内蒙古双尖子山银多金属矿床侏罗纪成矿的年代学证据及其找矿意义

The Jurassic Mineralization of the Shuangjianzisha Ag-polymetallic Deposit and its Significance in Prospecting:Evidence from Geochronology

双尖子山银多金属矿床是近年来在大兴安岭南段发现的一超大型规模矿床,其银资源量为2.6×10~4t,锌3.3Mt,铅1.1Mt,但矿床的成矿时代目前还未得到精确的厘定,这制约了对矿床成因、区域成矿规律及找矿方向的认识。本文在详细的矿床地质特征调查基础上,识别出了与双尖子山银多金属矿床成矿有关的斑状花岗岩。该岩体内部发育浸染状方铅矿和矿化的隐爆角砾岩,岩体外接触带硅化、绿泥石化及银铅锌矿化强烈,这指示双尖子山矿床的银铅锌矿化和蚀变作用与斑状花岗岩的侵入作用密切相关。鉴于此,作者对斑状花岗岩的锆石进行了LA-MC-ICP-MS U-Pb定年,获得该岩体16颗锆石的^(206)Pb/^(238)U加权平均年龄为159.3±2.3Ma,该年龄可近似代表双尖子山矿床的成矿年龄。上述结果表明,双尖子山银多金属矿床为与晚侏罗世斑状花岗岩有关的热液脉型矿床。尽管精确的成矿年龄数据已揭示大兴安岭南段存在侏罗纪成矿事件,但大多数的观点认为早白垩世(~135Ma)是大兴安岭南段内生金属矿床成矿作用的峰期,因此忽视了侏罗纪成矿事件的重要性。鉴于双尖子山矿床银、铅和锌的资源量均已达超大型矿床规模,作者认为,大兴安岭南段侏罗纪成矿作用的强度可能并不亚于早白垩世,应在今后的找矿勘察中得到足够的重视。

The Shuangjianzishan Ag-polymetallic deposit is a newly discovered super-large deposit in the southern Great Xing＇an Range （SGXR） with a resources of Ag 2.6×10^4t, Zn 3.3 Mr, and Pb 1. 1 Mt. However, its mineralization age which affects the understanding of its genesis as well as the regional metallogeny and prospecting in the SGXR, is still a pending question. In this study, based on detailed deposit geology survey, we have identified the mineralization-related porphyritic granite in the deposit. Disseminated galena and mineralized cryptoexplosive breccias are developed in this intrusion. Furthermore, in the outer contact zone of the porphyritic granite, silicification, chloritization, and Ag-Pb-Zn mineralization are developed extensively. Both suggest that the mineralization and alteration of the Shuangjianzishan deposit have a genetic relationship with the intrusion of porphyritic granite. Zircon U-Pb dating by LA-MC-INP-MS of the porphyritic granite gives a ^206Pb/^238 U weighted average age of 159.3 q- 2.3 Ma. It indicates that the Shuangjianzishan deposit is a vein-type deposit related to the Late Jurrasic porphyritic granite. Former studies suggested that mineralization in the SGXR peaked at the Early Cretaceous （～135 Ma）. Consequently, the importance of the Jurassic mineralization event in the SGXR is usually neglected, although precise ore-forming ages have revealed its existence. In the light of the size and mineralization age of the Shuangjianzishan deposit, we propose that the importance and intensity of the Jurassic mineralization in the SGXR may not be less than that in the Early Cretaceous, and must be taken into account in the upcoming prospecting works.