桂东南油麻坡钨钼矿床Re-Os同位素年龄及其地质意义

摘要

油麻坡钨钼矿床是近年来在桂东南岑溪-博白成矿带内新发现的一个大型矽卡岩矿床,矿体主要产在油麻坡复式花岗岩体与泥盆系钙泥质粉砂岩的接触带附近,呈似层状、透镜状和脉状产出,目前发现有30个矿体,探获WO3资源量＞8万吨,矿石品位WO30.191％～1.284％、M0 0.001％～0.019％,其中,工号主矿体资源量占探获总量的84％.本文在矿床地质特征研究基础上,对取自Ⅰ号主矿体内的6件矽卡岩钨钼矿石样品开展了辉钼矿Re-Os同位素测年,获得的等时线年龄为100.1±3.3Ma(MSWD=0.71)、模式年龄加权平均值为100.37±0.62Ma(MSWD=0.34),表明区内钨钼成矿时代为白垩纪.油麻坡复式岩体由主体花岗闪长岩和补体白云母花岗岩组成,前人研究表明,二者的成岩时代分别为109.0±0.3～109.7±1.1Ma和100.7±0.5～103.3±1.2Ma,且岩石学、岩石地球化学以及Sm-Nd同位素等方面存在明显差异,它们不是分离结晶演化关系而是两期不同岩浆活动产物,补体经历了比主体岩浆更为强烈的熔-流体相互作用,有利于萃取W、Mo等金属元素并使之富集成矿.本次Re-Os同位素测年获得的成矿年龄值与补体白云母花岗岩的成岩时代接近,而与主体花岗闪长岩存在约10Ma的时差,进一步佐证了钨钼成矿可能与补体白云母花岗岩关系更加密切的认识.钦杭成矿带是华南板内的一条重要的有色、贵金属成矿带,岑溪一博白成矿带位于其西南段,综合近年来发表的Re-Os、Ar-Ar和锡石原位LA-ICP-MS U-Pb法等高精度测年数据表明,钦杭成矿带内主成矿作用时代显示出自北东向南西,从中侏罗世→晚侏罗世→白垩纪逐渐变新的演化趋势.%The Youmapo tungsten-molybdenum deposit is a recently discovered large-sized skarn-type deposit in the Cenxi-Bobai metallogenic belt of southeast Guangxi.The orebodies occur as statiform,lentoid and veins in the contact zone between Youmapo complex granite and Devonian calcareous-argillaceous siltstone.Up to now,30 orebodies were discovered with more than 80 thousand tons of WOa metal resource.Grades of orebodies are 0.191％ ～1.284 ％ WOa,and 0.001％～0.019％ Mo.Of them,the No.I orebody accounts for 84 percent of the total resources.Based on deposit geology research,six molybdenite samples were separated from the skarn tungsten-molybdenum No.Ⅰ orebody.Re-Os isotopic dating yielded an isochron age of 100.1±3.3 Ma (MSWD=0.71),and a model weighted average age of 100.37±0.62 Ma (MSWD=0.34).It is suggested that tungsten-molybdenum mineralization happened in the Cretaceous.The Youmapo complex pluton consists dominantly of granodiorite and complementary muscovite granite.Previous studies show that both rocks formed at 109.0±0.3 Ma and 109.7 ± 1.1 Ma,but distinct differences occur in lithology,geochemistry and Sm-Nd isotopic compositions.This indicates that the both were not product of fractional crystallization but two different magmatic activities.More strong magmatism and fliud matallogenesis in the complementary intrusive rock are favorable in Tungsternmolybdenum extraction and mineralization.In this study,Re-Os isotopic dating yielded the W-Mo mineralization age close to that of Youmapo complementary intrusive rock,but the time difference of 10 Ma to main intrusive rock further proves that W-Mo mineralization has close relationship with complementary intrusive rock.The Qin-Hang (Qinzhou-Hangzhou) metallogenic belt is an important nonferrous and precious metal metallogenic zone in South China Plate,with the Cenxi-Bobai metallogenic belt in the southwestern part.The precise isotopic dating of Re-Os,Ar-Ar and cassiterite LA-ICP-MS UPb in recent years suggests that the main mineralization ages in the Qin-Hang metallogenic belt show a gradually young trend from middle Jurasic to late Jurasic to Cretaceous from the northeast to the southwest.