = 9.49～5.99,δEu = 0.19～0.41,and δCe = 0.82～0.79.REE distribution patterns demonstrate a significant negative Eu anomaly and a ＂swallow-type＂ shape.Except for P and Ti which are relatively slightly depleted relative to the primitive mantle,trace elements show enrichment in different degrees.Trace element spider diagrams exhibit the depletion of Nb,Ta,Sr,Ba,P,Ti,U,and the enrichment of Rb,Th,Pb.The right-inclination trend characteristics of spider diagram curves suggest that the content of trace elements and their compatibility are simultaneously decreasing and that the rocks have experienced crystallization differentiation of magmatic evolution.According to Zhang Qi＇s（2008） granite classification,Boroo granite is of the Nanling type;in combination with REE and trace element spider diagrams,Boroo granite should be a product of post-orogenic extensional setting.（^87Sr/^86Sr）ⅰ of Boroo biotite granite = 0.70986 ～ 0.70891,（^87Sr/^86Sr）ⅰof modern oceanic basalts = 0.702 ～ 0.706,（^87Sr/^86Sr）ⅰ average value of continental crust is 0.719,and（^87Sr/^86Sr）ⅰ value of Boroo granite is in the range of 0.706 ～ 0.719,indicating that the magma source of Boroo granite should have been mainly of crust-derivation,partly with contamination of mantle-derived materials.（^143Nd/^144Nd）i=0.512138～0.512115,with an average of 0.512127,and εNd（t） values range from 1.2 to 1.4,with an average of 1.3,showing positive values with characteristics of mantle source.Nd isotope two-stage model ages T2DM = 1070～1094Ma,with an average of 1082Ma,indicating that the source rocks of Boroo granite complex might have been products of Tuva-Mongolia massif in the late period of Middle Proterozoic.In the εNd（t）-（^87Sr/^86Sr）ⅰ diagram,the Boroo granite falls in Quadrant I.All these features indicate that the source rocks of Boroo biotite granite are possessed of both continental crust-derivation and the involvement of mantle-derived materials.Boroo granite was produced by crust-mantle interaction and the participation of new substances derived from the mantle and formed in a post-orogenic environment.Regional extension led to crust thinning,mantle uplifting and asthenospheric upwelling.Decompression resulted in mantle melting and the formation of basalt magma,whereas basaltic underplating gave rise to lower crust melting and the formation of granite.The concentrations of such elements as gold,silver,copper and zinc in Boroo biotite granite are lower than their respective Clark values except for lead,whose concentration is slightly higher than the Clark value.The ore-forming age of the Boroo gold deposit should be early Jurassic,indicating that Boroo granite complex only served as ore-bearing wall rocks of the Boroo gold deposit and provided ore-bearing space.The formation of the Boroo gold deposit was irrelevant to the formation of Boroo granite complex.