华北北缘张宣地区东坪金矿中的两期锆石:对成矿年龄的约束

Two periods of zircon from Dongping gold deposit in Zhangjiakou-Xuanhua area, northern margin of North China:Constraints on metallogenic chronology

东坪金矿床位于华北克拉通北部边缘的张宣地区,产于水泉沟碱性杂岩体南部的中细粒正长岩中,属于低硫化物钾长石石英脉型碲金矿床。金矿石包括低硫化物碲金石英脉型和钾质蚀变岩型两种类型。文章对产于东坪金矿1号脉的钾长石石英脉中的锆石进行了成因矿物学和LA-ICP-MSU-Pb年代学的系统研究,结果表明,矿脉中的锆石可以分成岩浆锆石和热液锆石两种成因类型。岩浆锆石具有自形到半自形结构、柱状晶型,在电子探针背散射电子图像(BSE)上呈暗灰色;在阴极发光图像(CL)上具有明显的岩浆振荡环带。热液锆石多呈不规则状充填在岩浆锆石中,也有呈他形晶独立存在的;在BSE图像上呈亮灰白色,而在CL图像上为深黑色(无阴极发光)。岩浆锆石的Th、U含量和Th/U比值相对较低,其U-Pb加权平均年龄为(380.5±2.6)Ma,说明水泉沟碱性杂岩体形成于晚泥盆世。热液锆石的Th、U含量和Th/U比值较岩浆锆石明显增高,U-Pb加权平均年龄为(140.2±1.3)Ma,说明东坪金矿床形成于早白垩世。因此,成岩与成矿年龄相差近240Ma,表明东坪金矿床的形成与水泉沟碱性杂岩体的岩浆侵位事件无直接关系。通过对区域地质背景的综合分析,作者认为成矿流体可能与该区燕山期的岩浆活动有关。

The Dongping gold deposit is located in Zhangjiakou-Xuanhua area of Hebei Province, on the northern margin of the North China Craton. This ore deposit is mainly hosted by the medium-fine grained syenite in the southern part of the Shuiquangou alkaline complex. There are two types of gold ores in this deposit, namely the telluride mineral-bearing and low-sulfide quartz veins and the intense K-feldspar alteration type. The authors performed a systematic mineralogical and U-Pb geochronologic study of zircon minerals from No.1 auriferous quartz vein. Two types of zircons were recognized in this vein, i.e., magmatic and hydrothermal ones. The magmatic zircon is characterized by euhedral-subhedral crystals with the prismatic form, dark-gray color in the backscattered electron （BSE） image, and clear magmatic oscillatory zoning in the cathodoluminescence （CL） image. The hydrothermal zircon occurs mainly in irregular forms and occasionally as anhedral grains, fills the magmatic zircon, and shows grayish white color in the BSE image and dark black color （non-luminescent） in the CL image. U-Pb analyses of the magmatic zircon indicate a Devonian event with a weighted mean ^206Pb/^238U age of （380.5±2.6） Ma. Therefore, the rock-forming process of the Shuiquangou alkaline complex is confidently constrained in Late Devonian. Compared with the magmatic zircon, the hydrothermal zircon has not only much higher Th,U content and Th/U ratios but also much younger U-Pb age of （140.2±1.3） Ma. Therefore, the ore-forming period of the Dongping gold deposit is confidently constrained in Early Cretaceous. Obviously,there is a time gap of about 240 Ma between the rock-forming process of the alkaline complex and the ore-forming process in the Dongping gold deposit. It is therefore concluded that there is no direct genetic relationship between the Shuiquangou alkaline complex and the Dongping gold deposit. According to local geological background, the authors hold that the ore-forming fluids might be genetically associated with Mesozoic magmatism in this area.