西藏波龙斑岩铜金矿床钾长石和绢云母^40Ar/^39Ar年龄及其地质意义

^40Ar/^39Ar dating of hydrothermal K-feldspar and hydrothermal sericite from Bolong porphyry Cu-Au deposit in Tibet

西藏波龙斑岩铜金矿床是新近在青藏高原中部发现的规模最大的斑岩型矿床。文章对该矿床内的蚀变钾长石和蚀变绢云母进行了^40Ar/^39Ar年代学测试,获得蚀变钾长石的^40Ar/^39Ar坪年龄为（118133±0160）Ma,反等时线年龄为（118149±0174）Ma（初始40Ar/36Ar=28611±814）,表明波龙斑岩铜金矿床的钾化蚀变年龄为118-119 Ma;蚀变绢云母的^40Ar/^39Ar坪年龄为（121161±0167）Ma,反等时线年龄为（12111±210）Ma（初始40Ar/36Ar=279±19）。由于蚀变绢云母测试样品内可能混入了斜长石,受其影响,蚀变绢云母测年结果的下限可能代表了该矿床绢英岩化蚀变年龄。这些蚀变钾长石和蚀变绢云母^40Ar/^39Ar测年结果与波龙矿床的成岩年龄值和成矿年龄值在误差范围内基本一致,表明该矿床的钾化和绢英岩化与成岩、成矿同期,该矿床的岩浆-热液活动过程的时限为121-118 Ma。

The Bolong porphyry copper-gold deposit is the largest porphyry copper deposit discovered recently in the middle of the Tibetan plateau.Hydrothermal K-feldspar from the potassic alteration zone and hydrothermal sericite from the phyllic alteration zone were analyzed by40Ar/39Ar dating technique.The plateau age of secondary K-feldspar is（118.33±0.60）Ma,which,together with its reverse isochron age of（118.49±0.74） Ma（initial40Ar/36Ar=286.1±8.4）,represents the potassic alteration age of the Bolong deposit.The plateau age of secondary sericite is（121.61±0.67）Ma,and its reverse isochron age is（121.1±2.0）Ma（initial40Ar/ 36Ar=279±19）.The high37Ar/39Ar ratio of the secondary sericite sample indicates that some magmatic plagioclase was mixed in the sample,which increased the dating value.Thus,the phyllic alteration might have occurre d around 119 Ma.Taking the deviation and uncertainties of geochronologic methods into account,the authorshave found that the40Ar/39Ar ages of hydrothermal K-feldspar and hydrothermal sericite are similar to the oreforming and rock-forming agesof the Bolong deposit,suggesting that the potassic alteration and mineralization in the Bolong porphyry deposit were coeval with the intrusion of porphyries and phyllic alteration,and the magmatic-hydrothermal duration was within 121-118 Ma.