豫西小秦岭地区秦南金矿床热液独居石U-Th-Pb定年及其地质意义  

Monazite U-Th-Pb Ages of the Qinnan Gold Deposit,Xiaoqinling District:Implications for Regional Metallogenesis and Tectonic Setting

作　　者：强山峰[1,2] 毕诗健[2,3] 邓晓东[2,3] 郭连巧 李建威[2,3] 

机构地区：[1]河南省灵宝市国土资源局,河南灵宝472500 [2]中国地质大学资源学院,湖北武汉430074 [3]中国地质大学地质过程与矿产资源国家重点实验室,湖北武汉430074 [4]安徽省地质实验研究所,安徽合肥230001

出　　处：《地球科学（中国地质大学学报）》2013年第1期43-56,共14页Earth Science-Journal of China University of Geosciences

基　　金：国家自然科学基金项目(Nos.41202052;90814004);中央高校基本科研业务费专项资金(Nos.CUG110833;CUG120102);地质过程与矿产资源国家重点实验室科技部专项基金(No.MSFGPMR201005)

摘　　要：小秦岭地区是我国最重要的金矿产区之一,目前对该区金矿床成矿作用存在两种不同的观点,即与秦岭造山带造山作用有关的早中生代成矿和与中国东部构造体制大转折有关的晚中生代成矿.在矿床地质研究的基础上,利用激光剥蚀ICP-MS方法对小秦岭矿集区北矿带秦南金矿床的热液独居石进行了U-Th-Pb定年,获得了高精度的年龄数据,为北矿带金矿床成矿时代和成矿构造背景提供了新的制约.岩相学研究表明,所研究的独居石具有完好的晶形,与载金矿物黄铁矿和石英的结构关系表明它们近于同时形成.13个独居石颗粒的U-Th-Pb同位素组成在206Pb/238U-207Pb/235U和206Pb/238U-208Pb/232Th图解上均位于谐和线上或其附近,其206Pb/238U和208Pb/233Th加权平均年龄分别为120.9±0.9Ma(MSWD=1.0)和122.6±1.9Ma(MSWD=2.6),在误差范围内完全一致.本次定年结果与小秦岭南矿带众多典型金矿床的Re-Os年龄和40Ar/39Ar年龄一致,表明独居石的U-Th-Pb同位素体系自矿物形成后一直处于封闭体系,其U-Pb和U-Th-Pb年龄可以解释为秦南金矿床的成矿时代,从而表明秦南金矿床形成于燕山期.对已有可靠年龄数据的统计分析表明,小秦岭地区绝大多数金矿床形成于早白垩世,证明该区金成矿作用集中发生于晚中生代,可能与华北克拉通岩石圈减薄作用有关.Two contrasting models have been proposed to explain the large-scale gold mineralization in the Xiaoqinling gold district, which is the second largest gold producer in China. The first model advocates that gold deposits in the Xiaoqinling district formed in the Triassic in relation to continental collision of the Qinling Orogen, whereas the second indicates that gold mineralization occurred in the Early Cretaceous and was genetically related to lithospheric thinning of the North China Craton. In this paper, we present high precision monazite U-Th-Pb ages of the Qinnan gold deposit in the north metallogenic belt of the Xiaoqinling district, by laser ablation inductively coupled plasma mass spectrometry （LA-ICP-MS）. Petrographic observations show the monazite occurs as euhedral grains and invariably intergrown with quartz and pyrite, suggesting that monazites are of hydrothermal origin and precipitate with gold-bearing minerals coevally. In the 206 pb/238 U-207 pb/235 U and 206 pb/238 U_208 Pb/z3z Th diagrams, all of the 13 analyses are concordant or nearly concordant, with weighted mean z06 pb/z38 U and 208 pb/232 Th ages of 120. 9±0. 9 Ma （MSW]）= 1.0） and 122.6± 1.9 Ma （MSWI）= 2.6）, respectively. These ages are reproducible with existing geochronological data for major gold deposits in the Xiaoqinling district, demonstrating that the U-Th-Pb isotopes of the monazite have remained closed since its precipitation, and thus the U-Th-Pb ages can be reliably interpreted as the mineralizing age of the Qinnan gold deposit. A synthesis of available age constraints suggests that gold mineralization throughout the Xiaoqinling district occurred with a relatively short time interval in the Early Cretaceous and was likely related to the lithospheric thinning of the North China Craton at that time.

关 键 词：小秦岭 热液独居石 U-Th-Pb定年 地质年代学 金矿床 岩石圈减薄 

分 类 号：P631[天文地球—地质矿产勘探]