再论内蒙古哈达门沟金矿床的成矿时限问题

Re-discussions on the time limitation of gold mineralization occurring within the Hadamenggou deposit,south-central Inner Mongolia autonomous region.

对内蒙古哈达门沟金矿化区黑云母-磁铁矿伟晶岩脉中黑云母、含金钾化蚀变岩和金矿石中的绢云母样品进行了详细^(40)Ar^(39)Ar 同位素年龄测定。每件样品各个加热阶段所获数据均构成一条相关性很好的直线,它们对应的同位素等时线年龄值分别为1992.73±38.94Ma、322.58±3.24Ma 和239.76±3.04Ma。3件样品^(40)Ar/^(39)Ar 初始值分别为299.7、279.6和286.6,低于或接近尼尔值(295.5)。考虑到所测样品均采自来受到明显后期构造-岩浆活动或其它热事件影响的岩(矿)体, 因此,1992Ma、322Ma 和239Ma 分别代表了黑云母-磁铁矿伟晶岩脉、含金钾化蚀变岩和金矿石的形成时代。根据野外地质证据,并且结合上述同位素年龄数值,可以推测,哈达门沟地区金矿化发生的时间分别为海西中期和印支早期,其中印支早期是金矿康的主要成矿时期。

Located at westernmost part of the northern margin of the North China craton, Hadamenggou is one of the most important gold deposits in Inner Mongolia, and has been considered as one of the key parts of the North China metallogenic belt. Gold mineralization at Hadamengou occurs within Archean metamorphic rocks of the Wulashan Group as gold-bearing quartz and quartz-K feldspar veins （gold ores） , but it shows intimate spatial relations to Proterozoic pegmatite dykes, Paleozoic K-altered rock zone and granitoid intrusions. Geochronological studies of the Hadamengou deposit and its related rocks have attracted geologists＇attention both at home and abroad. ^40Ar/^39Ar isotope data for biotite separates from the biotite-magnetite pegmatite dyke, sericite separates from the gold-bearing K-altered rock, and sericite separates from the gold-bearing quartz-K feldspar veins （gold ores ） of the Hadamengou deposit are presented in this paper. Nine stepwise heating Ar data of the biotite separates from the biotite-magnetite pegmatite dyke give an ^40Ar/^39Ar isochron age of 1992.73 ±38.94 Ma （2σ） and ^40Ar/^39Ar initial value of 279.60 ±49.64 （2σ）, which is closely to Nier＇ s value. This age may represent the crystallization time of the biotite from the biotite-magnetite pegmatite dyke, and has been considered as the peak of the Precambrian tectonic-magmatic events occurring within the Hadamengou district. For the gold-bearing-K- altered rock zone, six stepwise heating Ar data of the serieite separates from the K-ahered rock constitute an 40 At/39 Ar isochron line, which shows an age of 322.58± 3.24 Ma （2σ） and ^40Ar/^39Ar initial value of 299.73 ± 7.1 1 （2σ）, which is similar to Nier＇ s value. This age may represent the forming time of the gold-bearing K-altered rock, and has been regarded as time of the earliest gold mineralization event occurring within the Hadamengou district. Moreover, seven stepwise heating Ar data of the sericite separates from the gold-bearing quartz-K feldspar veins （gold ores ） give an ^40Ar/^39Ar Ar isochron age of 239.76 ±3,04 Ma （2σ） and 40 Ar/36 Ar initial value of 286.62 ± 18.18 （2σ） , which is closely to Nierg value. This age may represent the crystallization time of the sericite from the gold-bearing quartz-K feldspar veins （gold ores）. Since the sericite separate from the gold-bearing quartz-K feldspar veins （gold ores） show clear co-existing textures with gold-bearing sulfide and telluride, the isotopic age has also been considered as the ore-forming time of the Hadamengou deposit. Isotopic age data and field metrological observation indicate that the Hadamengou deposit was mainly formed during the Early Indosinian tectonic movement, but the earliest gold mineralization probably occurred during the time period of the Middle Hercynian orogeny.