赣西北大湖塘石门寺钨矿区花岗岩的成因及其对钨矿的指示意义

Petrogenesis of granites in Shimensi in northwestern Jiangxi Province and its implications for tungsten deposits

江西省晚中生代大湖塘钨矿是近年来中国勘查出的世界级钨矿产地之一,位于新元古代江南造山带的东段,地处赣北武宁、修水、靖安三县交界处。该区属于江南地块中生代钨铜钼等多金属成矿带,大规模出露晋宁期闪长岩和燕山期花岗岩。文章对赣西北大湖塘石门寺钨矿区新元古代和燕山期的4种花岗岩进行详细的岩相学,锆石和锡石U-Pb年代学、微量元素的研究。锆石U-Pb研究结果表明,新元古代黑云母花岗闪长岩的成岩年龄为(829.9±4.7)Ma;燕山期云英岩化似斑状黑云母花岗岩、细粒黑云母花岗岩和云英岩化黑云母花岗斑岩的成岩年龄分别是(148.3±2.6)Ma,(145.5±3.6)Ma和(147.7±1.5)Ma,与锡石所获得的U-Pb年龄相互吻合。主要含矿的似斑状黑云母花岗岩中锡石所含的钨含量明显高于其他燕山期花岗岩,暗示了似斑状黑云母花岗岩岩浆中高的钨含量。锆石钛温度计表明,燕山期的花岗岩均形成于较低的温度,加权平均值的变化范围为734~788°C。锆石的微量元素结果表明,似斑状黑云母花岗岩形成于较低的氧逸度环境,更有利于形成钨矿。

The Late Mesozoic Dahutang tungsten deposit,as one of the largest tungsten deposits in the world,is a recently discovered deposit in Jiangxi Province.It is situated in border area of Wuning,Xiushui and Jing’an counties,and also located in the eastern part of the Neoproterozoic Jiangnan orogen.This area belongs to Mesozoic W-Cu-Mo polymetallic ore-forming belt in South China Block with widespread Jinningian diorites and Yanshanian granites.In this study,the authors investigated the petrography,mineralogy,zircon and cassiterite U-Pb geochronology and trace element concentrations of the Neoproterozoic and Yanshannian granitic intrusions in the Shimenshi ore district.Zircon U-Pb dating results of the Neoproterozoic biotite granodiorites and Yanshannian biotite granite porphyry,greisenized fine-grained granites and greisenized biotite granite porphyry are(829.9±4.7)Ma,(145.5±3.6)Ma,(152.6±2.0)Ma and(154.2±1.7)Ma,respectively.In particular,the crystallization ages of the Yanshannian granites are consistent with the new cassiterite U-Pb age results obtained by the authors.The tungsten content in cassiterite from the ore-related fine-grained biotite granites is obviously higher then tungsten content of other Yanshannian intrusions.This implies a higher tungsten concentration in the fine-grained biotite granitic magmas.Apparent temperatures for zircon crystallization calculated by Ti-in-zircon thermometer suggest lower temperatures of Yanshannian magmas with weighted average temperature ranging from734°C to788°C.Trace element concentrations of zircon exhibit that fine-grained biotite granites were formed under low oxygen fugacity condition,which was favorable for the formation of tungsten ores.